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# Front end API

## Table of Contents

- 1. Coding conventions
  - 1.1. Memory allocation
- 2. Front end
  - 2.1. Error handling
    - 2.1.1. Decoding errors
      - 2.1.1.1. C source code
      - 2.1.1.2. Fortran interface
      - 2.1.1.3. Python interface
  - 2.2. Back ends
    - 2.2.1. C
    - 2.2.2. Fortran
    - 2.2.3. Python
  - 2.3. Read/write behavior
  - 2.4. TREXIO file type
  - 2.5. File opening
    - 2.5.1. C
    - 2.5.2. Fortran
    - 2.5.3. Python
    - 2.5.4. Zero-based versus one-based arrays of indices
  - 2.6. File closing
    - 2.6.1. C
    - 2.6.2. Fortran
    - 2.6.3. Python
  - 2.7. File flushing
    - 2.7.1. C
    - 2.7.2. Fortran
    - 2.7.3. Python
  - 2.8. File existence
    - 2.8.1. C
    - 2.8.2. Fortran
    - 2.8.3. Python
  - 2.9. File copy
    - 2.9.1. C
    - 2.9.2. Fortran
    - 2.9.3. Python
  - 2.10. File state
    - 2.10.1. C
    - 2.10.2. Fortran
    - 2.10.3. Python
  - 2.11. Tasks to be done before closing
- 3. Templates for front end
  - 3.1. Description
  - 3.2. Templates for front end hasgroup functions
    - 3.2.1. Introduction
    - 3.2.2. C templates for front end
      - 3.2.2.1. Function declarations
      - 3.2.2.2. Source code
    - 3.2.3. Fortran templates for front end
    - 3.2.4. Python templates for front end
  - 3.3. Templates for front end has/read/write a single numerical attribute
    - 3.3.1. Introduction
    - 3.3.2. C templates for front end
      - 3.3.2.1. Function declarations
      - 3.3.2.2. Source code for double precision functions
      - 3.3.2.3. Source code for single precision functions
      - 3.3.2.4. Source code for default functions
    - 3.3.3. Fortran templates for front end
    - 3.3.4. Python templates for front end
  - 3.4. Templates for front end has/read/write a dataset of numerical data
    - 3.4.1. Introduction
    - 3.4.2. C templates for front end
      - 3.4.2.1. Function declarations
      - 3.4.2.2. Source code for double precision functions
      - 3.4.2.3. Source code for single precision functions
      - 3.4.2.4. Source code for memory-safe functions
      - 3.4.2.5. Source code for default functions
    - 3.4.3. Fortran templates for front end
    - 3.4.4. Python templates for front end
  - 3.5. Templates for front end has/read/write a dataset of sparse data
    - 3.5.1. Introduction
    - 3.5.2. C templates for front end
      - 3.5.2.1. Function declarations
      - 3.5.2.2. Source code for default functions
    - 3.5.3. Fortran templates for front end
    - 3.5.4. Python templates for front end
  - 3.6. Templates for front end has/read/write a dataset of strings
    - 3.6.1. Introduction
    - 3.6.2. C templates for front end
      - 3.6.2.1. Function declarations
      - 3.6.2.2. Source code for default functions
    - 3.6.3. Fortran templates for front end
    - 3.6.4. Python templates for front end
  - 3.7. Templates for front end has/read/write a buffered vector
    - 3.7.1. C source code
    - 3.7.2. Fortran interface
    - 3.7.3. Python interface
  - 3.8. Templates for front end has/read/write a single string attribute
    - 3.8.1. Introduction
    - 3.8.2. C templates for front end
      - 3.8.2.1. Function declarations
      - 3.8.2.2. Source code for default functions
    - 3.8.3. Fortran templates for front end
    - 3.8.4. Python templates for front end
  - 3.9. Templates for front end delete an entire group (UNSAFE MODE)
    - 3.9.1. Introduction
    - 3.9.2. C templates for front end
    - 3.9.3. Fortran templates for front end
    - 3.9.4. Python templates for front end
- 4. Source code for the determinant part
  - - 4.0.1. C source code
    - 4.0.2. Fortran interface
    - 4.0.3. Python interface
- 5. General helper functions
  - 5.1. C
  - 5.2. Fortran
  - 5.3. Python
- 6. Fortran helper/wrapper functions

## 1 Coding conventions

- integer types will be defined using types given in `stdint.h`
- pointers are always initialized to `NULL`
- when memory is freed, the pointer is set to `NULL`
- `assert.h` should be used extensively
- variable names are in lower case
- `#define` constants are in upper case
- structs are suffixed by `_s`
- types are suffixed by `_t`
- API calls return `trexio_exit_code` (except for `trexio_open` function)

### 1.1 Memory allocation

Memory allocation of structures can be facilitated by using the following macros, which ensure that the size of the allocated object is the same as the size of the data type pointed by the pointer. For CALLOC, we allocate N+1 to avoid errors when N=0.

    #define MALLOC(T) (T*) malloc (sizeof(T))
    #define CALLOC(N,T) (T*) calloc ( (N)+1 , sizeof(T) )

When a pointer is freed, it should be set to `NULL`. This can be facilitated by the use of the following macro:

    #define FREE(X) { free(X) ; (X)=NULL; }

The maximum string size for the filenames is 4096 characters.

    #define TREXIO_MAX_FILENAME_LENGTH 4096

## 2 Front end

All calls to TREXIO are thread-safe. TREXIO front end is modular, which simplifies implementation of new back ends.

### 2.1 Error handling

Macro | Code | Description
--- | --- | ---
`TREXIO_FAILURE` | -1 | 'Unknown failure'
`TREXIO_SUCCESS` | 0 | 'Success'
`TREXIO_INVALID_ARG_1` | 1 | 'Invalid argument 1'
`TREXIO_INVALID_ARG_2` | 2 | 'Invalid argument 2'
`TREXIO_INVALID_ARG_3` | 3 | 'Invalid argument 3'
`TREXIO_INVALID_ARG_4` | 4 | 'Invalid argument 4'
`TREXIO_INVALID_ARG_5` | 5 | 'Invalid argument 5'
`TREXIO_END` | 6 | 'End of file'
`TREXIO_READONLY` | 7 | 'Read-only file'
`TREXIO_ERRNO` | 8 | strerror(errno)
`TREXIO_INVALID_ID` | 9 | 'Invalid ID'
`TREXIO_ALLOCATION_FAILED` | 10 | 'Allocation failed'
`TREXIO_HAS_NOT` | 11 | 'Element absent'
`TREXIO_INVALID_NUM` | 12 | 'Invalid (negative or 0) dimension'
`TREXIO_ATTR_ALREADY_EXISTS` | 13 | 'Attribute already exists'
`TREXIO_DSET_ALREADY_EXISTS` | 14 | 'Dataset already exists'
`TREXIO_OPEN_ERROR` | 15 | 'Error opening file'
`TREXIO_LOCK_ERROR` | 16 | 'Error locking file'
`TREXIO_UNLOCK_ERROR` | 17 | 'Error unlocking file'
`TREXIO_FILE_ERROR` | 18 | 'Invalid file'
`TREXIO_GROUP_READ_ERROR` | 19 | 'Error reading group'
`TREXIO_GROUP_WRITE_ERROR` | 20 | 'Error writing group'
`TREXIO_ELEM_READ_ERROR` | 21 | 'Error reading element'
`TREXIO_ELEM_WRITE_ERROR` | 22 | 'Error writing element'
`TREXIO_UNSAFE_ARRAY_DIM` | 23 | 'Access to memory beyond allocated'
`TREXIO_ATTR_MISSING` | 24 | 'Attribute does not exist in the file'
`TREXIO_DSET_MISSING` | 25 | 'Dataset does not exist in the file'
`TREXIO_BACK_END_MISSING` | 26 | 'Requested back end is disabled'
`TREXIO_INVALID_ARG_6` | 27 | 'Invalid argument 6'
`TREXIO_INVALID_ARG_7` | 28 | 'Invalid argument 7'
`TREXIO_INVALID_ARG_8` | 29 | 'Invalid argument 8'
`TREXIO_INVALID_STR_LEN` | 30 | 'Invalid maxstrlen'
`TREXIO_INT_SIZE_OVERFLOW` | 31 | 'Possible integer overflow'
`TREXIO_SAFE_MODE` | 32 | 'Unsafe operation in safe mode'
`TREXIO_INVALID_ELECTRON_NUM` | 33 | 'Inconsistent number of electrons'
`TREXIO_INVALID_DETERMINANT_NUM` | 34 | 'Inconsistent number of determinants'
`TREXIO_INVALID_STATE` | 35 | 'Inconsistent state of the file'
`TREXIO_VERSION_PARSING_ISSUE` | 36 | 'Failed to parse packageversion'
`TREXIO_PHASE_CHANGE` | 37 | 'The function succeeded with a change of sign'

IMPORTANT! The code below has to be executed within Emacs each time a new error code is added to the table above. Otherwise, the codes and the corresponding message are not propagated to the source code.

    """ This script generates the C and Fortran constants for the error
        codes from the org-mode table.
    """

    result = [ "#+begin_src c :tangle prefix_front.h :exports none" ]
    for (text, code,_) in table:
      text=text.replace("~","")
      result += [ f"#define {text:30s} ((trexio_exit_code) {code:d})" ]
    result += [ "#+end_src" ]

    result += [ "" ]

    result += [ "#+begin_src f90 :tangle prefix_fortran.f90 :exports none" ]
    for (text, code,_) in table:
      text=text.replace("~","")
      result += [ f"   integer(trexio_exit_code), parameter :: {text:30s} = {code:d}" ]
    result += [ "#+end_src" ]

    result += [ "" ]

    result += [ "#+begin_src python :tangle prefix_python.py :exports none" ]
    result += [ "# define TREXIO exit codes" ]
    for (text, code,_) in table:
      text=text.replace("~","")
      result += [ f"{text:30s} = {code:d}" ]
    result += [ "#+end_src" ]

    return '\n'.join(result)

#### 2.1.1 Decoding errors

The `trexio_string_of_error` converts an exit code into a string. The string is assumed to be large enough to contain the error message (typically 128 characters).

To decode the error messages, `trexio_string_of_error` converts an error code into a string.

    128

IMPORTANT! The code below has to be executed within Emacs each time a new error code is added to the table above. Otherwise, the codes and the corresponding message are not propagated to the source code.

##### 2.1.1.1 C source code

    const char*
    trexio_string_of_error (const trexio_exit_code error)
    {
      switch (error) {
      case TREXIO_FAILURE:
        return "Unknown failure";
        break;
      case TREXIO_SUCCESS:
        return "Success";
        break;
      case TREXIO_INVALID_ARG_1:
        return "Invalid argument 1";
        break;
      case TREXIO_INVALID_ARG_2:
        return "Invalid argument 2";
        break;
      case TREXIO_INVALID_ARG_3:
        return "Invalid argument 3";
        break;
      case TREXIO_INVALID_ARG_4:
        return "Invalid argument 4";
        break;
      case TREXIO_INVALID_ARG_5:
        return "Invalid argument 5";
        break;
      case TREXIO_END:
        return "End of file";
        break;
      case TREXIO_READONLY:
        return "Read-only file";
        break;
      case TREXIO_ERRNO:
        return strerror(errno);
        break;
      case TREXIO_INVALID_ID:
        return "Invalid ID";
        break;
      case TREXIO_ALLOCATION_FAILED:
        return "Allocation failed";
        break;
      case TREXIO_HAS_NOT:
        return "Element absent";
        break;
      case TREXIO_INVALID_NUM:
        return "Invalid (negative or 0) dimension";
        break;
      case TREXIO_ATTR_ALREADY_EXISTS:
        return "Attribute already exists";
        break;
      case TREXIO_DSET_ALREADY_EXISTS:
        return "Dataset already exists";
        break;
      case TREXIO_OPEN_ERROR:
        return "Error opening file";
        break;
      case TREXIO_LOCK_ERROR:
        return "Error locking file";
        break;
      case TREXIO_UNLOCK_ERROR:
        return "Error unlocking file";
        break;
      case TREXIO_FILE_ERROR:
        return "Invalid file";
        break;
      case TREXIO_GROUP_READ_ERROR:
        return "Error reading group";
        break;
      case TREXIO_GROUP_WRITE_ERROR:
        return "Error writing group";
        break;
      case TREXIO_ELEM_READ_ERROR:
        return "Error reading element";
        break;
      case TREXIO_ELEM_WRITE_ERROR:
        return "Error writing element";
        break;
      case TREXIO_UNSAFE_ARRAY_DIM:
        return "Access to memory beyond allocated";
        break;
      case TREXIO_ATTR_MISSING:
        return "Attribute does not exist in the file";
        break;
      case TREXIO_DSET_MISSING:
        return "Dataset does not exist in the file";
        break;
      case TREXIO_BACK_END_MISSING:
        return "Requested back end is disabled";
        break;
      case TREXIO_INVALID_ARG_6:
        return "Invalid argument 6";
        break;
      case TREXIO_INVALID_ARG_7:
        return "Invalid argument 7";
        break;
      case TREXIO_INVALID_ARG_8:
        return "Invalid argument 8";
        break;
      case TREXIO_INVALID_STR_LEN:
        return "Invalid max_str_len";
        break;
      case TREXIO_INT_SIZE_OVERFLOW:
        return "Possible integer overflow";
        break;
      case TREXIO_SAFE_MODE:
        return "Unsafe operation in safe mode";
        break;
      case TREXIO_INVALID_ELECTRON_NUM:
        return "Inconsistent number of electrons";
        break;
      case TREXIO_INVALID_DETERMINANT_NUM:
        return "Inconsistent number of determinants";
        break;
      case TREXIO_INVALID_STATE:
        return "Inconsistent state of the file";
        break;
      case TREXIO_VERSION_PARSING_ISSUE:
        return "Failed to parse package_version";
        break;
      case TREXIO_PHASE_CHANGE:
        return "The function succeeded with a change of sign";
        break;
      }
      return "Unknown error";
    }

    void
    trexio_string_of_error_f (const trexio_exit_code error, char result[128])
    {
      strncpy(result, trexio_string_of_error(error), 128);
    }

##### 2.1.1.2 Fortran interface

    interface
       subroutine trexio_string_of_error (error, string) bind(C, name='trexio_string_of_error_f')
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_exit_code), intent(in), value :: error
         character(kind=c_char), intent(out)          :: string(128)
       end subroutine trexio_string_of_error
    end interface

##### 2.1.1.3 Python interface

    class Error(Exception):
        """Base class for TREXIO errors.

        Attributes:
           error: int -- exit code returned by the call to TREXIO library;
           message: str -- decoded string corresponding to trexio_exit_code.

        """

        def __init__(self, trexio_return_code):
            self.error = trexio_return_code
            self.message = string_of_error(trexio_return_code)
            super().__init__(self.message)

    def string_of_error(return_code: int) -> str:
        """Decode the TREXIO exit code.

        Argument is an integer return code that correspond to one of the TREXIO errors.
        Returns a string that contains description of TREXIO ~return_code~.
        """

        return pytr.trexio_string_of_error(return_code)

### 2.2 Back ends

TREXIO has several back ends:

1. `TREXIO_HDF5` relies on extensive use of the HDF5 library and the associated file format. The HDF5 file is binary and tailored to high-performance I/O. This back end is the default one. HDF5 can be compiled with MPI for parallel I/O. Note, that HDF5 has to be downloaded and installed independently of TREXIO, which may cause some obstacles, especially when the user is not allowed to install external software. The produced files usually have `.h5` extension.
2. `TREXIO_TEXT` relies on basic file I/O in C, namely `fopen, fclose, fprintf, fscanf` etc. from `stdio.h` library. This back end is not optimized for performance. It is supposed to be used for debug purposes or, for example, when the user wants to modify some data manually within the file. This back end is supposed to work "out-of-the-box" since there are no external dependencies, which might be useful for users that do not have access to HDF5 library. The produced files usually have `.txt` extension.

`TREXIO_AUTO` can be provided as a back end when opening an existing TREXIO file in read-only `'r'` mode. In this case, TREXIO will try to automatically detect the back end, which should be used to open the file.

Additional back ends can be implemented thanks to the modular nature of the front end. This can be achieved by adding a new `case` (corresponding to the desired back end) in the front-end `switch`. Then the corresponding back-end `has/read/write` functions has to be implemented. For example, see the commented lines that correspond to the `TREXIO_JSON` back end (not implemented yet).

Note: It is important to increment the value of `TREXIO_INVALID_BACK_END` when a new back end is implemented. Otherwise, it will not be available.

#### 2.2.1 C

    typedef int32_t back_end_t;

    #define TREXIO_HDF5             ( (back_end_t) 0 )
    #define TREXIO_TEXT             ( (back_end_t) 1 )
    #define TREXIO_INVALID_BACK_END ( (back_end_t) 2 )
    #define TREXIO_AUTO             TREXIO_INVALID_BACK_END
    /*#define TREXIO_JSON             ( (back_end_t) 2 )*/

    #define TREXIO_DELIM            "\n"

The helper function `trexio_has_back_end` returns `true` if TREXIO compilation includes a back end provided as an argument; `false` otherwise. This is useful due to the fact that HDF5 back end can be disabled at configure step.

    bool trexio_has_backend(back_end_t back_end);
    bool trexio_has_back_end(back_end_t back_end);

    bool trexio_has_back_end(back_end_t back_end) {
      switch (back_end) {
        case TREXIO_TEXT:
          return true;
        case TREXIO_HDF5:
    #ifdef HAVE_HDF5
          return true;
    #else
          return false;
    #endif
      }
      return false;
    }

    bool trexio_has_backend(back_end_t back_end) {
      return trexio_has_back_end(back_end);
    }

#### 2.2.2 Fortran

      integer(trexio_back_end_t), parameter :: TREXIO_HDF5 = 0
      integer(trexio_back_end_t), parameter :: TREXIO_TEXT = 1
    ! integer(trexio_back_end_t), parameter :: TREXIO_JSON = 2
      integer(trexio_back_end_t), parameter :: TREXIO_INVALID_BACK_END = 2
      integer(trexio_back_end_t), parameter :: TREXIO_AUTO = TREXIO_INVALID_BACK_END

The function below is a Fortran interface for the aforementioned C-compatible `trexio_has_back_end` function.

    interface
       logical(c_bool) function trexio_has_back_end (back_end) bind(C)
         import
         integer(trexio_back_end_t), intent(in), value :: back_end
       end function trexio_has_back_end
    end interface

    interface
       logical(c_bool) function trexio_has_backend (back_end) bind(C)
         import
         integer(trexio_back_end_t), intent(in), value :: back_end
       end function trexio_has_backend
    end interface

Originally, the function was named `trexio_has_backend`. For consistency, in version 2.2 it was renamed `trexio_has_back_end`.

#### 2.2.3 Python

    # define TREXIO back ends
    TREXIO_HDF5 = 0
    TREXIO_TEXT = 1
    #TREXIO_JSON = 2
    TREXIO_INVALID_BACK_END = 2
    TREXIO_AUTO = TREXIO_INVALID_BACK_END

### 2.3 Read/write behavior

Every time a reading function is called, the data is read from the disk. If data needs to be cached, this is left to the user of the library.

Writing to TREXIO files is done with transactions (all-or-nothing effect). File writes are attempted by calling explicitly the write (`TREXIO_HDF5`) or flush (`TREXIO_TEXT`) function, or when the TREXIO file is closed. If writing is impossible because the data is not valid, no data is written.

The order in which the data is written is not necessarily consistent with the order in which the function calls were made.

The TREXIO files are supposed to be opened by only one program at a time: if the same TREXIO file is modified simultaneously by multiple concurrent programs, the behavior is not specified.

### 2.4 TREXIO file type

`trexio_s` is the the main type for TREXIO files, visible to the users of the library. This type is kept opaque, and all modifications to the files will be necessarily done through the use of functions, taking such a type as argument.

File creation and opening functions will return TREXIO file handles, namely pointers to `trexio_s` types. All functions accessing to the TREXIO files will have as a first argument the TREXIO file handle.

    typedef struct trexio_s trexio_t;

    struct trexio_s {
      char              file_name[TREXIO_MAX_FILENAME_LENGTH];
      pthread_mutex_t   thread_lock;
      back_end_t        back_end;
      char              mode;
      bool              one_based;
      int32_t           state;
      int16_t           version_major;
      int16_t           version_minor;
      int16_t           version_patch;
      char              version[16];
    };

File class for the Python API is defined below. Use of Python class make it more intuitive and more python-ic to work with TREXIO files.

    class File:
        """trexio.File class.

        General information about the TREXIO file.

        Parameters:

        filename: str
            Is a name of the full path to the TREXIO file.
        back_end: int
            One of the currently supported TREXIO back ends.
            For example, TREXIO_HDF5 (0) or TREXIO_TEXT (1).
        mode: str
            One of the currently supported TREXIO open modes.
            For example, 'r' or 'w'.
        state: int
            Active (excited or ground) state of the file.
            Default is 0.
        isOpen: bool
            Flag indicating whether the current object is still open for I/O
        pytrexio_s:
            A PyObject corresponding to SWIG proxy of the trexio_s struct in C.
            This argument is in fact a TREXIO file handle, which is required for
            communicating with the C back end.
        info: dict
            Dictionary of key-value pairs with additional information about the file.
        """

        def __init__(self, filename, mode='r', back_end=TREXIO_HDF5,
                     pytrexio_s=None, info=None):
            """TREXIO File class constructor."""
            self.filename = filename
            self.mode = mode
            self.state = 0

            self.isOpen = False
            self.exists = False
            if pytrexio_s is None:
                self.pytrexio_s = _open(filename, mode, back_end)
                self.isOpen = True
                self.exists = True
            else:
                self.pytrexio_s = pytrexio_s
                self.isOpen = None
                self.exists = None

            self.back_end = self.pytrexio_s.back_end
            self.info = info

        def __enter__(self):
            """Enter statement for with ... as ... handling."""
            return self

        def __exit__(self, *args):
            """Exit statement for with ... as ... handling."""
            if self.isOpen:
               self.close()

        def close(self):
            """Close a TREXIO File."""
            if self.isOpen:
                _close(self.pytrexio_s)
                self.isOpen = False
            else:
                raise Exception("TREXIO File object has not been opened.")

        def set_state(self, state):
            """Set the state of the TREXIO File."""
            if not isinstance(state, int):
                raise TypeError("state argument has to be int")

            rc = pytr.trexio_set_state(self.pytrexio_s, state)
            if rc != TREXIO_SUCCESS:
                raise Error(rc)

            self.state = state

        def get_state(self):
            """Get the state of the TREXIO File."""
            rc, state = pytr.trexio_get_state(self.pytrexio_s)
            if rc != TREXIO_SUCCESS:
                raise Error(rc)

            if state != self.state:
                raise Exception("Inconsistent state of the TREXIO file.")

            return self.state

        def inquire(self):
            """Inquire whether a TREXIO file exists."""
            self.exists = _inquire(self.filename)

        def __del__(self):
            """TREXIO File class destructor."""
            if self.isOpen:
                _close(self.pytrexio_s)
            elif self.isOpen is None:
                raise Exception("[WIP]: TREXIO file handle provided but what if the file is already closed?")
            else:
                pass

### 2.5 File opening

`trexio_open` creates a new TREXIO file or opens the existing one.

Input parameters:

1. `file_name` - string containing file name
2. `mode` - character containing open mode (see below)
   - `'w'` - (write) creates a new file as READWRITE
   - `'r'` - (read) opens existing file as READONLY
   - `'u'` - (unsafe) opens existing file as READWRITE with the possibility to overwrite blocks and delete full groups.
3. `back_end` - integer number (or the corresponding global parameter) specifying the back end
   - `TREXIO_HDF5` - for HDF5 back end (integer alternative: 0)
   - `TREXIO_TEXT` - for TEXT back end (integer alternative: 1)
   - `TREXIO_AUTO` - to automatically detect the applicable back end

Output:

- Pointer to the `trexio_t` (struct) file handle.

Upon creation of the TREXIO file, the `package_version` attribute of the `metadata` group is set to the `TREXIO_PACKAGE_VERSION` string. This attribute is not automatically updated when the file is modified with newer versions. However, this can be done manually by the user in unsafe mode.

Note: internal consistency is not guaranteed once the file has been modified in `'u'` (unsafe) mode. Upon the first unsafe call to `trexio_open` the `unsafe` attribute of the `metadata` group will be set to 1 (true) in order to indicate that the file might be inconsistent due to the use of `'u'` mode.

Note: the `file_name` in TEXT back end actually corresponds to the name of the directory where `.txt` data files are stored. The actual name of each `.txt` file corresponds to the group name provided in `trex.config` (e.g. `nucleus.txt` for nuclei-related data). These names are populated by the generator.py (i.e. they are hard-coded), which is why the user should tend to avoid renaming the `.txt` data files.

#### 2.5.1 C

    trexio_t*
    trexio_open(const char* file_name, const char mode,
                const back_end_t back_end, trexio_exit_code* const rc_open)
    {

      if (file_name == NULL || file_name[0] == '\0') {
        if (rc_open != NULL) *rc_open = TREXIO_INVALID_ARG_1;
        return NULL;
      }
      /* Check overflow in file_name */

      /* Check that the mode is valid */
      if (mode != 'r' && mode != 'w' && mode != 'u') {
        if (rc_open != NULL) *rc_open = TREXIO_INVALID_ARG_2;
        return NULL;
      }

      /* Check that the back end is valid in non-read mode */
      if ((back_end <  0 || back_end >= TREXIO_INVALID_BACK_END) && mode != 'r') {
        if (rc_open != NULL) *rc_open = TREXIO_INVALID_ARG_3;
        return NULL;
      }

      /* Check that the back end is valid in read-only mode */
      if ((back_end <  0 || back_end > TREXIO_INVALID_BACK_END) && mode == 'r') {
        if (rc_open != NULL) *rc_open = TREXIO_INVALID_ARG_3;
        return NULL;
      }

      back_end_t back_end_local = back_end;
      /* Try to determine the applicable backend if the back_end argument is TREXIO_AUTO */
      if (back_end == TREXIO_AUTO && mode == 'r') {
    #ifdef HAVE_HDF5
        /* Check if the TREXIO file exists and if it is a directory */
        trexio_exit_code rc_text = trexio_text_inquire(file_name);
        if (rc_text == TREXIO_SUCCESS) {
          back_end_local = TREXIO_TEXT;
        } else {
        /* If not, check if it is an HDF5 file */
          trexio_exit_code rc_hdf5 = trexio_hdf5_inquire(file_name);
          if (rc_hdf5 == TREXIO_SUCCESS) {
            back_end_local = TREXIO_HDF5;
          } else {
        /* File is neither a directory nor an HDF5 file -> return an error */
            if (rc_open != NULL) *rc_open = TREXIO_FILE_ERROR;
            return NULL;
          }
        }
    #else
        /* In the current implementation if HDF5 back end is not available - then there is only back end left */
        back_end_local = TREXIO_TEXT;
    #endif
      }

      trexio_t* result = NULL;
      void* result_tmp = NULL;

      /* Allocate data structures */
      switch (back_end_local) {

      case TREXIO_TEXT:
        result_tmp = malloc(sizeof(trexio_text_t));
        break;

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        result_tmp = malloc(sizeof(trexio_hdf5_t));
        break;
    #else
        if (rc_open != NULL) *rc_open = TREXIO_BACK_END_MISSING;
        return NULL;
    #endif

      default:
        if (rc_open != NULL) *rc_open = TREXIO_FILE_ERROR;
        return NULL;

    /*
      case TREXIO_JSON:
        result = (trexio_t*) malloc (sizeof(trexio_json_t));
        break;
    */
      }
      result = (trexio_t*) result_tmp;

      assert (result != NULL);    /* TODO: Error handling */

      /* Data for the parent type */

      strncpy(result->file_name, file_name, TREXIO_MAX_FILENAME_LENGTH);
      if (result->file_name[TREXIO_MAX_FILENAME_LENGTH-1] != '\0') {
        if (rc_open != NULL) *rc_open = TREXIO_INVALID_ARG_1;
        free(result);
        return NULL;
      }

      result->back_end    = back_end_local;
      result->mode        = mode;
      result->one_based   = false;  // Need to be flipped in Fortran interface
      result->state       = 0;      // By default the file corresponds to a ground state
      int irc = pthread_mutex_init ( &(result->thread_lock), NULL);
      if (irc != 0) {
        if (rc_open != NULL) *rc_open = TREXIO_FAILURE;
        free(result);
        return NULL;
      }
      // assert (irc == 0);

      trexio_exit_code rc;

      /* Back end initialization */

      rc = TREXIO_OPEN_ERROR;

      switch (back_end_local) {

      case TREXIO_TEXT:
        rc = trexio_text_init(result);
        break;

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        rc = trexio_hdf5_init(result);
        break;
    #else
        if (rc_open != NULL) *rc_open = TREXIO_BACK_END_MISSING;
        free(result);
        return NULL;
    #endif
    /*
      case TREXIO_JSON:
        rc = trexio_json_init(result);
        break;
    */
      }

      if (rc != TREXIO_SUCCESS) {
        if (rc_open != NULL) *rc_open = TREXIO_OPEN_ERROR;
        free(result);
        return NULL;
      }

      /* File locking */

      rc = TREXIO_LOCK_ERROR;

      switch (back_end_local) {

      case TREXIO_TEXT:
        rc = trexio_text_lock(result);
        break;
      /* HDF5 v.>=1.10 has file locking activated by default */
      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        rc = TREXIO_SUCCESS;
        break;
    #else
        if (rc_open != NULL) *rc_open = TREXIO_BACK_END_MISSING;
        free(result);
        return NULL;
    #endif
    /*
      case TREXIO_JSON:
        rc = trexio_json_lock(result);
        break;
    */
      }

      if (rc != TREXIO_SUCCESS) {
        if (rc_open != NULL) *rc_open = TREXIO_LOCK_ERROR;
        free(result);
        return NULL;
      }

      /* Write metadata (i.e. package version) upon creation */
      rc = trexio_has_metadata_package_version(result);
      if (rc == TREXIO_FAILURE) {
        if (rc_open != NULL) *rc_open = TREXIO_OPEN_ERROR;
        free(result);
        return NULL;
      }

      if (rc == TREXIO_HAS_NOT) {
      /* Write TREXIO_PACKAGE_VERSION upon creation of the file */
        switch (back_end_local) {

        case TREXIO_TEXT:
          rc = trexio_text_write_metadata_package_version(result, TREXIO_PACKAGE_VERSION);
          break;

        case TREXIO_HDF5:
    #ifdef HAVE_HDF5
          rc = trexio_hdf5_write_metadata_package_version(result, TREXIO_PACKAGE_VERSION);
          break;
    #else
          if (rc_open != NULL) *rc_open = TREXIO_BACK_END_MISSING;
          free(result);
          return NULL;
    #endif

        }

        if (rc != TREXIO_SUCCESS) {
          if (rc_open != NULL) *rc_open = rc;
          free(result);
          return NULL;
        }

        result->version_major = TREXIO_VERSION_MAJOR;
        result->version_minor = TREXIO_VERSION_MINOR;
        result->version_patch = TREXIO_VERSION_PATCH;
        strncpy(result->version, TREXIO_PACKAGE_VERSION, 16);

      } else {
      /* Otherwise read the metadata_package_version to get the TREXIO version upon creation of the file */
        char version_origin[16];

        switch (back_end_local) {

        case TREXIO_TEXT:
          rc = trexio_text_read_metadata_package_version(result, version_origin, 16);
          break;

        case TREXIO_HDF5:
    #ifdef HAVE_HDF5
          rc = trexio_hdf5_read_metadata_package_version(result, version_origin, 16);
          break;
    #else
          if (rc_open != NULL) *rc_open = TREXIO_BACK_END_MISSING;
          free(result);
          return NULL;
    #endif

        }

        if (rc != TREXIO_SUCCESS) {
          if (rc_open != NULL) *rc_open = rc;
          free(result);
          return NULL;
        }

        int16_t version_major, version_minor, version_patch;
        int rc_scan = sscanf(version_origin,
                             "%3" SCNd16 ".%5" SCNd16 ".%5" SCNd16,
                             &version_major, &version_minor, &version_patch);
        if (rc_scan != 3) {
          if (rc_open != NULL) *rc_open = TREXIO_VERSION_PARSING_ISSUE;
          free(result);
          return NULL;
        }

        result->version_major = version_major;
        result->version_minor = version_minor;
        result->version_patch = version_patch;
        strncpy(result->version, version_origin, 16);

      }

      if (result->version[15] != '\0' || result->version_major == 0) {
        if (rc_open != NULL) *rc_open = TREXIO_FAILURE;
        free(result);
        return NULL;
      }

      /* Mark the file as unsafe upon opening in UNSAFE 'u' mode */
      if (mode == 'u') {

        rc = trexio_has_metadata_unsafe(result);
        if (rc == TREXIO_FAILURE) {
          if (rc_open != NULL) *rc_open = TREXIO_OPEN_ERROR;
          free(result);
          return NULL;
        }

        if (rc == TREXIO_HAS_NOT) {
          int64_t unsafe_val = 1;
          switch (back_end) {

          case TREXIO_TEXT:
            rc = trexio_text_write_metadata_unsafe(result, unsafe_val);
            break;

          case TREXIO_HDF5:
    #ifdef HAVE_HDF5
            rc = trexio_hdf5_write_metadata_unsafe(result, unsafe_val);
            break;
    #else
            if (rc_open != NULL) *rc_open = TREXIO_BACK_END_MISSING;
            free(result);
            return NULL;
    #endif
          }
        }

        if (rc != TREXIO_SUCCESS) {
          if (rc_open != NULL) *rc_open = rc;
          free(result);
          return NULL;
        }

      }

      /* Exit upon success */
      if (rc_open != NULL) *rc_open = TREXIO_SUCCESS;

      return result;
    }

#### 2.5.2 Fortran

    interface
       integer(trexio_t) function trexio_open_c (filename, mode, back_end, rc_open) bind(C, name="trexio_open")
         use, intrinsic :: iso_c_binding
         import
         character(kind=c_char), dimension(*)          :: filename
         character(kind=c_char), intent(in), value     :: mode
         integer(trexio_back_end_t), intent(in), value :: back_end
         integer(trexio_exit_code), intent(out)        :: rc_open
       end function trexio_open_c
    end interface

#### 2.5.3 Python

    def _open(file_name: str, mode: str, back_end: int):
        """Create TREXIO file or open existing one.

        Parameters:

        file_name: str
            Name of the TREXIO file

        mode: str
            One of the currently supported ~open~ modes (e.g. 'w', 'r')

        back_end: int
            One of the currently supported TREXIO back ends (e.g. TREXIO_HDF5, TREXIO_TEXT)

        Return:
            SWIG object of type trexio_s.

        Examples:
        >>> from trexio import open as tr_open
        >>> trex_file = tr_open("example.h5", "w", TREXIO_HDF5)
        """

        # The new trexio_open function is capable of returning error code which SWIG can append to the output trexio_s file struct
        # However, if trexio_s* == NULL, then SWIG returns only an error code rc_open instead of a list [trexio_s, rc_open]
        # Thus, the following try/except sequence is needed
        try:
            return_obj = pytr.trexio_open(file_name, mode, back_end)
            assert return_obj is not None
            if isinstance(return_obj, int):
                raise Error(return_obj)
            else:
                rc_open = return_obj[1]
                # this is a sanity check in case the code evolves and SWIG issue is patched
                if rc_open == TREXIO_SUCCESS:
                    trexio_file = return_obj[0]
                    assert trexio_file is not None
        except AssertionError:
            raise Exception(f"Could not open TREXIO file {file_name} using trexio_open function. The return value is None (NULL pointer).")

        return trexio_file

#### 2.5.4 Zero-based versus one-based arrays of indices

Because arrays are zero-based in Fortran, we need to set a flag to know if we need to shift by 1 arrays of indices.

    trexio_exit_code trexio_set_one_based(trexio_t* file)
    {
      if (file == NULL) return TREXIO_FILE_ERROR;

      file->one_based = true;

      return TREXIO_SUCCESS;
    }

    interface
       integer(trexio_exit_code) function trexio_set_one_based(trex_file) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
       end function trexio_set_one_based
    end interface

### 2.6 File closing

`trexio_close` closes an existing `trexio_t` file.

input parameters: `file` – TREXIO file handle.

output: `trexio_exit_code` exit code.

#### 2.6.1 C

    trexio_exit_code
    trexio_close (trexio_t* file)
    {

      if (file == NULL) return TREXIO_FILE_ERROR;

      trexio_exit_code rc = TREXIO_FAILURE;

      assert(file->back_end < TREXIO_INVALID_BACK_END);

      /* Things to be done before closing the file in the back-end */
      rc = trexio_pre_close(file);
      if (rc != TREXIO_SUCCESS) return rc;

      /* Terminate the back end */
      switch (file->back_end) {

      case TREXIO_TEXT:
        rc = trexio_text_deinit(file);
        break;

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        rc = trexio_hdf5_deinit(file);
        break;
    #else
        FREE(file);
        return TREXIO_BACK_END_MISSING;
    #endif
    /*
      case TREXIO_JSON:
        rc = trexio_json_deinit(file);
        break;
    */
      }

      if (rc != TREXIO_SUCCESS) {
        FREE(file);
        return rc;
      }

      /* File unlocking */

      rc = TREXIO_UNLOCK_ERROR;

      switch (file->back_end) {

      case TREXIO_TEXT:
        rc = trexio_text_unlock(file);
        break;

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        rc = TREXIO_SUCCESS;
        break;
    #else
        FREE(file);
        return TREXIO_BACK_END_MISSING;
    #endif
    /*
      case TREXIO_JSON:
        rc = trexio_json_unlock(file);
        break;
    */
      }

      /* Terminate front end */

      int irc = pthread_mutex_destroy( &(file->thread_lock) );

      free(file);

      if (irc != 0) return TREXIO_ERRNO;
      if (rc != TREXIO_SUCCESS) return rc;

      return TREXIO_SUCCESS;
    }

#### 2.6.2 Fortran

    interface
       integer(trexio_exit_code) function trexio_close (trex_file) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
       end function trexio_close
    end interface

#### 2.6.3 Python

    def _close(trexio_file):
        """Close TREXIO file.

        Parameter is a ~trexio_file~ object that has been created by a call to ~open~ function.
        """

        rc = pytr.trexio_close(trexio_file)
        if rc != TREXIO_SUCCESS:
            raise Error(rc)

### 2.7 File flushing

`trexio_flush` flushes all buffers into the `trexio_t` file.

input parameters: `file` – TREXIO file handle.

output: `trexio_exit_code` exit code.

#### 2.7.1 C

    trexio_exit_code
    trexio_flush (trexio_t* file)
    {

      if (file == NULL) return TREXIO_FILE_ERROR;

      trexio_exit_code rc = TREXIO_FAILURE;

      assert(file->back_end < TREXIO_INVALID_BACK_END);

      /* Terminate the back end */
      switch (file->back_end) {

      case TREXIO_TEXT:
        rc = trexio_text_flush(file);
        break;

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        rc = trexio_hdf5_flush(file);
        break;
    #else
        return TREXIO_BACK_END_MISSING;
    #endif
    /*
      case TREXIO_JSON:
        rc = trexio_json_flush(file);
        break;
    */
      }

      return rc;
    }

#### 2.7.2 Fortran

    interface
       integer(trexio_exit_code) function trexio_flush (trex_file) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
       end function trexio_flush
    end interface

#### 2.7.3 Python

    def flush(trexio_file):
        """Flush buffers into the TREXIO file.

        Parameter is a ~trexio_file~ object that has been created by a call to ~open~ function.
        """

        rc = pytr.trexio_flush(trexio_file)
        if rc != TREXIO_SUCCESS:
            raise Error(rc)

### 2.8 File existence

`trexio_inquire` check whether TREXIO file exists.

input parameters: `file_name` - string containing file name

output: `trexio_exit_code` exit code.

It returns:

- `TREXIO_SUCCESS` if the `file_name` exists and if it correspond to a valid TREXIO file (i.e. a directory OR an HDF5 file)
- `TREXIO_FILE_ERROR` if `file_name` exists but it is not a TREXIO file
- `TREXIO_FAILURE` otherwise

Also, there is an implicit way to have the aforementioned functionality. Attempt to open a non-existing file with `trexio_open` function will result in a `TREXIO_OPEN_ERROR` exit code (see the modified value that has been passed as a 4-th argument to `trexio_open`).

You can see examples of both functionalities in `test_f.f90` (search for calls with `_void` suffix).

#### 2.8.1 C

    trexio_exit_code
    trexio_inquire (const char* file_name)
    {

      if (file_name == NULL || file_name[0] == '\0') return TREXIO_INVALID_ARG_1;

      /* First check if the TREXIO file exists and if it is a directory */
      trexio_exit_code rc_text = trexio_text_inquire(file_name);

    #ifdef HAVE_HDF5
      /* FILE_ERROR here means that the file exists but it is not a directory -> check with HDF5 */
      if (rc_text == TREXIO_FILE_ERROR) {
        trexio_exit_code rc_hdf5 = trexio_hdf5_inquire(file_name);
        return rc_hdf5;
      /* If rc_text is TREXIO_SUCCESS -> file is a TREXIO directory; TREXIO_FAILURE -> file/directory does not exist */
      } else {
        return rc_text;
      }
    #else
      return rc_text;
    #endif
    }

#### 2.8.2 Fortran

The function below is a C binding. The front end Fortran function for `trexio_inquire` can be found in the `Fortran helper/wrapper functions`.

    interface
       integer(trexio_exit_code) function trexio_inquire_c (filename) bind(C, name="trexio_inquire")
         use, intrinsic :: iso_c_binding
         import
         character(kind=c_char), dimension(*)       :: filename
       end function trexio_inquire_c
    end interface

#### 2.8.3 Python

    def _inquire(file_name: str) -> bool:
        """Check whether ~file_name~ TREXIO file exists.
        """
        rc = pytr.trexio_inquire(trexio_file_name)
        if rc == TREXIO_SUCCESS:
            return True
        elif rc == TREXIO_FAILURE:
            return False
        else:
            raise Error(rc)

### 2.9 File copy

`trexio_cp` copies a TREXIO file using `cp`.

Input parameters:

1. `source_file_name` - string containing the name of the source file
2. `destination_file_name` - string containing the name of the new file

Output:

- `trexio_exit_code`

#### 2.9.1 C

    trexio_exit_code
    trexio_cp(const char* source, const char* dest)
    {

      if (source == NULL || source[0] == '\0') {
        return TREXIO_INVALID_ARG_1;
      }

      if (dest == NULL || dest[0] == '\0') {
        return TREXIO_INVALID_ARG_2;
      }

      back_end_t back_end_local = TREXIO_AUTO;
      /* Try to determine the applicable backend if the back_end argument is TREXIO_AUTO */
      /* Check if the TREXIO file exists and if it is a directory */
      trexio_exit_code rc_text = trexio_text_inquire(source);
      if (rc_text == TREXIO_SUCCESS) {
        back_end_local = TREXIO_TEXT;
      }
    #ifdef HAVE_HDF5
      else {
        /* If not, check if it is an HDF5 file */
        trexio_exit_code rc_hdf5 = trexio_hdf5_inquire(source);
        if (rc_hdf5 == TREXIO_SUCCESS) {
          back_end_local = TREXIO_HDF5;
        } else {
          /* File is neither a directory nor an HDF5 file -> return an error */
          return TREXIO_FILE_ERROR;
        }
      }
    #else
      else {
        return TREXIO_FILE_ERROR;
      }
    #endif
      assert (back_end_local != TREXIO_AUTO);

      if (trexio_inquire(dest) == TREXIO_SUCCESS) {
        /* Destination file already exists */
        return TREXIO_FILE_ERROR;
      }

      /* Call cp */

    #ifndef CP_COMMAND
    #define CP_COMMAND "cp", "-r"
    #endif

      pid_t pid = fork();
      if (pid == 0) {
        execlp("cp", CP_COMMAND, source, dest, (char *)0);
      } else if (pid < 0) {
        return TREXIO_FILE_ERROR;
      } else {
        int wstatus;
        pid_t ws = waitpid( pid, &wstatus, 0);
        if (ws != pid || !WIFEXITED(wstatus) )
          return TREXIO_FILE_ERROR;
      }

      return TREXIO_SUCCESS;
    }

#### 2.9.2 Fortran

    interface
       integer(trexio_exit_code) function trexio_cp_c (source, destination) bind(C, name="trexio_cp")
         use, intrinsic :: iso_c_binding
         import
         character(kind=c_char), dimension(*)          :: source, destination
       end function trexio_cp_c
    end interface

#### 2.9.3 Python

    def _cp(source: str, destination: str):
        """Copies a TREXIO file

        Parameters:

        source: str
            Name of the source file

        destination: str
            Name of the destination file

        Examples:
        >>> from trexio import cp as tr_cp
        >>> tr_cp("example.h5", "new.h5")
        """

        from shutil import copytree, copyfile

        f = File(filename=source, mode='r', back_end=TREXIO_AUTO)
        if f.back_end == TREXIO_TEXT:
          copytree(source, destination)
        elif f.back_end == TREXIO_HDF5:
          copyfile(source, destination)

### 2.10 File state

Note: the use of the functions below is discouraged as of version 2.3.0. Please use `trexio_write_state_id` and `trexio_read_state_id` to get the state corresponding to a given `trexio_t` file. There can be only one `state_id` per file, namely data for excited state wave functions have to go in a file different from the ground state one (see the `state` group description for more info about how you can link the ground state TREXIO file with its excited states).

`trexio_set_state` set an existing `trexio_t` file handle to a given state and write it as `state_id` attribute. `trexio_get_state` returns current state of the `trexio_t` file handle.

input parameters: `file` – TREXIO file handle. `state` – `int32_t` ID of a state (0 for ground state).

output: `trexio_exit_code` exit code.

#### 2.10.1 C

    trexio_exit_code
    trexio_set_state (trexio_t* file, const int32_t num)
    {
      if (file == NULL) return TREXIO_INVALID_ARG_1;

      /* Write state_id in the file (as of v.2.3.0) */
      trexio_exit_code rc = trexio_has_state_id(file);
      if (rc == TREXIO_HAS_NOT || file->mode == 'u') {
        trexio_exit_code rc_w = trexio_write_state_id(file, num);
        if (rc_w != TREXIO_SUCCESS) return rc_w;
      }

      file->state = num;

      return TREXIO_SUCCESS;
    }

    trexio_exit_code
    trexio_get_state (trexio_t* file, int32_t* const num)
    {
      if (file == NULL) return TREXIO_INVALID_ARG_1;
      if (num  == NULL) return TREXIO_INVALID_ARG_2;

      /* Read state_id from the file (as of v.2.3.0) */
      int32_t state_id = 0;
      trexio_exit_code rc = trexio_has_state_id(file);
      if (rc == TREXIO_SUCCESS) {
        trexio_exit_code rc_r = trexio_read_state_id(file, &state_id);
        if (rc_r != TREXIO_SUCCESS) return rc_r;
      }

      /* If the state is not in a file then state_id=0, i.e. ground state */
      *num = state_id;

      return TREXIO_SUCCESS;
    }

#### 2.10.2 Fortran

    interface
       integer(trexio_exit_code) function trexio_set_state (trex_file, state) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         integer(c_int32_t), intent(in), value :: state
       end function trexio_set_state
    end interface

    interface
       integer(trexio_exit_code) function trexio_get_state (trex_file, state) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         integer(c_int32_t), intent(out)       :: state
       end function trexio_get_state
    end interface

#### 2.10.3 Python

See TREXIO File Python class.

### 2.11 Tasks to be done before closing

    trexio_exit_code
    trexio_pre_close (trexio_t* file)
    {

      if (file == NULL) return TREXIO_FILE_ERROR;

      trexio_exit_code rc;

      /* Check consistency between number of determinants and coefficients stored in the file */

      if (file->version_major >= 2 && file->version_minor >= 2) {

        bool has_det   = (trexio_has_determinant_list(file) == TREXIO_SUCCESS);
        bool has_coeff = (trexio_has_determinant_coefficient(file) == TREXIO_SUCCESS);
        int64_t ndet, ncoeff;

        if (has_det && has_coeff) {
          rc = trexio_read_determinant_num_64(file, &ndet);
          if (rc != TREXIO_SUCCESS) return rc;

          rc = trexio_read_determinant_coefficient_size(file, &ncoeff);
          if (rc != TREXIO_SUCCESS) return rc;

          /* Maybe be even more direct and use assert here so that the user's code crushes in case of inconsistency */
          if (ndet != ncoeff) return TREXIO_INVALID_DETERMINANT_NUM;
        }

      }
      /* Up-spin and down-spin electrons */

      int32_t nup, ndn, nelec;
      bool has_up   = (trexio_has_electron_up_num(file) == TREXIO_SUCCESS);
      bool has_dn   = (trexio_has_electron_dn_num(file) == TREXIO_SUCCESS);
      bool has_updn = (trexio_has_electron_num(file) == TREXIO_SUCCESS);

      if (file->mode != 'r') {
        if (has_updn && has_up && has_dn) {
          rc = trexio_read_electron_up_num(file, &nup);
          if (rc != TREXIO_SUCCESS) return rc;

          rc = trexio_read_electron_dn_num(file, &ndn);
          if (rc != TREXIO_SUCCESS) return rc;

          rc = trexio_read_electron_num(file, &nelec);
          if (rc != TREXIO_SUCCESS) return rc;

          if (nelec != nup + ndn) {
            if (file->mode == 'u') {
              nelec = nup + ndn;
              rc = trexio_write_electron_num(file, nelec);
              if (rc != TREXIO_SUCCESS) return rc;
            } else {
              return TREXIO_INVALID_ELECTRON_NUM;
            }
          }
        } else if (has_up && has_dn) {
          rc = trexio_read_electron_up_num(file, &nup);
          if (rc != TREXIO_SUCCESS) return rc;

          rc = trexio_read_electron_dn_num(file, &ndn);
          if (rc != TREXIO_SUCCESS) return rc;

          nelec = nup + ndn;
          rc = trexio_write_electron_num(file, nelec);
          if (rc != TREXIO_SUCCESS) return rc;
        } else if (has_up) {
          rc = trexio_read_electron_up_num(file, &nup);
          if (rc != TREXIO_SUCCESS) return rc;

          ndn = 0;
          rc = trexio_write_electron_dn_num(file, ndn);
          if (rc != TREXIO_SUCCESS) return rc;

          nelec = nup;
          rc = trexio_write_electron_num(file, nelec);
          if (rc != TREXIO_SUCCESS) return rc;
        } else if (has_dn) {
          rc = trexio_read_electron_dn_num(file, &ndn);
          if (rc != TREXIO_SUCCESS) return rc;

          nup = 0;
          rc = trexio_write_electron_up_num(file, nup);
          if (rc != TREXIO_SUCCESS) return rc;

          nelec = ndn;
          rc = trexio_write_electron_num(file, nelec);
          if (rc != TREXIO_SUCCESS) return rc;
        }
      }

      return TREXIO_SUCCESS;
    }

## 3 Templates for front end

### 3.1 Description

Consider the following block of `trex.json`:

    {
      "nucleus": {
          "num"                : [ "int"  , [                     ] ]
        , "charge"             : [ "float", [ "nucleus.num"       ] ]
        , "coord"              : [ "float", [ "nucleus.num", "3"  ] ]
        , "label"              : [ "str" ,  [ "nucleus.num"       ] ]
        , "point_group"        : [ "str"  , [                     ] ]
      }
    }

`TREXIO` is generated automatically by the `generator.py` Python script based on the tree-like configuration provided in the `trex.json` file. Because of that, generalized templates can be implemented and re-used. This approach minimizes the number of bugs as compared with manual copy-paste-modify scheme.

All templates presented below use the `$var$` notation to indicate the variable, which will be replaced by the `generator.py`. Sometimes the upper case is used, i.e. `$VAR$` (for example, in `#define` statements). More detailed description of each variable can be found below:

Template variable | Description | Example
--- | --- | ---
`$group$` | Name of the group | `nucleus`
`$group_num$` | Name of the numerical attribute (scalar) | `nucleus_num`
`$group_str$` | Name of the string attribute (scalar) | `nucleus_point_group`
`$group_dset$` | Name of the dataset (vector/matrix/tensor) | `nucleus_coord`
`$group_dset_rank$` | Rank of the dataset | `2`
`$group_dset_dim$` | Selected dimension of the dataset | `nucleus_num`
`$group_dset_dim_list$` | All dimensions of the dataset | `{nucleus_num, 3}`
`$group_dset_dtype$` | Basic type of the dataset (int/float/char) | `float`
`$group_dset_h5_dtype$` | Type of the dataset in HDF5 | `double`
`$group_dset_format_scanf$` | Input type of the dataset in TEXT [fscanf] | `%lf`
`$group_dset_format_printf$` | Output type of the dataset in TEXT [fprintf] | `%24.16e`
`$group_dset_dtype_default$` | Default datatype of the dataset [C] | `double/int32_t`
`$group_dset_dtype_single$` | Single precision datatype of the dataset [C] | `float/int32_t`
`$group_dset_dtype_double$` | Double precision datatype of the dataset [C] | `double/int64_t`
`$group_dset_f_dtype_default$` | Default datatype of the dataset [Fortran] | `real(c_double)/integer(c_int32_t)`
`$group_dset_f_dtype_single$` | Single precision datatype of the dataset [Fortran] | `real(c_float)/integer(c_int32_t)`
`$group_dset_f_dtype_double$` | Double precision datatype of the dataset [Fortran] | `real(c_double)/integer(c_int64_t)`
`$group_dset_f_dims$` | Dimensions in Fortran | `(:,:)`
`$group_dset_py_dtype$` | Standard datatype of the dataset [Python] | `float/int`
`$default_prec$` | Default precision for read/write without suffix [C] | `64/32`
`$is_index$` | Expands to `true` if dataset has a type `index` [C] | `true/false`

Some of the aforementioned template variables with `group_dset` prefix are duplicated with `group_num` prefix, e.g. you might find \(group_num_dtype_double\) in the templates corresponding to numerical attributes. The expanding values are the same as for `group_dset` and thus are not listed in the table above.

Note: parent group name is always added to the child objects upon construction of TREXIO (e.g. `num` of `nucleus` group becomes `nucleus_num` and should be accessed accordingly within TREXIO).

TREXIO generator parses the `trex.json` file. TREXIO operates with names of variables based on the 1-st (parent group) and 2-nd (child object) levels of `trex.json` . The parsed data is divided in 2 parts:

1. Single attributes. These can be numerical values or strings.
2. Datasets. These can be vectors, matrices or tensors. The types are indicated in `trex.json`. Currently supported data types: int, float and strings.

For each of the aforementioned objects, TREXIO provides has, read and write functionality. TREXIO supports I/O with single or double precision for integer and floating point numbers.

Note: single integer attributes that contain `num` in their name (e.g. `nucleus_num`) are considered dimensioning variables and cannot be negative or 0. An attempt to write negative or 0 value will result in `TREXIO_INVALID_ARG_2` exit code.

### 3.2 Templates for front end hasgroup functions

#### 3.2.1 Introduction

This section concerns API calls related to TREXIO groups

Function name | Description
--- | ---
`trexio_has_$group$` | Check if a group exists in a file

#### 3.2.2 C templates for front end

The `C` templates that correspond to each of the abovementioned functions can be found below. First parameter is the `TREXIO` file handle.

##### 3.2.2.1 Function declarations

##### 3.2.2.2 Source code

    trexio_exit_code
    trexio_has_$group$ (trexio_t* const file)
    {

      if (file == NULL) return TREXIO_INVALID_ARG_1;

      assert(file->back_end < TREXIO_INVALID_BACK_END);

      switch (file->back_end) {

      case TREXIO_TEXT:
        return trexio_text_has_$group$(file);

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        return trexio_hdf5_has_$group$(file);
    #else
        return TREXIO_BACK_END_MISSING;
    #endif
    /*
      case TREXIO_JSON:
        return trexio_json_has_$group$(file);
        break;
    */
      }

      return TREXIO_FAILURE;
    }

#### 3.2.3 Fortran templates for front end

The `Fortran` templates that provide an access to the `C` API calls from Fortran. These templates are based on the use of `iso_c_binding`. Pointers have to be passed by value.

    interface
       integer(trexio_exit_code) function trexio_has_$group$ (trex_file) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
       end function trexio_has_$group$
    end interface

#### 3.2.4 Python templates for front end

    def has_$group$(trexio_file) -> bool:
        """Check that $group$ group exists in the TREXIO file.

        Parameter is a ~TREXIO File~ object that has been created by a call to ~open~ function.

        Returns:
              True if the variable exists, False otherwise

        Raises:
            - trexio.Error if TREXIO return code ~rc~ is TREXIO_FAILURE and prints the error message using string_of_error.
            - Exception from some other error (e.g. RuntimeError).
        """

        rc = pytr.trexio_has_$group$(trexio_file.pytrexio_s)
        if rc == TREXIO_FAILURE:
            raise Error(rc)

        return rc == TREXIO_SUCCESS

### 3.3 Templates for front end has/read/write a single numerical attribute

#### 3.3.1 Introduction

This section concerns API calls related to numerical attributes, namely single value of int/float types.

Function name | Description | Precision
--- | --- | ---
`trexio_has_$group_num$` | Check if an attribute exists in a file | ---
`trexio_read_$group_num$` | Read a attribute | Single
`trexio_write_$group_num$` | Write a attribute | Single
`trexio_read_$group_num$_32` | Read a attribute | Single
`trexio_write_$group_num$_32` | Write a attribute | Single
`trexio_read_$group_num$_64` | Read a attribute | Double
`trexio_write_$group_num$_64` | Write a attribute | Double

#### 3.3.2 C templates for front end

The `C` templates that correspond to each of the abovementioned functions can be found below. First parameter is the `TREXIO` file handle. Second parameter is the variable to be written/read to/from the `TREXIO` file (except for `trexio_has_` functions). Suffixes `_32` and `_64` correspond to API calls dealing with single and real(cdouble), respectively. The basic (non-suffixed) API call on dimensioning variables deals with single precision (see Table above).

##### 3.3.2.1 Function declarations

##### 3.3.2.2 Source code for double precision functions

    trexio_exit_code
    trexio_read_$group_num$_64 (trexio_t* const file, $group_num_dtype_double$* const num)
    {
      if (file == NULL) return TREXIO_INVALID_ARG_1;
      if (num  == NULL) return TREXIO_INVALID_ARG_2;
      if (trexio_has_$group_num$(file) != TREXIO_SUCCESS) return TREXIO_ATTR_MISSING;

      switch (file->back_end) {

      case TREXIO_TEXT:
        return trexio_text_read_$group_num$(file, num);

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        return trexio_hdf5_read_$group_num$(file, num);
    #else
        return TREXIO_BACK_END_MISSING;
    #endif
    /*
      case TREXIO_JSON:
        return trexio_json_read_$group_num$(file, num);
    */
      }

      return TREXIO_FAILURE;
    }

    trexio_exit_code
    trexio_write_$group_num$_64 (trexio_t* const file, const $group_num_dtype_double$ num)
    {
      if (file == NULL) return TREXIO_INVALID_ARG_1;
      //if (num <= 0L) return TREXIO_INVALID_NUM; /* this line is uncommented by the generator for dimensioning variables; do NOT remove! */
      if (trexio_has_$group_num$(file) == TREXIO_SUCCESS && file->mode != 'u') return TREXIO_ATTR_ALREADY_EXISTS;

      switch (file->back_end) {

      case TREXIO_TEXT:
        return trexio_text_write_$group_num$(file, num);

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        return trexio_hdf5_write_$group_num$(file, num);
    #else
        return TREXIO_BACK_END_MISSING;
    #endif
    /*
      case TREXIO_JSON:
        return trexio_json_write_$group_num$(file, num);
    */
      }

      return TREXIO_FAILURE;
    }

##### 3.3.2.3 Source code for single precision functions

    trexio_exit_code
    trexio_read_$group_num$_32 (trexio_t* const file, $group_num_dtype_single$* const num)
    {
      if (file == NULL) return TREXIO_INVALID_ARG_1;
      if (num  == NULL) return TREXIO_INVALID_ARG_2;
      if (trexio_has_$group_num$(file) != TREXIO_SUCCESS) return TREXIO_ATTR_MISSING;

      $group_num_dtype_double$ num_64 = 0;
      trexio_exit_code rc = TREXIO_GROUP_READ_ERROR;

      switch (file->back_end) {

      case TREXIO_TEXT:
        rc = trexio_text_read_$group_num$(file, &num_64);
        break;

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        rc = trexio_hdf5_read_$group_num$(file, &num_64);
        break;
    #else
        rc = TREXIO_BACK_END_MISSING;
        break ;
    #endif
    /*
      case TREXIO_JSON:
        rc =trexio_json_read_$group_num$(file, &num_64);
        break;
    */
      }

      if (rc != TREXIO_SUCCESS) return rc;

      *num = ($group_num_dtype_single$) num_64;
      return TREXIO_SUCCESS;
    }

    trexio_exit_code
    trexio_write_$group_num$_32 (trexio_t* const file, const $group_num_dtype_single$ num)
    {

      if (file == NULL) return TREXIO_INVALID_ARG_1;
      //if (num <= 0) return TREXIO_INVALID_NUM; /* this line is uncommented by the generator for dimensioning variables; do NOT remove! */
      if (trexio_has_$group_num$(file) == TREXIO_SUCCESS && file->mode != 'u') return TREXIO_ATTR_ALREADY_EXISTS;

      switch (file->back_end) {

      case TREXIO_TEXT:
        return trexio_text_write_$group_num$(file, ($group_num_dtype_double$) num);

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        return trexio_hdf5_write_$group_num$(file, ($group_num_dtype_double$) num);
    #else
        return TREXIO_BACK_END_MISSING;
    #endif
    /*
      case TREXIO_JSON:
        return trexio_json_write_$group_num$(file, ($group_num_dtype_double$) num);
        break;
    */
      }

      return TREXIO_FAILURE;
    }

##### 3.3.2.4 Source code for default functions

    trexio_exit_code
    trexio_read_$group_num$ (trexio_t* const file, $group_num_dtype_default$* const num)
    {
      return trexio_read_$group_num$_$default_prec$(file, num);
    }

    trexio_exit_code
    trexio_write_$group_num$ (trexio_t* const file, const $group_num_dtype_default$ num)
    {
      return trexio_write_$group_num$_$default_prec$(file, num);
    }

    trexio_exit_code
    trexio_has_$group_num$ (trexio_t* const file)
    {

      if (file == NULL) return TREXIO_INVALID_ARG_1;

      assert(file->back_end < TREXIO_INVALID_BACK_END);

      switch (file->back_end) {

      case TREXIO_TEXT:
        return trexio_text_has_$group_num$(file);

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        return trexio_hdf5_has_$group_num$(file);
    #else
        return TREXIO_BACK_END_MISSING;
    #endif
    /*
      case TREXIO_JSON:
        return trexio_json_has_$group_num$(file);
        break;
    */
      }

      return TREXIO_FAILURE;
    }

#### 3.3.3 Fortran templates for front end

The `Fortran` templates that provide an access to the `C` API calls from Fortran. These templates are based on the use of `iso_c_binding`. Pointers have to be passed by value.

    interface
       integer(trexio_exit_code) function trexio_write_$group_num$_64 (trex_file, num) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         $group_num_f_dtype_double$, intent(in), value :: num
       end function trexio_write_$group_num$_64
    end interface

    interface
       integer(trexio_exit_code) function trexio_read_$group_num$_64 (trex_file, num) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         $group_num_f_dtype_double$, intent(out) :: num
       end function trexio_read_$group_num$_64
    end interface

    interface
       integer(trexio_exit_code) function trexio_write_$group_num$_32 (trex_file, num) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         $group_num_f_dtype_single$, intent(in), value :: num
       end function trexio_write_$group_num$_32
    end interface

    interface
       integer(trexio_exit_code) function trexio_read_$group_num$_32 (trex_file, num) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         $group_num_f_dtype_single$, intent(out) :: num
       end function trexio_read_$group_num$_32
    end interface

    interface
       integer(trexio_exit_code) function trexio_write_$group_num$ (trex_file, num) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         $group_num_f_dtype_default$, intent(in), value :: num
       end function trexio_write_$group_num$
    end interface

    interface
       integer(trexio_exit_code) function trexio_read_$group_num$ (trex_file, num) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         $group_num_f_dtype_default$, intent(out) :: num
       end function trexio_read_$group_num$
    end interface

    interface
       integer(trexio_exit_code) function trexio_has_$group_num$ (trex_file) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
       end function trexio_has_$group_num$
    end interface

#### 3.3.4 Python templates for front end

    def write_$group_num$(trexio_file, num_w: $group_num_py_dtype$) -> None:
        """Write the $group_num$ variable in the TREXIO file.

        Parameters:

        trexio_file:
            TREXIO File object.

        num_w: int
            Value of the $group_num$ variable to be written.

        Raises:
            - Exception from AssertionError if TREXIO return code ~rc~ is different from TREXIO_SUCCESS and prints the error message using trexio_string_of_error.
            - Exception from some other error (e.g. RuntimeError).
        """

        rc = pytr.trexio_write_$group_num$(trexio_file.pytrexio_s, num_w)
        if rc != TREXIO_SUCCESS:
            raise Error(rc)

    def read_$group_num$(trexio_file) -> $group_num_py_dtype$:
        """Read the $group_num$ variable from the TREXIO file.

        Parameter is a ~TREXIO File~ object that has been created by a call to ~open~ function.

        Returns:
            ~num_r~: int
            Integer value of $group_num$ variable read from ~trexio_file~.

        Raises:
            - Exception from AssertionError if TREXIO return code ~rc~ is different from TREXIO_SUCCESS and prints the error message using trexio_string_of_error.
            - Exception from some other error (e.g. RuntimeError).
        """

        rc, num_r = pytr.trexio_read_$group_num$(trexio_file.pytrexio_s)
        if rc != TREXIO_SUCCESS:
            raise Error(rc)

        return num_r

    def has_$group_num$(trexio_file) -> bool:
        """Check that $group_num$ variable exists in the TREXIO file.

        Parameter is a ~TREXIO File~ object that has been created by a call to ~open~ function.

        Returns:
              True if the variable exists, False otherwise

        Raises:
            - trexio.Error if TREXIO return code ~rc~ is TREXIO_FAILURE and prints the error message using string_of_error.
            - Exception from some other error (e.g. RuntimeError).
        """

        rc = pytr.trexio_has_$group_num$(trexio_file.pytrexio_s)
        if rc == TREXIO_FAILURE:
            raise Error(rc)

        return rc == TREXIO_SUCCESS

### 3.4 Templates for front end has/read/write a dataset of numerical data

#### 3.4.1 Introduction

This section concerns API calls related to datasets.

Function name | Description | Precision
--- | --- | ---
`trexio_has_$group_dset$` | Check if a dataset exists in a file | ---
`trexio_read_$group_dset$` | Read a dataset in default precision | Double/Single for float/int
`trexio_write_$group_dset$` | Write a dataset in default precision | Double/Single for float/int
`trexio_read_safe_$group_dset$` | Read a bounded dataset | Double
`trexio_write_safe_$group_dset$` | Write a bounded dataset | Double
`trexio_read_$group_dset$_32` | Read a dataset in single precision | Single
`trexio_write_$group_dset$_32` | Write a dataset in single precision | Single
`trexio_read_$group_dset$_64` | Read a dataset in double precision | Double
`trexio_write_$group_dset$_64` | Write a dataset in double precision | Double

#### 3.4.2 C templates for front end

The C templates that correspond to each of the abovementioned functions can be found below. First parameter is the `TREXIO` file handle. Second parameter is the variable to be written/read to/from the `TREXIO` file (except for `trexio_has_` functions). Suffixes `_32` and `_64` correspond to API calls dealing with single and double precision, respectively. The basic (non-suffixed) API call on datasets deals with real(cdouble) (see Table above).

##### 3.4.2.1 Function declarations

##### 3.4.2.2 Source code for double precision functions

    trexio_exit_code
    trexio_read_$group_dset$_64 (trexio_t* const file, $group_dset_dtype_double$* const $group_dset$)
    {

      if (file  == NULL) return TREXIO_INVALID_ARG_1;
      if ($group_dset$ == NULL) return TREXIO_INVALID_ARG_2;
      if (trexio_has_$group_dset$(file) != TREXIO_SUCCESS) return TREXIO_DSET_MISSING;

      trexio_exit_code rc;
      int64_t $group_dset_dim$ = 0;

      /* Error handling for this call is added by the generator */
      rc = trexio_read_$group_dset_dim$_64(file, &($group_dset_dim$));

      if ($group_dset_dim$ == 0L) return TREXIO_INVALID_NUM;

      uint32_t rank = $group_dset_rank$;
      uint64_t dims[$group_dset_rank$] = {$group_dset_dim_list$};

      assert(file->back_end < TREXIO_INVALID_BACK_END);

      rc = TREXIO_FAILURE;
      switch (file->back_end) {

      case TREXIO_TEXT:
        rc = trexio_text_read_$group_dset$(file, $group_dset$, rank, dims);
        break;

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        rc = trexio_hdf5_read_$group_dset$(file, $group_dset$, rank, dims);
        break;
    #else
        rc = TREXIO_BACK_END_MISSING;
        break;
    #endif
    /*
      case TREXIO_JSON:
        rc = trexio_json_read_$group_dset$(file, $group_dset$, rank, dims);
        break;
    */
      }
      if (rc != TREXIO_SUCCESS) return rc;

      /* Handle index type */
      if ($is_index$) {
        uint64_t dim_size = 1;
        for (uint32_t i=0; i<rank; ++i){
            dim_size *= dims[i];
        }
        for (uint64_t i=0; i<dim_size; ++i){
           $group_dset$[i] += ($group_dset_dtype_double$) 1;
        }
      }

      return TREXIO_SUCCESS;
    }

    trexio_exit_code
    trexio_write_$group_dset$_64 (trexio_t* const file, const $group_dset_dtype_double$* $group_dset$)
    {

      if (file  == NULL) return TREXIO_INVALID_ARG_1;
      assert(file->back_end < TREXIO_INVALID_BACK_END);

      if ($group_dset$ == NULL) return TREXIO_INVALID_ARG_2;
      if (trexio_has_$group_dset$(file) == TREXIO_SUCCESS && file->mode != 'u') return TREXIO_DSET_ALREADY_EXISTS;

      trexio_exit_code rc;
      int64_t $group_dset_dim$ = 0;

      /* Error handling for this call is added by the generator */
      rc = trexio_read_$group_dset_dim$_64(file, &($group_dset_dim$));

      if ($group_dset_dim$ == 0L) return TREXIO_INVALID_NUM;

      uint32_t rank = $group_dset_rank$;
      uint64_t dims[$group_dset_rank$] = {$group_dset_dim_list$};

      if ($is_index$) {
        /* Handle index type : is_index = $is_index$ */
        uint64_t dim_size = 1;
        for (uint32_t i=0; i<rank; ++i){
          dim_size *= dims[i];
        }

        $group_dset_dtype_double$* $group_dset$_p =
           CALLOC(dim_size, $group_dset_dtype_double$);

        if ($group_dset$_p == NULL) return TREXIO_ALLOCATION_FAILED;

        for (uint64_t i=0; i<dim_size; ++i){
           $group_dset$_p[i] = $group_dset$[i] - ($group_dset_dtype_double$) 1;
        }

        rc = TREXIO_FAILURE;
        switch (file->back_end) {

        case TREXIO_TEXT:
          rc = trexio_text_write_$group_dset$(file,
                                              (const $group_dset_dtype_double$*) $group_dset$_p,
                                              rank, dims);
          break;

        case TREXIO_HDF5:
    #ifdef HAVE_HDF5
          rc = trexio_hdf5_write_$group_dset$(file,
                                              (const $group_dset_dtype_double$*) $group_dset$_p,
                                              rank, dims);
          break;
    #else
          rc = TREXIO_BACK_END_MISSING;
          break;
    #endif
          /*
            case TREXIO_JSON:
            rc = trexio_json_write_$group_dset$(file, $group_dset$_p, rank, dims);
            break;
          */
          FREE($group_dset$_p);

        }

      } else {

        rc = TREXIO_FAILURE;
        switch (file->back_end) {

        case TREXIO_TEXT:
          rc = trexio_text_write_$group_dset$(file, $group_dset$, rank, dims);
          break;

        case TREXIO_HDF5:
    #ifdef HAVE_HDF5
          rc = trexio_hdf5_write_$group_dset$(file, $group_dset$, rank, dims);
          break;
    #else
          rc = TREXIO_BACK_END_MISSING;
          break;
    #endif
          /*
            case TREXIO_JSON:
            rc = trexio_json_write_$group_dset$(file, $group_dset$, rank, dims);
            break;
          */
        }
      }

      return rc;
    }

##### 3.4.2.3 Source code for single precision functions

    trexio_exit_code
    trexio_read_$group_dset$_32 (trexio_t* const file, $group_dset_dtype_single$* const $group_dset$)
    {

      if (file  == NULL) return TREXIO_INVALID_ARG_1;
      if ($group_dset$ == NULL) return TREXIO_INVALID_ARG_2;
      if (trexio_has_$group_dset$(file) != TREXIO_SUCCESS) return TREXIO_DSET_MISSING;

      trexio_exit_code rc;
      int64_t $group_dset_dim$ = 0;

      /* Error handling for this call is added by the generator */
      rc = trexio_read_$group_dset_dim$_64(file, &($group_dset_dim$));

      if ($group_dset_dim$ == 0L) return TREXIO_INVALID_NUM;

      uint32_t rank = $group_dset_rank$;
      uint64_t dims[$group_dset_rank$] = {$group_dset_dim_list$};

      uint64_t dim_size = 1;
      for (uint32_t i=0; i<rank; ++i){
        dim_size *= dims[i];
      }

      $group_dset_dtype_double$* $group_dset$_64 = CALLOC(dim_size, $group_dset_dtype_double$);
      if ($group_dset$_64 == NULL) return TREXIO_ALLOCATION_FAILED;

      assert(file->back_end < TREXIO_INVALID_BACK_END);

      rc = TREXIO_FAILURE;

      switch (file->back_end) {

      case TREXIO_TEXT:
        rc = trexio_text_read_$group_dset$(file, $group_dset$_64, rank, dims);
        break;

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        rc = trexio_hdf5_read_$group_dset$(file, $group_dset$_64, rank, dims);
        break;
    #else
        rc = TREXIO_BACK_END_MISSING;
        break;
    #endif
    /*
      case TREXIO_JSON:
        rc = trexio_json_read_$group_dset$(file, $group_dset$_64, rank, dims);
        break;
    */
      }

      if (rc != TREXIO_SUCCESS){
        FREE($group_dset$_64);
        return rc;
      }

      if ($is_index$) {
        for (uint64_t i=0; i<dim_size; ++i){
          $group_dset$[i] = ($group_dset_dtype_single$) $group_dset$_64[i] + 1;
        }
      } else {
        for (uint64_t i=0; i<dim_size; ++i){
          $group_dset$[i] = ($group_dset_dtype_single$) $group_dset$_64[i];
        }
      }

      FREE($group_dset$_64);
      return TREXIO_SUCCESS;
    }

    trexio_exit_code
    trexio_write_$group_dset$_32 (trexio_t* const file, const $group_dset_dtype_single$* $group_dset$)
    {

      if (file  == NULL) return TREXIO_INVALID_ARG_1;
      if ($group_dset$ == NULL) return TREXIO_INVALID_ARG_2;
      if (trexio_has_$group_dset$(file) == TREXIO_SUCCESS && file->mode != 'u') return TREXIO_DSET_ALREADY_EXISTS;

      trexio_exit_code rc;
      int64_t $group_dset_dim$ = 0;

      /* Error handling for this call is added by the generator */
      rc = trexio_read_$group_dset_dim$_64(file, &($group_dset_dim$));

      if ($group_dset_dim$ == 0L) return TREXIO_INVALID_NUM;

      uint32_t rank = $group_dset_rank$;
      uint64_t dims[$group_dset_rank$] = {$group_dset_dim_list$};

      uint64_t dim_size = 1;
      for (uint32_t i=0; i<rank; ++i){
        dim_size *= dims[i];
      }

      $group_dset_dtype_double$* $group_dset$_64 = CALLOC(dim_size, $group_dset_dtype_double$);
      if ($group_dset$_64 == NULL) return TREXIO_ALLOCATION_FAILED;

      /* A type conversion from single precision to double required since back end only accepts 64-bit data */
      if ($is_index$) {
        for (uint64_t i=0; i<dim_size; ++i){
          $group_dset$_64[i] = ($group_dset_dtype_double$) $group_dset$[i] - ($group_dset_dtype_double$) 1;
        }
      } else {
        for (uint64_t i=0; i<dim_size; ++i){
          $group_dset$_64[i] = ($group_dset_dtype_double$) $group_dset$[i];
        }
      }

      assert(file->back_end < TREXIO_INVALID_BACK_END);

      rc = TREXIO_FAILURE;
      switch (file->back_end) {

      case TREXIO_TEXT:
        rc = trexio_text_write_$group_dset$(file, $group_dset$_64, rank, dims);
        break;

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        rc = trexio_hdf5_write_$group_dset$(file, $group_dset$_64, rank, dims);
        break;
    #else
        rc = TREXIO_BACK_END_MISSING;
        break;
    #endif
    /*
      case TREXIO_JSON:
        rc = trexio_json_write_$group_dset$(file, $group_dset$_64, rank, dims);
        break;
    */
      }

      FREE($group_dset$_64);

      if (rc != TREXIO_SUCCESS) return rc;

      return TREXIO_SUCCESS;
    }

##### 3.4.2.4 Source code for memory-safe functions

    trexio_exit_code rc;
    int64_t $group_dset_dim$ = 0;

    /* Error handling for this call is added by the generator */
    rc = trexio_read_$group_dset_dim$_64(file, &($group_dset_dim$));

    if ($group_dset_dim$ == 0L) return TREXIO_INVALID_NUM;

    uint32_t rank = $group_dset_rank$;
    uint64_t dims[$group_dset_rank$] = {$group_dset_dim_list$};

    /* The block below is specific to safe API as it checks the boundaries */
    uint64_t dim_size = 1;
    for (uint32_t i=0; i<rank; ++i){
        dim_size *= dims[i];
    }

    trexio_exit_code
    trexio_read_safe_$group_dset$_32 (trexio_t* const file, $group_dset_dtype_single$* const dset_out, const int64_t dim_out)
    {

      if (file  == NULL) return TREXIO_INVALID_ARG_1;
      if (dset_out == NULL) return TREXIO_INVALID_ARG_2;
      if (trexio_has_$group_dset$(file) != TREXIO_SUCCESS) return TREXIO_DSET_MISSING;

    trexio_exit_code rc;
    int64_t $group_dset_dim$ = 0;

    /* Error handling for this call is added by the generator */
    rc = trexio_read_$group_dset_dim$_64(file, &($group_dset_dim$));

    if ($group_dset_dim$ == 0L) return TREXIO_INVALID_NUM;

    uint32_t rank = $group_dset_rank$;
    uint64_t dims[$group_dset_rank$] = {$group_dset_dim_list$};

    /* The block below is specific to safe API as it checks the boundaries */
    uint64_t dim_size = 1;
    for (uint32_t i=0; i<rank; ++i){
        dim_size *= dims[i];
    }

      if (dim_out > (int64_t) dim_size) return TREXIO_UNSAFE_ARRAY_DIM;

      return trexio_read_$group_dset$_32(file, dset_out);
    }

    trexio_exit_code
    trexio_write_safe_$group_dset$_32 (trexio_t* const file, const $group_dset_dtype_single$* dset_in, const int64_t dim_in)
    {

      if (file  == NULL) return TREXIO_INVALID_ARG_1;
      if (dset_in == NULL) return TREXIO_INVALID_ARG_2;
      if (trexio_has_$group_dset$(file) == TREXIO_SUCCESS && file->mode != 'u') return TREXIO_DSET_ALREADY_EXISTS;

    trexio_exit_code rc;
    int64_t $group_dset_dim$ = 0;

    /* Error handling for this call is added by the generator */
    rc = trexio_read_$group_dset_dim$_64(file, &($group_dset_dim$));

    if ($group_dset_dim$ == 0L) return TREXIO_INVALID_NUM;

    uint32_t rank = $group_dset_rank$;
    uint64_t dims[$group_dset_rank$] = {$group_dset_dim_list$};

    /* The block below is specific to safe API as it checks the boundaries */
    uint64_t dim_size = 1;
    for (uint32_t i=0; i<rank; ++i){
        dim_size *= dims[i];
    }

      if (dim_in > (int64_t) dim_size) return TREXIO_UNSAFE_ARRAY_DIM;

      return trexio_write_$group_dset$_32(file, dset_in);
    }

    trexio_exit_code
    trexio_read_safe_$group_dset$_64 (trexio_t* const file, $group_dset_dtype_double$* const dset_out, const int64_t dim_out)
    {

      if (file  == NULL) return TREXIO_INVALID_ARG_1;
      if (dset_out == NULL) return TREXIO_INVALID_ARG_2;
      if (trexio_has_$group_dset$(file) != TREXIO_SUCCESS) return TREXIO_DSET_MISSING;

    trexio_exit_code rc;
    int64_t $group_dset_dim$ = 0;

    /* Error handling for this call is added by the generator */
    rc = trexio_read_$group_dset_dim$_64(file, &($group_dset_dim$));

    if ($group_dset_dim$ == 0L) return TREXIO_INVALID_NUM;

    uint32_t rank = $group_dset_rank$;
    uint64_t dims[$group_dset_rank$] = {$group_dset_dim_list$};

    /* The block below is specific to safe API as it checks the boundaries */
    uint64_t dim_size = 1;
    for (uint32_t i=0; i<rank; ++i){
        dim_size *= dims[i];
    }

      if (dim_out > (int64_t) dim_size) return TREXIO_UNSAFE_ARRAY_DIM;

      return trexio_read_$group_dset$_64(file, dset_out);
    }

    trexio_exit_code
    trexio_write_safe_$group_dset$_64 (trexio_t* const file, const $group_dset_dtype_double$* dset_in, const int64_t dim_in)
    {

      if (file  == NULL) return TREXIO_INVALID_ARG_1;
      if (dset_in == NULL) return TREXIO_INVALID_ARG_2;
      if (trexio_has_$group_dset$(file) == TREXIO_SUCCESS && file->mode != 'u') return TREXIO_DSET_ALREADY_EXISTS;

    trexio_exit_code rc;
    int64_t $group_dset_dim$ = 0;

    /* Error handling for this call is added by the generator */
    rc = trexio_read_$group_dset_dim$_64(file, &($group_dset_dim$));

    if ($group_dset_dim$ == 0L) return TREXIO_INVALID_NUM;

    uint32_t rank = $group_dset_rank$;
    uint64_t dims[$group_dset_rank$] = {$group_dset_dim_list$};

    /* The block below is specific to safe API as it checks the boundaries */
    uint64_t dim_size = 1;
    for (uint32_t i=0; i<rank; ++i){
        dim_size *= dims[i];
    }

      if (dim_in > (int64_t) dim_size) return TREXIO_UNSAFE_ARRAY_DIM;

      return trexio_write_$group_dset$_64(file, dset_in);
    }

##### 3.4.2.5 Source code for default functions

    trexio_exit_code
    trexio_read_safe_$group_dset$ (trexio_t* const file, $group_dset_dtype_default$* const $group_dset$, const int64_t dim_out)
    {
      return trexio_read_safe_$group_dset$_$default_prec$(file, $group_dset$, dim_out);
    }

    trexio_exit_code
    trexio_write_safe_$group_dset$ (trexio_t* const file, const $group_dset_dtype_default$* $group_dset$, const int64_t dim_in)
    {
      return trexio_write_safe_$group_dset$_$default_prec$(file, $group_dset$, dim_in);
    }

    trexio_exit_code
    trexio_read_$group_dset$ (trexio_t* const file, $group_dset_dtype_default$* const $group_dset$)
    {
      return trexio_read_$group_dset$_$default_prec$(file, $group_dset$);
    }

    trexio_exit_code
    trexio_write_$group_dset$ (trexio_t* const file, const $group_dset_dtype_default$* $group_dset$)
    {
      return trexio_write_$group_dset$_$default_prec$(file, $group_dset$);
    }

    trexio_exit_code
    trexio_has_$group_dset$ (trexio_t* const file)
    {

      if (file  == NULL) return TREXIO_INVALID_ARG_1;

      assert(file->back_end < TREXIO_INVALID_BACK_END);

      switch (file->back_end) {

      case TREXIO_TEXT:
        return trexio_text_has_$group_dset$(file);

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        return trexio_hdf5_has_$group_dset$(file);
    #else
        return TREXIO_BACK_END_MISSING;
    #endif
    /*
      case TREXIO_JSON:
        return trexio_json_has_$group_dset$(file);
    */
      }
      return TREXIO_FAILURE;
    }

#### 3.4.3 Fortran templates for front end

The `Fortran` templates that provide an access to the `C` API calls from `Fortran`. These templates are based on the use of `iso_c_binding`. Pointers have to be passed by value.

    interface
       integer(trexio_exit_code) function trexio_write_$group_dset$_64 (trex_file, dset) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         $group_dset_f_dtype_double$, intent(in) :: dset$group_dset_f_dims$
       end function trexio_write_$group_dset$_64
    end interface

    interface
       integer(trexio_exit_code) function trexio_read_$group_dset$_64 (trex_file, dset) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         $group_dset_f_dtype_double$, intent(out) :: dset$group_dset_f_dims$
       end function trexio_read_$group_dset$_64
    end interface

    interface
       integer(trexio_exit_code) function trexio_write_$group_dset$_32 (trex_file, dset) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         $group_dset_f_dtype_single$, intent(in) :: dset$group_dset_f_dims$
       end function trexio_write_$group_dset$_32
    end interface

    interface
       integer(trexio_exit_code) function trexio_read_$group_dset$_32 (trex_file, dset) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         $group_dset_f_dtype_single$, intent(out) :: dset$group_dset_f_dims$
       end function trexio_read_$group_dset$_32
    end interface

    interface
       integer(trexio_exit_code) function trexio_write_$group_dset$ (trex_file, dset) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         $group_dset_f_dtype_default$, intent(in) :: dset$group_dset_f_dims$
       end function trexio_write_$group_dset$
    end interface

    interface
       integer(trexio_exit_code) function trexio_read_$group_dset$ (trex_file, dset) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         $group_dset_f_dtype_default$, intent(out) :: dset$group_dset_f_dims$
       end function trexio_read_$group_dset$
    end interface

    interface
       integer(trexio_exit_code) function trexio_has_$group_dset$ (trex_file) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
       end function trexio_has_$group_dset$
    end interface

#### 3.4.4 Python templates for front end

    def write_$group_dset$(trexio_file, dset_w) -> None:
        """Write the $group_dset$ array of numbers in the TREXIO file.

        Parameters:

        trexio_file:
            TREXIO File object.

        dset_w: list OR numpy.ndarray
            Array of $group_dset$ values to be written. If array data type does not correspond to int64 or float64, the conversion is performed.

        Raises:
            - trexio.Error if TREXIO return code ~rc~ is different from TREXIO_SUCCESS and prints the error message.
            - Exception from some other error (e.g. RuntimeError).
        """

        doConversion = False
        doFlatten = False
        if not isinstance(dset_w, (list, tuple)):
           # if input array is not a list or tuple then it is probably a numpy array
           if isinstance(dset_w, np.ndarray) and (not dset_w.dtype==np.int64 or not dset_w.dtype==np.float64):
               doConversion = True

           if len(dset_w.shape) > 1:
               doFlatten = True
               if doConversion:
                   dset_64 = np.$group_dset_py_dtype$64(dset_w).flatten()
               else:
                   dset_flat = np.array(dset_w, dtype=np.$group_dset_py_dtype$64).flatten()
           else:
               if doConversion:
                   dset_64 = np.$group_dset_py_dtype$64(dset_w)

        else:
           # if input array is a multidimensional list or tuple, we have to convert it
           try:
               doFlatten = True
               ncol = len(dset_w[0])
               dset_flat = np.array(dset_w, dtype=np.$group_dset_py_dtype$64).flatten()
           except TypeError:
               doFlatten = False
               pass

        if doConversion:
            rc = pytr.trexio_write_safe_$group_dset$_64(trexio_file.pytrexio_s, dset_64)
        elif doFlatten:
            rc = pytr.trexio_write_safe_$group_dset$_64(trexio_file.pytrexio_s, dset_flat)
        else:
            rc = pytr.trexio_write_safe_$group_dset$_64(trexio_file.pytrexio_s, dset_w)

        if rc != TREXIO_SUCCESS:
            raise Error(rc)

    def read_$group_dset$(trexio_file, dim = None, doReshape = None, dtype = None):
        """Read the $group_dset$ array of numbers from the TREXIO file.

        Parameters:

        trexio_file:
            TREXIO File object.

        dim (Optional): int
            Size of the block to be read from the file (i.e. how many items of $group_dset$ will be returned)
            If None, the function will read all necessary array dimensions from the file.

        dtype (Optional): type
            NumPy data type of the output (e.g. np.int32|int16 or np.float32|float16). If specified, the output array will be converted from the default double precision.

        doReshape (Optional): bool
            Flag to determine whether the output NumPy array has be reshaped or not. Be default, reshaping is performed
            based on the dimensions from the ~trex.json~ file. Otherwise, ~shape~ array (list or tuple) is used if provided by the user.

        Returns:
            ~dset_64~ if dtype is None or ~dset_converted~ otherwise: numpy.ndarray
            1D NumPy array with ~dim~ elements corresponding to $group_dset$ values read from the TREXIO file.

        Raises:
            - trexio.Error if TREXIO return code ~rc~ is different from TREXIO_SUCCESS and prints the error message.
            - Exception from some other error (e.g. RuntimeError).
    """

        if doReshape is None:
            doReshape = True

        # if dim is not specified, read dimensions from the TREXIO file
        dims_list = None
        if doReshape:
            $group_dset_dim$ = read_$group_dset_dim$(trexio_file)

            dims_list = [$group_dset_dim_list$]
            dim_real  = 1
            for i in range($group_dset_rank$):
                dim_real *= dims_list[i]

        if dim:
            if dim_real != dim:
                raise Error(TREXIO_UNSAFE_ARRAY_DIM)
        else:
            dim = dim_real

        shape = tuple(dims_list)
        if shape is None and doReshape:
            raise ValueError("Reshaping failure: shape is None.")

        rc, dset_64 = pytr.trexio_read_safe_$group_dset$_64(trexio_file.pytrexio_s, dim)
        if rc != TREXIO_SUCCESS:
            raise Error(rc)

        isConverted = False
        dset_converted = None
        if dtype is not None:
            try:
                assert isinstance(dtype, type)
            except AssertionError:
                raise TypeError("dtype argument has to be an instance of the type class (e.g. np.float32).")

            if not dtype==np.int64 or not dtype==np.float64:
                dset_converted = np.array(dset_64, dtype=dtype)
                isConverted = True

        # additional assert can be added here to check that read_safe functions returns numpy array of proper dimension

        if doReshape:
            # in-place reshaping did not work so I have to make a copy
            if isConverted:
                dset_reshaped = np.reshape(dset_converted, shape, order='C')
            else:
                dset_reshaped = np.reshape(dset_64, shape, order='C')

        if isConverted:
            return dset_converted
        elif doReshape:
            return dset_reshaped
        else:
            return dset_64

    def has_$group_dset$(trexio_file) -> bool:
        """Check that $group_dset$ variable exists in the TREXIO file.

        Parameter is a ~TREXIO File~ object that has been created by a call to ~open~ function.

        Returns:
              True if the variable exists, False otherwise

        Raises:
            - trexio.Error if TREXIO return code ~rc~ is TREXIO_FAILURE and prints the error message using string_of_error.
            - Exception from some other error (e.g. RuntimeError).
        """

        rc = pytr.trexio_has_$group_dset$(trexio_file.pytrexio_s)
        if rc == TREXIO_FAILURE:
            raise Error(rc)

        return rc == TREXIO_SUCCESS

### 3.5 Templates for front end has/read/write a dataset of sparse data

#### 3.5.1 Introduction

Sparse data structures are used typically for large tensors such as two-electron integrals. For example, in the `trex.json` file sparse arrays appear as for the `eri` :

    "ao_2e_int"  : {
      "eri"      : [ "float sparse", [ "ao.num", "ao.num", "ao.num", "ao.num" ] ]
    }

The electron repulsion integral (eri) \(\langle ij | kl \rangle\) is represented as a quartet of integers \((i,j,k,l)\) and a floating point value.

To store \(N\) integrals in the file, we store

- An array of quartets of integers
- An array of values (floats)

Both arrays have the same size, \(N\), the number of non-zero integrals. Knowing the maximum dimensions allows to check that the integers are in a valid range, and also lets the library choose the smallest integer representation to compress the storage.

Fortran uses 1-based array indexing, while C uses 0-based indexing. Internally, we use a 0-based representation but the Fortran binding does the appropriate conversion when reading or writing.

As the number of integrals to store can be prohibitively large, we provide the possibility to read/write the integrals in chunks. So the functions take two extra parameters:

- `offset_file` : how many integrals in the file should be skipped when reading/writing. An offset of zero implies to read the first integral.
- `buffer_size` : the number of integrals to read/write. If EOF is encountered upon reading, the `buffer_size` is overwritten with the number of integrals that have been read before EOF and the `trexio_read_` function return `TREXIO_END` exit code instead of `TREXIO_SUCCESS`.

The storage of `int` indices is internally compressed based on the maximum possible value of an index, which is derived from the corresponding dimension of the sparse array (e.g. `ao_num` is the upper bound of indices in the aforementioned `ao_2e_int_eri` dataset). The upper bounds for different `int` types (e.g. `uint16_t`) can be found in the in the `stdint.h` C library. Currently implemented list of compressions based on the upper bound of indices can be found below:

Max value of indices | Internal representation (in the TREXIO file)
--- | ---
`UINT8_MAX` (e.g. \(< 255\)) | 8-bit unsigned int
`UINT16_MAX` (e.g. \(< 65535\)) | 16-bit unsigned int
Otherwise (e.g. \(\ge 65535\)) | 32-bit signed int

This section concerns API calls related to sparse data structures.

Function name | Description | Precision
--- | --- | ---
`trexio_has_$group_dset$` | Check if a sparse dset is present in a file | ---
`trexio_read_$group_dset$` | Read indices and values of a sparse dset | Single/Double for indices/values
`trexio_read_$group_dset$_size` | Read the number of sparse data elements stored in the file | Double for size
`trexio_write_$group_dset$` | Write indices and values of a sparse dset | Single/Double for indices/values
`trexio_read_safe_$group_dset$` | Safe (bounded) read of indices and values (for Python API) | Single/Double for indices/values
`trexio_write_safe_$group_dset$` | Safe (bounded) write of indices and values (for Python API) | Single/Double for indices/values

#### 3.5.2 C templates for front end

##### 3.5.2.1 Function declarations

##### 3.5.2.2 Source code for default functions

    trexio_exit_code trexio_read_safe_$group_dset$(trexio_t* const file,
                                                   const int64_t offset_file,
                                                   int64_t* const buffer_size_read,
                                                   int32_t* const index_sparse_read,
                                                   const int64_t size_index_read,
                                                   double* const value_sparse_read,
                                                   const int64_t size_value_read
                                                   )
    {
      return trexio_read_$group_dset$(file, offset_file, buffer_size_read, index_sparse_read, value_sparse_read);
    }

    trexio_exit_code
    trexio_read_$group_dset$(trexio_t* const file,
                             const int64_t offset_file,
                             int64_t* const buffer_size,
                             int32_t* const index_sparse,
                             double* const value_sparse
                             )
    {
      if (file  == NULL) return TREXIO_INVALID_ARG_1;
      if (offset_file < 0L) return TREXIO_INVALID_ARG_2;
      if (*buffer_size <= 0L) return TREXIO_INVALID_ARG_3;
      if (index_sparse == NULL) return TREXIO_INVALID_ARG_4;
      if (value_sparse == NULL) return TREXIO_INVALID_ARG_5;
      if (trexio_has_$group_dset$(file) != TREXIO_SUCCESS) return TREXIO_DSET_MISSING;

      const uint32_t rank = $group_dset_rank$;  // To be set by generator : number of indices

      int64_t size_max;         // Max number of integrals (already in the file)
      trexio_exit_code rc;

      /* Read the max number of integrals stored in the file */
      rc = trexio_read_$group_dset$_size(file, &size_max);
      if (rc != TREXIO_SUCCESS) return rc;

      /* To be set by generator : number of unique dimensions
         (e.g. 1 for ERI in AO basis because only ao_num is present in the list of dimensions) */
    #define unique_rank $group_dset_unique_rank$
      int64_t unique_dims[$group_dset_unique_rank$];

      // Below part is populated by the generator when unique_rank > 1
      rc = trexio_read_$group_dset_unique_dim$_64(file, &unique_dims[$dim_id$]); if (rc != TREXIO_SUCCESS) return rc;

      /* Find the maximal value along all dimensions to define the compression technique in the back end */
      int64_t max_dim = unique_dims[0];
    #if (unique_rank != 1)
      for (uint32_t i = 1; i < unique_rank; i++) {
        if (unique_dims[i] > max_dim) max_dim = unique_dims[i];
      }
    #endif
    #undef unique_rank

      // introduce a new variable which will be modified with the number of integrals being read if EOF is encountered
      int64_t eof_read_size = 0L;

      switch (file->back_end) {

      case TREXIO_TEXT:
        rc = trexio_text_read_$group_dset$(file, offset_file, *buffer_size, max_dim, &eof_read_size, index_sparse, value_sparse);
        break;

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        rc = trexio_hdf5_read_$group_dset$(file, offset_file, *buffer_size, max_dim, &eof_read_size, index_sparse, value_sparse);
        break;
    #else
        rc = TREXIO_BACK_END_MISSING;
        break;
    #endif
    /*
      case TREXIO_JSON:
        return trexio_json_read_$group_dset$(...);
        break;
    */
      default:
        rc = TREXIO_FAILURE;  /* Impossible case */
        break;
      }

      if (rc != TREXIO_SUCCESS && rc != TREXIO_END) return rc;

      if (rc == TREXIO_END) *buffer_size = eof_read_size;

      // shift indices to be one-based if Fortran API is used
      if (file->one_based) {
        // if EOF is reached - shift only indices that have been read, not an entire buffer
        uint64_t index_size = rank*(*buffer_size) ;
        for (uint64_t i=0; i<index_size; ++i){
          index_sparse[i] += 1;
        }
      }

      return rc;
    }

    trexio_exit_code
    trexio_read_$group_dset$_size(trexio_t* const file, int64_t* const size_max)
    {
      if (file  == NULL) return TREXIO_INVALID_ARG_1;
      if (size_max == NULL) return TREXIO_INVALID_ARG_2;
      if (trexio_has_$group_dset$(file) != TREXIO_SUCCESS) return TREXIO_DSET_MISSING;

      switch (file->back_end) {

      case TREXIO_TEXT:
        return trexio_text_read_$group_dset$_size(file, size_max);
        break;

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        return trexio_hdf5_read_$group_dset$_size(file, size_max);
        break;
    #else
        return TREXIO_BACK_END_MISSING;
    #endif
    /*
      case TREXIO_JSON:
        return trexio_json_read_$group_dset$_size(...);
        break;
    */
      default:
        return TREXIO_FAILURE;  /* Impossible case */
      }
    }

    trexio_exit_code trexio_write_safe_$group_dset$(trexio_t* const file,
                                                    const int64_t offset_file,
                                                    const int64_t buffer_size,
                                                    const int32_t* index_sparse_write,
                                                    const int64_t size_index_write,
                                                    const double* value_sparse_write,
                                                    const int64_t size_value_write
                                                    )
    {
      return trexio_write_$group_dset$(file, offset_file, buffer_size, index_sparse_write, value_sparse_write);
    }

    trexio_exit_code
    trexio_write_$group_dset$(trexio_t* const file,
                              const int64_t offset_file,
                              const int64_t buffer_size,
                              const int32_t* index_sparse,
                              const double* value_sparse
                              )
    {
      if (file  == NULL) return TREXIO_INVALID_ARG_1;
      if (offset_file < 0L) return TREXIO_INVALID_ARG_2;
      if (buffer_size <= 0L) return TREXIO_INVALID_ARG_3;
      if (index_sparse == NULL) return TREXIO_INVALID_ARG_4;
      if (value_sparse == NULL) return TREXIO_INVALID_ARG_5;

      /* To be set by generator : number of indices */
      const uint32_t rank = $group_dset_rank$;

      int64_t size_max = 0L; // Max number of integrals (already in the file)
      trexio_exit_code rc;

      /* Read the max number of integrals stored in the file */
      rc = trexio_read_$group_dset$_size(file, &size_max);
      if (rc != TREXIO_SUCCESS && rc != TREXIO_DSET_MISSING) return rc;
      if (rc == TREXIO_DSET_MISSING) size_max = 0L;

      /* To be set by generator : number of unique dimensions
         (e.g. 1 for ERI in AO basis because only ao_num is present in the list of dimensions) */
    #define unique_rank $group_dset_unique_rank$
      int64_t unique_dims[$group_dset_unique_rank$];

      // Below part is populated by the generator when unique_rank > 1
      rc = trexio_read_$group_dset_unique_dim$_64(file, &unique_dims[$dim_id$]); if (rc != TREXIO_SUCCESS) return rc;

      /* Find the maximal value along all dimensions to define the compression technique in the back end */
      int64_t max_dim = unique_dims[0];
    #if (unique_rank != 1)
      for (uint32_t i = 1; i < unique_rank; i++) {
        if (unique_dims[i] > max_dim) max_dim = unique_dims[i];
      }
    #endif
    #undef unique_rank

      // shift indices to be zero-based if Fortran API is used
      if (file->one_based) {

        uint64_t index_size = rank * buffer_size;

        int32_t* index_sparse_p = CALLOC(index_size, int32_t);
        if (index_sparse_p == NULL) return TREXIO_ALLOCATION_FAILED;

        for (uint64_t i=0; i<index_size; ++i){
          index_sparse_p[i] = index_sparse[i] - 1;
        }

        switch (file->back_end) {

        case TREXIO_TEXT:
          rc = trexio_text_write_$group_dset$(file, offset_file, buffer_size, max_dim,
                                              size_max, index_sparse_p, value_sparse);
          break;

        case TREXIO_HDF5:
    #ifdef HAVE_HDF5
          rc = trexio_hdf5_write_$group_dset$(file, offset_file, buffer_size, max_dim,
                                              index_sparse_p, value_sparse);
          break;
    #else
          rc = TREXIO_BACK_END_MISSING;
          break;
    #endif
          /*
            case TREXIO_JSON:
            rc = trexio_json_write_$group_dset$(...);
            break;
          */
        default:
          rc = TREXIO_FAILURE;  /* Impossible case */
          break;
        }

        FREE(index_sparse_p);

      }
      else {

        switch (file->back_end) {

        case TREXIO_TEXT:
          rc = trexio_text_write_$group_dset$(file, offset_file, buffer_size, max_dim,
                                              size_max, index_sparse, value_sparse);
          break;

        case TREXIO_HDF5:
    #ifdef HAVE_HDF5
          rc = trexio_hdf5_write_$group_dset$(file, offset_file, buffer_size, max_dim,
                                              index_sparse, value_sparse);
          break;
    #else
          rc = TREXIO_BACK_END_MISSING;
          break;
    #endif
          /*
            case TREXIO_JSON:
            rc = trexio_json_write_$group_dset$(...);
            break;
          */
        default:
          rc = TREXIO_FAILURE;  /* Impossible case */
          break;
        }

      }

      return rc;
    }

    trexio_exit_code
    trexio_has_$group_dset$ (trexio_t* const file)
    {

      if (file  == NULL) return TREXIO_INVALID_ARG_1;

      assert(file->back_end < TREXIO_INVALID_BACK_END);

      switch (file->back_end) {

      case TREXIO_TEXT:
        return trexio_text_has_$group_dset$(file);

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        return trexio_hdf5_has_$group_dset$(file);
    #else
        return TREXIO_BACK_END_MISSING;
    #endif
    /*
      case TREXIO_JSON:
        return trexio_json_has_$group_dset$(file);
        break;
    */
      }

      return TREXIO_FAILURE;
    }

#### 3.5.3 Fortran templates for front end

The `Fortran` templates that provide an access to the `C` API calls from `Fortran`. These templates are based on the use of `iso_c_binding`. Pointers have to be passed by value.

    interface
       integer(trexio_exit_code) function trexio_write_$group_dset$ (trex_file, &
                                                   offset_file, buffer_size, &
                                                   index_sparse, value_sparse) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         integer(c_int64_t), intent(in), value :: offset_file
         integer(c_int64_t), intent(in), value :: buffer_size
         integer(c_int32_t), intent(in)        :: index_sparse(*)
         real(c_double), intent(in)            :: value_sparse(*)
       end function trexio_write_$group_dset$
    end interface

    interface
       integer(trexio_exit_code) function trexio_write_safe_$group_dset$ (trex_file, &
                                                        offset_file, buffer_size, &
                                                        index_sparse, index_size, &
                                                        value_sparse, value_size) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         integer(c_int64_t), intent(in), value :: offset_file
         integer(c_int64_t), intent(in), value :: buffer_size
         integer(c_int32_t), intent(in)        :: index_sparse(*)
         integer(c_int64_t), intent(in), value :: index_size
         real(c_double), intent(in)            :: value_sparse(*)
         integer(c_int64_t), intent(in), value :: value_size
       end function trexio_write_safe_$group_dset$
    end interface

    interface
       integer(trexio_exit_code) function trexio_read_$group_dset$ (trex_file, &
                                                  offset_file, buffer_size, &
                                                  index_sparse, value_sparse) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         integer(c_int64_t), intent(in), value :: offset_file
         integer(c_int64_t), intent(inout)     :: buffer_size
         integer(c_int32_t), intent(out)       :: index_sparse(*)
         real(c_double), intent(out)           :: value_sparse(*)
       end function trexio_read_$group_dset$
    end interface

    interface
       integer(trexio_exit_code) function trexio_read_safe_$group_dset$ (trex_file, &
                                                       offset_file, buffer_size, &
                                                       index_sparse, index_size, &
                                                       value_sparse, value_size) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         integer(c_int64_t), intent(in), value :: offset_file
         integer(c_int64_t), intent(inout)     :: buffer_size
         integer(c_int32_t), intent(out)       :: index_sparse(*)
         integer(c_int64_t), intent(in), value :: index_size
         real(c_double), intent(out)           :: value_sparse(*)
         integer(c_int64_t), intent(in), value :: value_size
       end function trexio_read_safe_$group_dset$
    end interface

    interface
       integer(trexio_exit_code) function trexio_read_$group_dset$_size (trex_file, &
                                                       size_max) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         integer(c_int64_t), intent(out) :: size_max
       end function trexio_read_$group_dset$_size
    end interface

    interface
       integer(trexio_exit_code) function trexio_has_$group_dset$ (trex_file) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
       end function trexio_has_$group_dset$
    end interface

#### 3.5.4 Python templates for front end

    def write_$group_dset$(trexio_file: File, offset_file: int, buffer_size: int, indices: list, values: list) -> None:
        """Write the $group_dset$ indices and values in the TREXIO file.

        Parameters:

        trexio_file:
            TREXIO File object.

        offset_file: int
            The number of integrals to be skipped in the file when writing.

        buffer_size: int
            The number of integrals to write in the file from the provided sparse arrays.

        indices: list OR numpy.ndarray
            Array of $group_dset$ indices to be written. If array data type does not correspond to int32, the conversion is performed.

        values: list OR numpy.ndarray
            Array of $group_dset$ values to be written. If array data type does not correspond to float64, the conversion is performed.

        Raises:
            - trexio.Error if TREXIO return code ~rc~ is different from TREXIO_SUCCESS and prints the error message.
            - Exception from some other error (e.g. RuntimeError).
        """

        if not isinstance(offset_file, int):
            raise TypeError("offset_file argument has to be an integer.")
        if not isinstance(buffer_size, int):
            raise TypeError("buffer_size argument has to be an integer.")
        if not isinstance(indices, (list, tuple, np.ndarray)):
            raise TypeError("indices argument has to be an array (list, tuple or NumPy ndarray).")
        if not isinstance(values, (list, tuple, np.ndarray)):
            raise TypeError("values argument has to be an array (list, tuple or NumPy ndarray).")

        convertIndices = False
        convertValues = False
        flattenIndices = False
        if isinstance(indices, np.ndarray):
           # convert to int32 if input indices are in a different precision
           if not indices.dtype==np.int32:
               convertIndices = True

           if len(indices.shape) > 1:
               flattenIndices = True
               if convertIndices:
                   indices_32 = np.int32(indices).flatten()
               else:
                   indices_32 = np.array(indices, dtype=np.int32).flatten()
           else:
               if convertIndices:
                   indices_32 = np.int32(indices)
        else:
           # if input array is a multidimensional list or tuple, we have to convert it
           try:
               doFlatten = True
               # if list of indices is flat - the attempt to compute len(indices[0]) will raise a TypeError
               ncol = len(indices[0])
               indices_32 = np.array(indices, dtype=np.int32).flatten()
           except TypeError:
               doFlatten = False
               pass

        if isinstance(values, np.ndarray):
           # convert to float64 if input values are in a different precision
           if not values.dtype==np.float64:
               convertValues = True
           if convertValues:
               values_64 = np.float64(values)

        if (convertIndices or flattenIndices) and convertValues:
            rc = pytr.trexio_write_safe_$group_dset$(trexio_file.pytrexio_s, offset_file, buffer_size, indices_32, values_64)
        elif (convertIndices or flattenIndices) and not convertValues:
            rc = pytr.trexio_write_safe_$group_dset$(trexio_file.pytrexio_s, offset_file, buffer_size, indices_32, values)
        elif not (convertIndices or flattenIndices) and convertValues:
            rc = pytr.trexio_write_safe_$group_dset$(trexio_file.pytrexio_s, offset_file, buffer_size, indices, values_64)
        else:
            rc = pytr.trexio_write_safe_$group_dset$(trexio_file.pytrexio_s, offset_file, buffer_size, indices, values)

        if rc != TREXIO_SUCCESS:
            raise Error(rc)

    def read_$group_dset$(trexio_file: File, offset_file: int, buffer_size: int) -> tuple:
        """Read the $group_dset$ indices and values from the TREXIO file.

        Parameters:

        trexio_file:
            TREXIO File object.

        offset_file: int
            The number of integrals to be skipped in the file when reading.

        buffer_size: int
            The number of integrals to read from the file.

        Returns:
            (indices, values, n_int_read, eof_flag) tuple where
              - indices and values are NumPy arrays [numpy.ndarray] with the default int32 and float64 precision, respectively;
              - n_int_read [int] is the number of integrals read from the trexio_file
                (either strictly equal to buffer_size or less than buffer_size if EOF has been reached);
              - eof_flag [bool] is True when EOF has been reached (i.e. when call to low-level pytrexio API returns TREXIO_END)
                                   False otherwise.

        Raises:
            - Exception from AssertionError if TREXIO return code ~rc~ is different from TREXIO_SUCCESS
                and prints the error message using trexio_string_of_error.
            - Exception from some other error (e.g. RuntimeError).
        """

        if not isinstance(offset_file, int):
            raise TypeError("offset_file argument has to be an integer.")
        if not isinstance(buffer_size, int):
            raise TypeError("buffer_size argument has to be an integer.")

        # read the number of integrals already in the file
        integral_num = read_$group_dset$_size(trexio_file)

        # additional modification needed to avoid allocating more memory than needed if EOF will be reached during read
        overflow = offset_file + buffer_size - integral_num
        eof_flag = False
        if overflow > 0:
            verified_size = buffer_size - overflow
            eof_flag = True
        else:
            verified_size = buffer_size

        # main call to the low-level (SWIG-wrapped) trexio_read function, which also requires the sizes of the output to be provided
        # as the last 2 arguments (for numpy arrays of indices and values, respectively)
        # read_buf_size contains the number of elements being read from the file, useful when EOF has been reached
        rc, n_int_read, indices, values = pytr.trexio_read_safe_$group_dset$(trexio_file.pytrexio_s,
                                                                             offset_file,
                                                                             verified_size,
                                                                             verified_size * $group_dset_rank$,
                                                                             verified_size)
        if rc != TREXIO_SUCCESS:
            raise Error(rc)
        if n_int_read == 0:
            raise ValueError("No integrals have been read from the file.")
        if indices is None or values is None:
            raise ValueError("Returned NULL array from the low-level pytrexio API.")

        # conversion to custom types can be performed on the user side, here we only reshape the returned flat array of indices according to group_dset_rank
        shape = tuple([verified_size, $group_dset_rank$])
        indices_reshaped = np.reshape(indices, shape, order='C')

        return (indices_reshaped, values, n_int_read, eof_flag)

    def read_$group_dset$_size(trexio_file) -> int:
        """Read the number of $group_dset$ integrals stored in the TREXIO file.

        Parameter is a ~TREXIO File~ object that has been created by a call to ~open~ function.

        Returns:
            ~num_integral~: int
            Integer value of corresponding to the size of the $group_dset$ sparse array from ~trexio_file~.

        Raises:
            - Exception from AssertionError if TREXIO return code ~rc~ is different from TREXIO_SUCCESS and prints the error message using trexio_string_of_error.
            - Exception from some other error (e.g. RuntimeError).
        """

        rc, num_integral = pytr.trexio_read_$group_dset$_size(trexio_file.pytrexio_s)
        if rc != TREXIO_SUCCESS:
            raise Error(rc)

        return num_integral

    def has_$group_dset$(trexio_file) -> bool:
        """Check that $group_dset$ variable exists in the TREXIO file.

        Parameter is a ~TREXIO File~ object that has been created by a call to ~open~ function.

        Returns:
              True if the variable exists, False otherwise

        Raises:
            - trexio.Error if TREXIO return code ~rc~ is TREXIO_FAILURE and prints the error message using string_of_error.
            - Exception from some other error (e.g. RuntimeError).
        """

        rc = pytr.trexio_has_$group_dset$(trexio_file.pytrexio_s)
        if rc == TREXIO_FAILURE:
            raise Error(rc)

        return rc == TREXIO_SUCCESS

### 3.6 Templates for front end has/read/write a dataset of strings

#### 3.6.1 Introduction

This section concerns API calls related to datasets of strings.

Function name | Description
--- | ---
`trexio_has_$group_dset$` | Check if a dataset exists in a file
`trexio_read_$group_dset$` | Read a dataset
`trexio_write_$group_dset$` | Write a dataset

#### 3.6.2 C templates for front end

First parameter is the `TREXIO` file handle. Second parameter is the variable to be written/read to/from the `TREXIO` file (except for `trexio_has_` functions).

##### 3.6.2.1 Function declarations

##### 3.6.2.2 Source code for default functions

    trexio_exit_code
    trexio_read_$group_dset$_low (trexio_t* const file, char* dset_out, const int32_t max_str_len)
    {

      if (file  == NULL) return TREXIO_INVALID_ARG_1;
      if (dset_out == NULL) return TREXIO_INVALID_ARG_2;
      if (max_str_len <= 0) return TREXIO_INVALID_ARG_3;
      if (trexio_has_$group_dset$(file) != TREXIO_SUCCESS) return TREXIO_DSET_MISSING;

      trexio_exit_code rc;
      int64_t $group_dset_dim$ = 0;

      /* Error handling for this call is added by the generator */
      rc = trexio_read_$group_dset_dim$_64(file, &($group_dset_dim$));

      if ($group_dset_dim$ == 0L) return TREXIO_INVALID_NUM;

      uint32_t rank = $group_dset_rank$;
      uint64_t dims[$group_dset_rank$] = {$group_dset_dim_list$};

      assert(file->back_end < TREXIO_INVALID_BACK_END);
      switch (file->back_end) {

      case TREXIO_TEXT:
        return trexio_text_read_$group_dset$(file, dset_out, rank, dims, (uint32_t) max_str_len);

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        return trexio_hdf5_read_$group_dset$(file, dset_out, rank, dims, (uint32_t) max_str_len);
    #else
        return TREXIO_BACK_END_MISSING;
    #endif
    /*
      case TREXIO_JSON:
        rc = trexio_json_read_$group_dset$(file, dset_out, rank, dims);
    */
      }

      return TREXIO_FAILURE;
    }

    trexio_exit_code
    trexio_read_$group_dset$ (trexio_t* const file, char** dset_out, const int32_t max_str_len)
    {

      if (file  == NULL) return TREXIO_INVALID_ARG_1;
      if (dset_out == NULL) return TREXIO_INVALID_ARG_2;
      if (max_str_len <= 0) return TREXIO_INVALID_ARG_3;
      if (trexio_has_$group_dset$(file) != TREXIO_SUCCESS) return TREXIO_DSET_MISSING;

      assert(file->back_end < TREXIO_INVALID_BACK_END);

      trexio_exit_code rc;
      int64_t dset_dim = 0;

      /* Error handling for this call is added by the generator */
      rc = trexio_read_$group_dset_dim$_64(file, &(dset_dim));

      if (dset_dim == 0L) return TREXIO_INVALID_NUM;

      char* str_compiled = CALLOC(dset_dim*(max_str_len+1) + 1, char);
      if (str_compiled == NULL) return TREXIO_ALLOCATION_FAILED;

      rc = trexio_read_$group_dset$_low(file, str_compiled, max_str_len);
      if (rc != TREXIO_SUCCESS) {
        FREE(str_compiled);
        return rc;
      }

      for (uint64_t i=0; i < (uint64_t) dset_dim; i++) {

        char * pch;
        pch = i == 0 ? strtok(str_compiled, TREXIO_DELIM) : strtok(NULL, TREXIO_DELIM) ;
        if (pch == NULL) {
          FREE(str_compiled);
          return TREXIO_FAILURE;
        }

        strcpy(dset_out[i], "");
        strcat(dset_out[i], pch);

      }

      FREE(str_compiled);
      return TREXIO_SUCCESS;

    }

    trexio_exit_code
    trexio_write_$group_dset$_low (trexio_t* const file, char* dset_in, const int32_t max_str_len)
    {

      if (file  == NULL) return TREXIO_INVALID_ARG_1;
      if (dset_in == NULL) return TREXIO_INVALID_ARG_2;
      if (max_str_len <= 0) return TREXIO_INVALID_ARG_3;
      if (trexio_has_$group_dset$(file) == TREXIO_SUCCESS && file->mode != 'u') return TREXIO_DSET_ALREADY_EXISTS;

      trexio_exit_code rc;
      int64_t $group_dset_dim$ = 0;

      /* Error handling for this call is added by the generator */
      rc = trexio_read_$group_dset_dim$_64(file, &($group_dset_dim$));

      if ($group_dset_dim$ == 0L) return TREXIO_INVALID_NUM;

      uint32_t rank = $group_dset_rank$;
      uint64_t dims[$group_dset_rank$] = {$group_dset_dim_list$};

      assert(file->back_end < TREXIO_INVALID_BACK_END);

      char*  tmp_str  = CALLOC(dims[0]*(max_str_len+1), char);
      if (tmp_str == NULL) return TREXIO_ALLOCATION_FAILED;
      char** dset_str = CALLOC(dims[0], char*);
      if (dset_str == NULL) {
        FREE(tmp_str);
        return TREXIO_ALLOCATION_FAILED;
      }

      /* parse the string using strtok */
      for(uint64_t i=0; i<dims[0]; i++) {

        char* pch;
        pch = i == 0 ? strtok(dset_in, TREXIO_DELIM) : strtok(NULL, TREXIO_DELIM) ;

        if (pch == NULL) {
          FREE(dset_str[0]);
          FREE(dset_str);
          return TREXIO_FAILURE;
        }

        size_t pch_len = strlen(pch) + 1;

        if (pch_len > (size_t) max_str_len) {
          FREE(dset_str[0]);
          FREE(dset_str);
          return TREXIO_INVALID_STR_LEN;
        }

        dset_str[i] = tmp_str;
        strncpy(tmp_str, pch, pch_len);
        tmp_str += pch_len + 1;
      }

      rc = TREXIO_FAILURE;
      switch (file->back_end) {

      case TREXIO_TEXT:
        rc = trexio_text_write_$group_dset$(file, (const char**) dset_str, rank, dims);
        break;

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        rc = trexio_hdf5_write_$group_dset$(file, (const char**) dset_str, rank, dims);
        break;
    #else
        rc =TREXIO_BACK_END_MISSING;
        break;
    #endif
    /*
      case TREXIO_JSON:
        rc = trexio_json_write_$group_dset$(file, dset, rank, dims);
        break;
    */
      }

      FREE(dset_str[0]);
      FREE(dset_str);

      return rc;

    }

    trexio_exit_code
    trexio_write_$group_dset$ (trexio_t* const file, const char** dset_in, const int32_t max_str_len)
    {

      if (file  == NULL) return TREXIO_INVALID_ARG_1;
      if (dset_in == NULL) return TREXIO_INVALID_ARG_2;
      if (max_str_len <= 0) return TREXIO_INVALID_ARG_3;
      if (trexio_has_$group_dset$(file) == TREXIO_SUCCESS && file->mode != 'u') return TREXIO_DSET_ALREADY_EXISTS;

      assert(file->back_end < TREXIO_INVALID_BACK_END);

      trexio_exit_code rc;
      int64_t dset_dim = 0;

      /* Error handling for this call is added by the generator */
      rc = trexio_read_$group_dset_dim$_64(file, &(dset_dim));

      if (dset_dim == 0L) return TREXIO_INVALID_NUM;

      char* str_compiled = CALLOC(dset_dim*max_str_len + 1, char);
      if (str_compiled == NULL) return TREXIO_ALLOCATION_FAILED;

      strcpy(str_compiled, "");
      for (uint64_t i=0; i < (uint64_t) dset_dim; i++) {
        strcat(str_compiled, dset_in[i]);
        strcat(str_compiled, TREXIO_DELIM);
      }

      rc = trexio_write_$group_dset$_low(file, str_compiled, max_str_len);

      FREE(str_compiled);

      return rc;
    }

    trexio_exit_code
    trexio_has_$group_dset$ (trexio_t* const file)
    {

      if (file  == NULL) return TREXIO_INVALID_ARG_1;

      assert(file->back_end < TREXIO_INVALID_BACK_END);

      switch (file->back_end) {

      case TREXIO_TEXT:
        return trexio_text_has_$group_dset$(file);

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        return trexio_hdf5_has_$group_dset$(file);
    #else
        return TREXIO_BACK_END_MISSING;
    #endif
    /*
      case TREXIO_JSON:
        return trexio_json_has_$group_dset$(file);
        break;
    */
      }

      return TREXIO_FAILURE;
    }

#### 3.6.3 Fortran templates for front end

The `Fortran` templates that provide an access to the `C` API calls from `Fortran`. These templates are based on the use of `iso_c_binding`. Pointers have to be passed by value.

    interface
       integer(trexio_exit_code) function trexio_write_$group_dset$_low (trex_file, dset, max_str_len) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         character(kind=c_char), intent(in)    :: dset(*)
         integer(c_int32_t), intent(in), value :: max_str_len
       end function trexio_write_$group_dset$_low
    end interface

    interface
       integer(trexio_exit_code) function trexio_read_$group_dset$_low (trex_file, dset, max_str_len) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(c_int64_t), intent(in), value  :: trex_file
         character(kind=c_char), intent(out)    :: dset(*)
         integer(c_int32_t), intent(in), value  :: max_str_len
       end function trexio_read_$group_dset$_low
    end interface

    interface
       integer(trexio_exit_code) function trexio_has_$group_dset$ (trex_file) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
       end function trexio_has_$group_dset$
    end interface

    integer(trexio_exit_code) function trexio_read_$group_dset$ (trex_file, dset, max_str_len)
      implicit none
      integer(trexio_t), intent(in), value :: trex_file
      integer(c_int32_t), intent(in), value :: max_str_len
      character(len=*), intent(inout) :: dset(*)

      character, allocatable :: str_compiled(:)
      integer(c_int64_t) :: $group_dset_dim$
      integer(trexio_exit_code) :: rc

      rc = trexio_read_$group_dset_dim$_64(trex_file, $group_dset_dim$)
      if (rc /= TREXIO_SUCCESS) trexio_read_$group_dset$ = rc

      allocate(str_compiled($group_dset_dim$*(max_str_len+1)+1))

      rc = trexio_read_$group_dset$_low(trex_file, str_compiled, max_str_len)
      if (rc /= TREXIO_SUCCESS) then
        deallocate(str_compiled)
        trexio_read_$group_dset$ = rc
      else
        call trexio_str2strarray(str_compiled, $group_dset_dim$, max_str_len, dset)
        deallocate(str_compiled)
        trexio_read_$group_dset$ = TREXIO_SUCCESS
      endif

    end function trexio_read_$group_dset$

    integer(trexio_exit_code) function trexio_write_$group_dset$ (trex_file, dset, max_str_len)
      implicit none
      integer(trexio_t), intent(in), value :: trex_file
      integer(c_int32_t), intent(in), value :: max_str_len
      character(len=*), intent(in) :: dset(*)

      character(len=:), allocatable :: str_compiled
      integer(c_int64_t) :: $group_dset_dim$
      integer(trexio_exit_code) :: rc

      rc = trexio_read_$group_dset_dim$_64(trex_file, $group_dset_dim$)
      if (rc /= TREXIO_SUCCESS) then
        trexio_write_$group_dset$ = rc
      else
        call trexio_strarray2str(dset, $group_dset_dim$, max_str_len, str_compiled)
        trexio_write_$group_dset$ = trexio_write_$group_dset$_low(trex_file, str_compiled, max_str_len)
      endif

    end function trexio_write_$group_dset$

#### 3.6.4 Python templates for front end

    def write_$group_dset$(trexio_file, dset_w: list) -> None:
        """Write the $group_dset$ array of strings in the TREXIO file.

        Parameters:

        trexio_file:
            TREXIO File object.

        dset_w: list
            Array of $group_dset$ strings to be written.

        Raises:
            - Exception from AssertionError if TREXIO return code ~rc~ is different from TREXIO_SUCCESS and prints the error message using trexio_string_of_error.
            - Exception from some other error (e.g. RuntimeError).
        """

        max_str_length = len(max(dset_w, key=len)) + 1

        rc = pytr.trexio_write_$group_dset$(trexio_file.pytrexio_s, dset_w, max_str_length)
        if rc != TREXIO_SUCCESS:
            raise Error(rc)

    def read_$group_dset$(trexio_file, dim = None) -> list:
        """Read the $group_dset$ array of strings from the TREXIO file.

        Parameters:

        trexio_file:
            TREXIO File object.

        dim (Optional): int
            Size of the block to be read from the file (i.e. how many items of $group_dset$ will be returned)
            If None, the function will read all necessary array dimensions from the file.

        Returns:
            ~dset_r~: list
            1D list with ~dim~ elements corresponding to $group_dset$ strings read from the TREXIO file.

        Raises:
            - Exception from AssertionError if TREXIO return code ~rc~ is different from TREXIO_SUCCESS and prints the error message using trexio_string_of_error.
            - Exception from some other error (e.g. RuntimeError).
        """

        $group_dset_dim$ = read_$group_dset_dim$(trexio_file)

        dims_list = [$group_dset_dim_list$]
        dim_real  = 1
        for i in range($group_dset_rank$):
            dim_real *= dims_list[i]

        if dim:
            if dim_real != dim:
                raise Error(TREXIO_UNSAFE_ARRAY_DIM)
        else:
            dim = dim_real

        rc, dset_1d_r = pytr.trexio_read_$group_dset$_low(trexio_file.pytrexio_s, PYTREXIO_MAX_STR_LENGTH)
        if rc != TREXIO_SUCCESS:
            raise Error(rc)

        dset_full = dset_1d_r.split(pytr.TREXIO_DELIM)
        dset_2d_r = [dset_full[i] for i in range(dim) if dset_full[i]]
        if not dset_2d_r:
            raise ValueError(f"Output of {read_$group_dset$.__name__} function cannot be an empty list.")

        return dset_2d_r

    def has_$group_dset$(trexio_file) -> bool:
        """Check that $group_dset$ variable exists in the TREXIO file.

        Parameter is a ~TREXIO File~ object that has been created by a call to ~open~ function.

        Returns:
              True if the variable exists, False otherwise

        Raises:
            - trexio.Error if TREXIO return code ~rc~ is TREXIO_FAILURE and prints the error message using string_of_error.
            - Exception from some other error (e.g. RuntimeError).
        """

        rc = pytr.trexio_has_$group_dset$(trexio_file.pytrexio_s)
        if rc == TREXIO_FAILURE:
            raise Error(rc)

        return rc == TREXIO_SUCCESS

### 3.7 Templates for front end has/read/write a buffered vector

This corresponds to the `buffer` data type and is particularly useful for incremental additiona of values like it was done for `sparse` data but without the need to supply tuples of indices.

Function name | Description | Precision
--- | --- | ---
`trexio_has_$group_dset$` | Check if a buffered dset is present in a file | ---
`trexio_read_$group_dset$` | Read values of a vector in buffers | Double
`trexio_read_$group_dset$_size` | Read the number of elements stored in the file | Double
`trexio_write_$group_dset$` | Write values of a vector in buffers | Double
`trexio_read_safe_$group_dset$` | Safe (bounded) read (for Python API) | Double
`trexio_write_safe_$group_dset$` | Safe (bounded) write (for Python API) | Double

#### 3.7.1 C source code

    trexio_exit_code
    trexio_read_$group_dset$ (trexio_t* const file, const int64_t offset_file, int64_t* const buffer_size_read, double* const dset)
    {

      if (file  == NULL) return TREXIO_INVALID_ARG_1;
      if (dset  == NULL) return TREXIO_INVALID_ARG_2;
      if (trexio_has_$group_dset$(file) != TREXIO_SUCCESS) return TREXIO_DSET_MISSING;

      trexio_exit_code rc;

      uint32_t rank = 1;
      uint64_t det_size = (uint64_t) (*buffer_size_read);
      uint64_t dims[1] = {det_size};

      // introduce a new variable which will be modified with the number of integrals being read if EOF is encountered
      int64_t eof_read_size = 0L;

      switch (file->back_end) {

      case TREXIO_TEXT:
        rc = trexio_text_read_$group_dset$(file, offset_file, rank, dims, &eof_read_size, dset);
        break;

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        rc = trexio_hdf5_read_$group_dset$(file, offset_file, rank, dims, &eof_read_size, dset);
        break;
    #else
        rc = TREXIO_BACK_END_MISSING;
        break;
    #endif
    /*
      case TREXIO_JSON:
        return trexio_json_read_$group_dset$(...);
        break;
    */
      default:
        rc = TREXIO_FAILURE;  /* Impossible case */
        break;
      }

      if (rc != TREXIO_SUCCESS && rc != TREXIO_END) return rc;

      if (rc == TREXIO_END) *buffer_size_read = eof_read_size;

      return rc;
    }

    trexio_exit_code
    trexio_read_$group_dset$_size(trexio_t* const file, int64_t* const size_max)
    {

      if (file  == NULL) return TREXIO_INVALID_ARG_1;
      if (size_max == NULL) return TREXIO_INVALID_ARG_2;
      if (trexio_has_$group_dset$(file) != TREXIO_SUCCESS) return TREXIO_DSET_MISSING;

      switch (file->back_end) {

      case TREXIO_TEXT:
        return trexio_text_read_$group_dset$_size(file, size_max);
        break;

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        return trexio_hdf5_read_$group_dset$_size(file, size_max);
        break;
    #else
        return TREXIO_BACK_END_MISSING;
    #endif
    /*
      case TREXIO_JSON:
        return trexio_json_read_
        break;
    */
      default:
        return TREXIO_FAILURE;  /* Impossible case */
      }
    }

    trexio_exit_code
    trexio_read_safe_$group_dset$ (trexio_t* const file, const int64_t offset_file, int64_t* const buffer_size_read, double* const dset_out, const int64_t dim_out)
    {
      return trexio_read_$group_dset$(file, offset_file, buffer_size_read, dset_out);
    }

    trexio_exit_code
    trexio_write_$group_dset$ (trexio_t* const file, const int64_t offset_file, const int64_t buffer_size, const double* dset)
    {

      if (file == NULL) return TREXIO_INVALID_ARG_1;
      if (dset == NULL) return TREXIO_INVALID_ARG_2;

      uint32_t rank = 1;
      uint64_t dims[1] = {buffer_size};

      assert(file->back_end < TREXIO_INVALID_BACK_END);

      switch (file->back_end) {

      case TREXIO_TEXT:
        return trexio_text_write_$group_dset$(file, offset_file, rank, dims, dset);
        break;

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        return trexio_hdf5_write_$group_dset$(file, offset_file, rank, dims, dset);
        break;
    #else
        return TREXIO_BACK_END_MISSING;
        break;
    #endif
    /*
      case TREXIO_JSON:
        rc = trexio_json_read_
        break;
    */
      }

      return TREXIO_FAILURE;
    }

    trexio_exit_code
    trexio_write_safe_$group_dset$ (trexio_t* const file, const int64_t offset_file, const int64_t buffer_size, const double* dset_in, const int64_t dim_in)
    {
      return trexio_write_$group_dset$(file, offset_file, buffer_size, dset_in);
    }

    trexio_exit_code
    trexio_has_$group_dset$ (trexio_t* const file)
    {

      if (file == NULL) return TREXIO_INVALID_ARG_1;

      assert(file->back_end < TREXIO_INVALID_BACK_END);

      switch (file->back_end) {

      case TREXIO_TEXT:
        return trexio_text_has_$group_dset$(file);

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        return trexio_hdf5_has_$group_dset$(file);
    #else
        return TREXIO_BACK_END_MISSING;
    #endif
    /*
      case TREXIO_JSON:
        return trexio_json_has_
        break;
    */
      }

      return TREXIO_FAILURE;
    }

#### 3.7.2 Fortran interface

The `Fortran` templates that provide an access to the `C` API calls from Fortran. These templates are based on the use of `iso_c_binding`. Pointers have to be passed by value.

    interface
       integer(trexio_exit_code) function trexio_write_$group_dset$(trex_file, &
                                                   offset_file, buffer_size, dset) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         integer(c_int64_t), intent(in), value :: offset_file
         integer(c_int64_t), intent(in), value :: buffer_size
         real(c_double), intent(in)            :: dset(*)
       end function trexio_write_$group_dset$
    end interface

    interface
       integer(trexio_exit_code) function trexio_write_safe_$group_dset$ (trex_file, &
                                                        offset_file, buffer_size, &
                                                        dset, dset_size) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         integer(c_int64_t), intent(in), value :: offset_file
         integer(c_int64_t), intent(in), value :: buffer_size
         real(c_double), intent(in)            :: dset(*)
         integer(c_int64_t), intent(in), value :: dset_size
       end function trexio_write_safe_$group_dset$
    end interface

    interface
       integer(trexio_exit_code) function trexio_read_safe_$group_dset$ (trex_file, &
                                                       offset_file, buffer_size, &
                                                       dset, dset_size) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         integer(c_int64_t), intent(in), value :: offset_file
         integer(c_int64_t), intent(inout)     :: buffer_size
         real(c_double), intent(out)           :: dset(*)
         integer(c_int64_t), intent(in), value :: dset_size
       end function trexio_read_safe_$group_dset$
    end interface

    interface
       integer(trexio_exit_code) function trexio_read_$group_dset$(trex_file, &
                                                  offset_file, buffer_size, dset) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         integer(c_int64_t), intent(in), value :: offset_file
         integer(c_int64_t), intent(inout)     :: buffer_size
         real(c_double), intent(out)           :: dset(*)
       end function trexio_read_$group_dset$
    end interface

    interface
       integer(trexio_exit_code) function trexio_read_$group_dset$_size (trex_file, &
                                                       size_max) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         integer(c_int64_t), intent(out) :: size_max
       end function trexio_read_$group_dset$_size
    end interface

    interface
       integer(trexio_exit_code) function trexio_has_$group_dset$ (trex_file) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
       end function trexio_has_$group_dset$
    end interface

#### 3.7.3 Python interface

    def write_$group_dset$(trexio_file: File, offset_file: int, buffer_size: int, dset) -> None:
        """Write the $group_dset$ in the TREXIO file.

        Parameters:

        trexio_file:
            TREXIO File object.

        offset_file: int
            The number of values to be skipped in the file when writing.

        buffer_size: int
            The number of values to write in the file.

        dset: list OR numpy.ndarray
            Array of $group_dset$ to be written. If array data type does not correspond to int64, the conversion is performed.

        Raises:
            - trexio.Error if TREXIO return code ~rc~ is different from TREXIO_SUCCESS and prints the error message.
            - Exception from some other error (e.g. RuntimeError).
        """

        if not isinstance(offset_file, int):
            raise TypeError("offset_file argument has to be an integer.")
        if not isinstance(buffer_size, int):
            raise TypeError("buffer_size argument has to be an integer.")
        if not isinstance(dset, (list, tuple, np.ndarray)):
            raise TypeError("dset argument has to be an array (list, tuple or NumPy ndarray).")

        if isinstance(dset, np.ndarray) and not coefficients.dtype==np.float64:
            # convert to float64 if input is in a different precision
            dset_64 = np.float64(dset)
            rc = pytr.trexio_write_safe_$group_dset$(trexio_file.pytrexio_s, offset_file, buffer_size, dset_64)
        else:
            rc = pytr.trexio_write_safe_$group_dset$(trexio_file.pytrexio_s, offset_file, buffer_size, dset)

        if rc != TREXIO_SUCCESS:
            raise Error(rc)

    def read_$group_dset$(trexio_file: File, offset_file: int, buffer_size: int) -> tuple:
        """Read $group_dset$ from the TREXIO file.

        Parameters:

        trexio_file:
            TREXIO File object.

        offset_file: int
            The number of values to be skipped in the file when reading.

        buffer_size: int
            The number of values to read from the file.

        Returns:
            (dset, n_int_read, eof_flag) tuple where
              - dset is the NumPy array [numpy.ndarray] with the default int64 precision;
              - n_int_read [int] is the number of coefficients read from the trexio_file
                (either strictly equal to buffer_size or less than buffer_size if EOF has been reached);
              - eof_flag [bool] is True when EOF has been reached (i.e. when call to low-level pytrexio API returns TREXIO_END)
                                   False otherwise.

        Raises:
            - trexio.Error if TREXIO return code ~rc~ is different from TREXIO_SUCCESS and prints the error message.
            - Exception from some other error (e.g. RuntimeError).
        """

        if not isinstance(offset_file, int):
            raise TypeError("offset_file argument has to be an integer.")
        if not isinstance(buffer_size, int):
            raise TypeError("buffer_size argument has to be an integer.")

        # read the number of values already in the file
        det_num = read_$group_dset$_size(trexio_file)

        # additional modification needed to avoid allocating more memory than needed if EOF will be reached during read
        overflow = offset_file + buffer_size - det_num
        eof_flag = False
        if overflow > 0:
            verified_size = buffer_size - overflow
            eof_flag = True
        else:
            verified_size = buffer_size

        # main call to the low-level (SWIG-wrapped) trexio_read function, which also requires the sizes of the output to be provided
        # read_buf_size contains the number of elements being read from the file, useful when EOF has been reached
        rc, n_int_read, dset = pytr.trexio_read_safe_$group_dset$(trexio_file.pytrexio_s, offset_file, verified_size, verified_size)

        if rc != TREXIO_SUCCESS:
            raise Error(rc)
        if n_int_read == 0:
            raise ValueError("No integrals have been read from the file.")
        if dset is None:
            raise ValueError("Returned NULL array from the low-level pytrexio API.")

        return (dset, n_int_read, eof_flag)

    def read_$group_dset$_size(trexio_file) -> int:
        """Read the number of elements stored in the TREXIO file.

        Parameter is a ~TREXIO File~ object that has been created by a call to ~open~ function.

        Returns:
            ~num~: int
            Integer value of corresponding to the size of the $group_dset$ array from ~trexio_file~.

        Raises:
            - Exception from AssertionError if TREXIO return code ~rc~ is different from TREXIO_SUCCESS and prints the error message using trexio_string_of_error.
            - Exception from some other error (e.g. RuntimeError).
        """

        rc, num = pytr.trexio_read_$group_dset$_size(trexio_file.pytrexio_s)
        if rc != TREXIO_SUCCESS:
            raise Error(rc)

        return num

    def has_$group_dset$(trexio_file) -> bool:
        """Check that $group_dset$ exists in the TREXIO file.

        Parameter is a ~TREXIO File~ object that has been created by a call to ~open~ function.

        Returns:
              True if the variable exists, False otherwise

        Raises:
            - trexio.Error if TREXIO return code ~rc~ is TREXIO_FAILURE and prints the error message using string_of_error.
            - Exception from some other error (e.g. RuntimeError).
        """

        rc = pytr.trexio_has_$group_dset$(trexio_file.pytrexio_s)
        if rc == TREXIO_FAILURE:
            raise Error(rc)

        return rc == TREXIO_SUCCESS

### 3.8 Templates for front end has/read/write a single string attribute

#### 3.8.1 Introduction

This section concerns API calls related to string attributes.

Function name | Description
--- | ---
`trexio_has_$group_str$` | Check if a string attribute exists in a file
`trexio_read_$group_str$` | Read a string attribute
`trexio_write_$group_str$` | Write a string attribute

#### 3.8.2 C templates for front end

##### 3.8.2.1 Function declarations

##### 3.8.2.2 Source code for default functions

    trexio_exit_code
    trexio_read_$group_str$ (trexio_t* const file, char* const str_out, const int32_t max_str_len)
    {

      if (file == NULL) return TREXIO_INVALID_ARG_1;
      if (str_out  == NULL) return TREXIO_INVALID_ARG_2;
      if (max_str_len <= 0) return TREXIO_INVALID_ARG_3;
      if (trexio_has_$group_str$(file) != TREXIO_SUCCESS) return TREXIO_ATTR_MISSING;

      switch (file->back_end) {

      case TREXIO_TEXT:
        return trexio_text_read_$group_str$(file, str_out, (uint32_t) max_str_len);

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        return trexio_hdf5_read_$group_str$(file, str_out, (uint32_t) max_str_len);
    #else
        return TREXIO_BACK_END_MISSING;
    #endif
    /*
      case TREXIO_JSON:
        return trexio_json_read_$group_str$(file, str);
        break;
    */
      }

      return TREXIO_FAILURE;
    }

    trexio_exit_code
    trexio_write_$group_str$ (trexio_t* const file, const char* str, const int32_t max_str_len)
    {

      if (file == NULL) return TREXIO_INVALID_ARG_1;
      if (str  == NULL) return TREXIO_INVALID_ARG_2;
      if (max_str_len <= 0) return TREXIO_INVALID_ARG_3;
      if (trexio_has_$group_str$(file) == TREXIO_SUCCESS && file->mode != 'u') return TREXIO_ATTR_ALREADY_EXISTS;

      size_t len_write = strlen(str);
      if ((size_t) max_str_len < len_write) return TREXIO_INVALID_STR_LEN;

      switch (file->back_end) {

      case TREXIO_TEXT:
        return trexio_text_write_$group_str$(file, str);

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        return trexio_hdf5_write_$group_str$(file, str);
    #else
        return TREXIO_BACK_END_MISSING;
    #endif
    /*
      case TREXIO_JSON:
        return trexio_json_write_$group_str$(file, str);
    */
      }

      return TREXIO_FAILURE;
    }

    trexio_exit_code
    trexio_has_$group_str$ (trexio_t* const file)
    {

      if (file == NULL) return TREXIO_INVALID_ARG_1;

      assert(file->back_end < TREXIO_INVALID_BACK_END);

      switch (file->back_end) {

      case TREXIO_TEXT:
        return trexio_text_has_$group_str$(file);

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        return trexio_hdf5_has_$group_str$(file);
    #else
        return TREXIO_BACK_END_MISSING;
    #endif
    /*
      case TREXIO_JSON:
        return trexio_json_has_$group_str$(file);
    */
      }

      return TREXIO_FAILURE;
    }

#### 3.8.3 Fortran templates for front end

The `Fortran` templates that provide an access to the `C` API calls from Fortran. These templates are based on the use of `iso_c_binding`. Pointers have to be passed by value.

    interface
       integer(trexio_exit_code) function trexio_write_$group_str$_c (trex_file, str, max_str_len) &
               bind(C, name="trexio_write_$group_str$")
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value  :: trex_file
         character(kind=c_char), intent(in)    :: str(*)
         integer(c_int32_t), intent(in), value :: max_str_len
       end function trexio_write_$group_str$_c
    end interface

    interface
       integer(trexio_exit_code) function trexio_read_$group_str$_c (trex_file, str, max_str_len) &
               bind(C, name="trexio_read_$group_str$")
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value  :: trex_file
         character(kind=c_char), intent(out)   :: str(*)
         integer(c_int32_t), intent(in), value :: max_str_len
       end function trexio_read_$group_str$_c
    end interface

    interface
       integer(trexio_exit_code) function trexio_has_$group_str$ (trex_file) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value  :: trex_file
       end function trexio_has_$group_str$
    end interface

    integer(trexio_exit_code) function trexio_read_$group_str$ (trex_file, str, max_str_len)
      implicit none
      integer(trexio_t), intent(in), value  :: trex_file
      integer(c_int32_t), intent(in), value :: max_str_len
      character, intent(out) :: str(*)

      trexio_read_$group_str$ = trexio_read_$group_str$_c(trex_file, str, max_str_len)

    end function trexio_read_$group_str$

    integer(trexio_exit_code) function trexio_write_$group_str$ (trex_file, str, max_str_len)
      use, intrinsic :: iso_c_binding, only : c_null_char
      implicit none
      integer(trexio_t), intent(in), value  :: trex_file
      integer(c_int32_t), intent(in), value :: max_str_len
      character(len=*), intent(in) :: str

      character(len=len_trim(str)+1) :: str_c

      str_c = trim(str) // c_null_char

      trexio_write_$group_str$ = trexio_write_$group_str$_c(trex_file, str_c, max_str_len)

    end function trexio_write_$group_str$

#### 3.8.4 Python templates for front end

    def write_$group_str$(trexio_file, str_w: str) -> None:
        """Write the $group_str$ variable in the TREXIO file.

        Parameters:

        trexio_file:
            TREXIO File object.

        str_w: str
            String corresponding to the $group_str$ variable to be written.

        Raises:
            - Exception from AssertionError if TREXIO return code ~rc~ is different from TREXIO_SUCCESS and prints the error message using trexio_string_of_error.
            - Exception from some other error (e.g. RuntimeError).
        """

        max_str_length = len(str_w) + 1

        rc = pytr.trexio_write_$group_str$(trexio_file.pytrexio_s, str_w, max_str_length)
        if rc != TREXIO_SUCCESS:
            raise Error(rc)

    def read_$group_str$(trexio_file) -> str:
        """Read the $group_str$ variable from the TREXIO file.

        Parameter is a ~TREXIO File~ object that has been created by a call to ~open~ function.

        Returns:
            ~str_r~: str
            String corresponding to the $group_str$ variable read from ~trexio_file~.

        Raises:
            - Exception from AssertionError if TREXIO return code ~rc~ is different from TREXIO_SUCCESS and prints the error message using trexio_string_of_error.
            - Exception from some other error (e.g. RuntimeError).
        """

        rc, str_r = pytr.trexio_read_$group_str$(trexio_file.pytrexio_s, PYTREXIO_MAX_STR_LENGTH)
        if rc != TREXIO_SUCCESS:
            raise Error(rc)

        return str_r

    def has_$group_str$(trexio_file) -> bool:
        """Check that $group_str$ variable exists in the TREXIO file.

        Parameter is a ~TREXIO File~ object that has been created by a call to ~open~ function.

        Returns:
              True if the variable exists, False otherwise

        Raises:
            - trexio.Error if TREXIO return code ~rc~ is TREXIO_FAILURE and prints the error message using string_of_error.
            - Exception from some other error (e.g. RuntimeError).
        """

        rc = pytr.trexio_has_$group_str$(trexio_file.pytrexio_s)
        if rc == TREXIO_FAILURE:
            raise Error(rc)

        return rc == TREXIO_SUCCESS

### 3.9 Templates for front end delete an entire group (UNSAFE MODE)

#### 3.9.1 Introduction

This section concerns API calls related to string attributes.

Function name | Description
--- | ---
`trexio_delete_$group$` | Delete a given group from the TREXIO file

#### 3.9.2 C templates for front end

    trexio_exit_code
    trexio_delete_$group$ (trexio_t* const file)
    {

      if (file == NULL) return TREXIO_INVALID_ARG_1;
      if (file->mode != 'u') return TREXIO_SAFE_MODE;

      switch (file->back_end) {

      case TREXIO_TEXT:
        return trexio_text_delete_$group$(file);

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        return trexio_hdf5_delete_$group$(file);
    #else
        return TREXIO_BACK_END_MISSING;
    #endif
    /*
      case TREXIO_JSON:
        return trexio_json_delete_$group$(file);
        break;
    */
      }

      return TREXIO_FAILURE;
    }

#### 3.9.3 Fortran templates for front end

The `Fortran` templates that provide an access to the `C` API calls from Fortran. These templates are based on the use of `iso_c_binding`. Pointers have to be passed by value.

    interface
       integer(trexio_exit_code) function trexio_delete_$group$ (trex_file) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value  :: trex_file
       end function trexio_delete_$group$
    end interface

#### 3.9.4 Python templates for front end

    def delete_$group$(trexio_file) -> None:
        """Delete the entire $group$ group from the TREXIO file.

        Parameters:

        trexio_file:
            TREXIO File object.

        Raises:
            - Exception from AssertionError if TREXIO return code ~rc~ is different from TREXIO_SUCCESS and prints the error message using trexio_string_of_error.
            - Exception from some other error (e.g. RuntimeError).
        """

        rc = pytr.trexio_delete_$group$(trexio_file.pytrexio_s)
        if rc != TREXIO_SUCCESS:
            raise Error(rc)

## 4 Source code for the determinant part

Storage of the determinants is a particular case, which requires special treatment, but has to be coded only once (since there is only one group that corresponds to it). Thus, there is no need to auto-generate this part via templates.

This section concerns API calls related to Slater determinants.

Function name | Description
--- | ---
`trexio_has_determinant_list` | Check if an attribute exists in a file
`trexio_write_determinant_list` | Write an attribute
`trexio_read_determinant_list` | Read an attribute
`trexio_get_int64_num` | Get the number of int64 bit fields per determinant

#### 4.0.1 C source code

    trexio_exit_code
    trexio_get_int64_num(trexio_t* const file, int32_t* const num)
    {

      if (file  == NULL) return TREXIO_INVALID_ARG_1;
      if (num   == NULL) return TREXIO_INVALID_ARG_2;

      /* Read the number of mos */
      int64_t mo_num = 0L;
      trexio_exit_code rc = trexio_read_mo_num_64(file, &mo_num);
      if (rc != TREXIO_SUCCESS) return rc;
      if (mo_num == 0L) return TREXIO_INVALID_NUM;

      /* Compute how many integer numbers is needed to represent a determinant */
      int32_t int_num = 0;
      int_num = (mo_num - 1L)/64 + 1;

      *num = int_num;

      return TREXIO_SUCCESS;
    }

    trexio_exit_code
    trexio_read_determinant_list (trexio_t* const file, const int64_t offset_file, int64_t* const buffer_size_read, int64_t* const dset)
    {

      if (file  == NULL) return TREXIO_INVALID_ARG_1;
      if (dset  == NULL) return TREXIO_INVALID_ARG_2;
      if (trexio_has_determinant_list(file) != TREXIO_SUCCESS) return TREXIO_DSET_MISSING;

      /* Get the number of int bit fields per determinant */
      int32_t int_num = 0;
      trexio_exit_code rc = trexio_get_int64_num(file, &int_num);
      if (rc != TREXIO_SUCCESS) return rc;

      uint32_t rank = 2;
      uint64_t det_size = (uint64_t) (*buffer_size_read);
      uint64_t dims[2] = {det_size, int_num*2UL};

      // introduce a new variable which will be modified with the number of integrals being read if EOF is encountered
      int64_t eof_read_size = 0L;

      switch (file->back_end) {

      case TREXIO_TEXT:
        rc = trexio_text_read_determinant_list(file, offset_file, rank, dims, &eof_read_size, dset);
        break;

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        rc = trexio_hdf5_read_determinant_list(file, offset_file, rank, dims, &eof_read_size, dset);
        break;
    #else
        rc = TREXIO_BACK_END_MISSING;
        break;
    #endif
    /*
      case TREXIO_JSON:
        return trexio_json_read_$group_dset$(...);
        break;
    */
      default:
        rc = TREXIO_FAILURE;  /* Impossible case */
        break;
      }

      if (rc != TREXIO_SUCCESS && rc != TREXIO_END) return rc;

      if (rc == TREXIO_END) *buffer_size_read = eof_read_size;

      return rc;
    }

    trexio_exit_code
    trexio_read_safe_determinant_list (trexio_t* const file, const int64_t offset_file, int64_t* const buffer_size_read, int64_t* const dset_out, const int64_t dim_out)
    {
      return trexio_read_determinant_list(file, offset_file, buffer_size_read, dset_out);
    }

    trexio_exit_code
    trexio_write_determinant_list (trexio_t* const file, const int64_t offset_file, const int64_t buffer_size, const int64_t* dset)
    {

      if (file == NULL) return TREXIO_INVALID_ARG_1;
      if (dset == NULL) return TREXIO_INVALID_ARG_2;

      /* Get the number of int bit fields per determinant */
      int32_t int_num = 0;
      trexio_exit_code rc = trexio_get_int64_num(file, &int_num);
      if (rc != TREXIO_SUCCESS) return rc;

      uint32_t rank = 2;
      uint64_t dims[2] = {buffer_size, int_num*2UL};

      assert(file->back_end < TREXIO_INVALID_BACK_END);

      switch (file->back_end) {

      case TREXIO_TEXT:
        rc = trexio_text_write_determinant_list(file, offset_file, rank, dims, dset);
        break;

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        rc = trexio_hdf5_write_determinant_list(file, offset_file, rank, dims, dset);
        break;
    #else
        return TREXIO_BACK_END_MISSING;
        break;
    #endif
    /*
      case TREXIO_JSON:
        rc = trexio_json_read_
        break;
    */
      }

      if (rc != TREXIO_SUCCESS) return rc;

      // Update the determinant_num value with the number of determinants written
      int64_t det_num = 0L;
      // Read the determinant_num if it exists already
      if (trexio_has_determinant_num(file) == TREXIO_SUCCESS) {
         rc = trexio_read_determinant_num_64(file, &det_num);
         if (rc != TREXIO_SUCCESS) return rc;
      }
      // Check for the INT64 overflow before writing an updated value
      if (INT64_MAX - det_num > buffer_size) {
        det_num += buffer_size;
      } else {
        return TREXIO_INT_SIZE_OVERFLOW;
      }
      // Overwrite previous value. Here we have to temporarily set the file->mode to 'u' to trick the API
      //   in order to overwrite existing determinant_num. Otherwise the API returns TREXIO_NUM_ALREADY_EXISTS.
      char mode_tmp = file->mode;
      file->mode = 'u';
      rc = trexio_write_determinant_num_64(file, det_num);
      file->mode = mode_tmp;
      if (rc != TREXIO_SUCCESS) return rc;

      return TREXIO_SUCCESS;
    }

    trexio_exit_code
    trexio_write_safe_determinant_list (trexio_t* const file, const int64_t offset_file, const int64_t buffer_size, const int64_t* dset_in, const int64_t dim_in)
    {
      return trexio_write_determinant_list(file, offset_file, buffer_size, dset_in);
    }

    trexio_exit_code
    trexio_has_determinant_list (trexio_t* const file)
    {

      if (file == NULL) return TREXIO_INVALID_ARG_1;

      assert(file->back_end < TREXIO_INVALID_BACK_END);

      switch (file->back_end) {

      case TREXIO_TEXT:
        return trexio_text_has_determinant_list(file);

      case TREXIO_HDF5:
    #ifdef HAVE_HDF5
        return trexio_hdf5_has_determinant_list(file);
    #else
        return TREXIO_BACK_END_MISSING;
    #endif
    /*
      case TREXIO_JSON:
        return trexio_json_has_
        break;
    */
      }

      return TREXIO_FAILURE;
    }

#### 4.0.2 Fortran interface

The `Fortran` templates that provide an access to the `C` API calls from Fortran. These templates are based on the use of `iso_c_binding`. Pointers have to be passed by value.

    interface
       integer(trexio_exit_code) function trexio_write_determinant_list (trex_file, &
                                                   offset_file, buffer_size, list) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         integer(c_int64_t), intent(in), value :: offset_file
         integer(c_int64_t), intent(in), value :: buffer_size
         integer(c_int64_t), intent(in)        :: list(*)
       end function trexio_write_determinant_list
    end interface

    interface
       integer(trexio_exit_code) function trexio_write_safe_determinant_list (trex_file, &
                                                        offset_file, buffer_size, &
                                                        list, list_size) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         integer(c_int64_t), intent(in), value :: offset_file
         integer(c_int64_t), intent(in), value :: buffer_size
         integer(c_int64_t), intent(in)        :: list(*)
         integer(c_int64_t), intent(in), value :: list_size
       end function trexio_write_safe_determinant_list
    end interface

    interface
       integer(trexio_exit_code) function trexio_read_determinant_list(trex_file, &
                                                  offset_file, buffer_size, list) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         integer(c_int64_t), intent(in), value :: offset_file
         integer(c_int64_t), intent(inout)     :: buffer_size
         integer(c_int64_t), intent(out)       :: list(*)
       end function trexio_read_determinant_list
    end interface

    interface
       integer(trexio_exit_code) function trexio_read_safe_determinant_list (trex_file, &
                                                       offset_file, buffer_size, &
                                                       list, list_size) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         integer(c_int64_t), intent(in), value :: offset_file
         integer(c_int64_t), intent(inout)     :: buffer_size
         integer(c_int64_t), intent(out)       :: list(*)
         integer(c_int64_t), intent(in), value :: list_size
       end function trexio_read_safe_determinant_list
    end interface

    interface
       integer(trexio_exit_code) function trexio_has_determinant_list (trex_file) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
       end function trexio_has_determinant_list
    end interface

    interface
       integer(trexio_exit_code) function trexio_get_int64_num (trex_file, num) bind(C)
         use, intrinsic :: iso_c_binding
         import
         integer(trexio_t), intent(in), value :: trex_file
         integer(c_int32_t), intent(out) :: num
       end function trexio_get_int64_num
    end interface

#### 4.0.3 Python interface

    def write_determinant_list(trexio_file: File, offset_file: int, buffer_size: int, determinants: list) -> None:
        """Write the determinant list in the TREXIO file.

        Parameters:

        trexio_file:
            TREXIO File object.

        offset_file: int
            The number of determinants to be skipped in the file when writing.

        buffer_size: int
            The number of determinants to write in the file.

        determinants: list OR numpy.ndarray
            Array of determinant_list to be written. If array data type does not correspond to int64, the conversion is performed.

        Raises:
            - trexio.Error if TREXIO return code ~rc~ is different from TREXIO_SUCCESS and prints the error message.
            - Exception from some other error (e.g. RuntimeError).
        """

        if not isinstance(offset_file, int):
            raise TypeError("offset_file argument has to be an integer.")
        if not isinstance(buffer_size, int):
            raise TypeError("buffer_size argument has to be an integer.")
        if not isinstance(determinants, (list, tuple, np.ndarray)):
            raise TypeError("determinants argument has to be an array (list, tuple or NumPy ndarray).")

        convert = False
        flatten = False
        if isinstance(determinants, np.ndarray):
           # convert to int64 if input determinants are in a different precision
           if not determinants.dtype==np.int64:
               convert= True

           if len(determinants.shape) > 1:
               flatten = True
               if convert:
                   dets_64 = np.int64(determinants).flatten()
               else:
                   dets_64 = np.array(determinants, dtype=np.int64).flatten()
           else:
               if convert:
                   dets_64 = np.int64(determinants)
        else:
           # if input array is a multidimensional list or tuple, we have to convert it
           try:
               # if list is flat - the attempt to compute len() will raise a TypeError
               _ = len(determinants[0])
               dets_64 = np.array(determinants, dtype=np.int64).flatten()
               flatten = True
           except TypeError:
               pass

        if flatten or convert:
            rc = pytr.trexio_write_safe_determinant_list(trexio_file.pytrexio_s, offset_file, buffer_size, dets_64)
        else:
            rc = pytr.trexio_write_safe_determinant_list(trexio_file.pytrexio_s, offset_file, buffer_size, determinants)

        if rc != TREXIO_SUCCESS:
            raise Error(rc)

    def read_determinant_list(trexio_file: File, offset_file: int, buffer_size: int) -> tuple:
        """Read determinant_list from the TREXIO file.

        Parameters:

        trexio_file:
            TREXIO File object.

        offset_file: int
            The number of determinants to be skipped in the file when reading.

        buffer_size: int
            The number of determinants to read from the file.

        Returns:
            (determinants, n_int_read, eof_flag) tuple where
              - determinants are NumPy arrays [numpy.ndarray] with the default int64 precision;
              - n_int_read [int] is the number of determinants read from the trexio_file
                (either strictly equal to buffer_size or less than buffer_size if EOF has been reached);
              - eof_flag [bool] is True when EOF has been reached (i.e. when call to low-level pytrexio API returns TREXIO_END)
                                   False otherwise.

        Raises:
            - trexio.Error if TREXIO return code ~rc~ is different from TREXIO_SUCCESS and prints the error message.
            - Exception from some other error (e.g. RuntimeError).
        """

        if not isinstance(offset_file, int):
            raise TypeError("offset_file argument has to be an integer.")
        if not isinstance(buffer_size, int):
            raise TypeError("buffer_size argument has to be an integer.")

        # read the number of determinants already in the file
        det_num = read_determinant_num(trexio_file)
        # calculate the int_num (number of int bit fields per determinant)
        int_num = 2 * get_int64_num(trexio_file)

        # additional modification needed to avoid allocating more memory than needed if EOF will be reached during read
        overflow = offset_file + buffer_size - det_num
        eof_flag = False
        if overflow > 0:
            verified_size = buffer_size - overflow
            eof_flag = True
        else:
            verified_size = buffer_size

        # main call to the low-level (SWIG-wrapped) trexio_read function, which also requires the sizes of the output to be provided
        # read_buf_size contains the number of elements being read from the file, useful when EOF has been reached
        rc, n_int_read, determinants = pytr.trexio_read_safe_determinant_list(trexio_file.pytrexio_s,
                                                                              offset_file,
                                                                              verified_size,
                                                                              verified_size * int_num)
        if rc != TREXIO_SUCCESS:
            raise Error(rc)
        if n_int_read == 0:
            raise ValueError("No integrals have been read from the file.")
        if determinants is None:
            raise ValueError("Returned NULL array from the low-level pytrexio API.")

        # conversion to custom types can be performed on the user side, here we only reshape the returned flat array according to int_num
        dets_reshaped = np.reshape(determinants, (verified_size, int_num), order='C')

        return (dets_reshaped, n_int_read, eof_flag)

    def get_int64_num(trexio_file) -> int:
        """Compute the number of int64 bit fields corresponding to the TREXIO file.

        Parameter is a ~TREXIO File~ object that has been created by a call to ~open~ function.

        Returns:
            ~num~: int
            Number of int64 bit fields per determinant.

        Raises:
            - Exception from AssertionError if TREXIO return code ~rc~ is different from TREXIO_SUCCESS and prints the error message using trexio_string_of_error.
            - Exception from some other error (e.g. RuntimeError).
        """

        rc, num = pytr.trexio_get_int64_num(trexio_file.pytrexio_s)
        if rc != TREXIO_SUCCESS:
            raise Error(rc)

        return num

    def has_determinant_list(trexio_file) -> bool:
        """Check that determinant_list exists in the TREXIO file.

        Parameter is a ~TREXIO File~ object that has been created by a call to ~open~ function.

        Returns:
              True if the variable exists, False otherwise

        Raises:
            - trexio.Error if TREXIO return code ~rc~ is TREXIO_FAILURE and prints the error message using string_of_error.
            - Exception from some other error (e.g. RuntimeError).
        """

        rc = pytr.trexio_has_determinant_list(trexio_file.pytrexio_s)
        if rc == TREXIO_FAILURE:
            raise Error(rc)

        return rc == TREXIO_SUCCESS

## 5 General helper functions

This section contains general helper functions like `trexio_info`.

`trexio_info` prints information about the TREXIO configuration (see `config.h` file). In particular:

1. `TREXIO_PACKAGE_VERSION` [string]
2. `HAVE_HDF5` [bool]
3. `HDF5_VERSION` [string] (optional, only if `HAVE_HDF5` is `true`)
4. `TREXIO_GIT_HASH` [string]

`trexio_mark_safety` checks if the file has been open in UNSAFE mode. If it was, the `metadata_unsafe` attribute can be overwritten with the value provided in a second argument of the function. Since `metadata_unsafe` is set to `1` (`true`) upon the first opening of the file in UNSAFE mode, this value is immutable. However, if the user validated that the file is correct (e.g. using `trexio-tools`), then value of the `metadata_unsafe` attribute can be changed using the aforementioned function.

`trexio_to_orbital_list` function converts the list of integer bit fields of a given determinant into a list of indices of the occupied orbitals (for one spin component). `trexio_to_orbital_list_up_dn` function does the same but for both up- and down-spin components of the determinant and returns two list of orbitals each corresponding to a different component.

`trexio_to_bitfield_list` function converts the list of occupied orbitals (up- or down-spin) into the corresponding `int64_t` bitfield representation of the determinant. If the creation of the bitfield requires a change of sign, the return code is `TREXIO_PHASE_CHANGE`.

### 5.1 C

    trexio_exit_code trexio_info(void);
    trexio_exit_code trexio_mark_safety(trexio_t* const file, const int32_t safety_flag);

    typedef int64_t  bitfield_t;

    #define TREXIO_ORBITAL_SHIFT      1
    #define TREXIO_INT_SIZE           64
    #define TREXIO_NORB_PER_INT       ( 8*sizeof(bitfield_t) )
    #define TREXIO_NORB_PER_INT_SHIFT ( trailz( TREXIO_NORB_PER_INT ) )

    trexio_exit_code trexio_to_orbital_list (const int32_t N_int, const bitfield_t* d1, int32_t* const list, int32_t* const occupied_num);
    trexio_exit_code trexio_to_orbital_list_up_dn (const int32_t N_int, const bitfield_t* d1, int32_t* const list_up, int32_t* const list_dn, int32_t* const occ_num_up, int32_t* const occ_num_dn);
    trexio_exit_code trexio_safe_to_orbital_list (const int32_t N_int, const bitfield_t* dset_in, const int64_t dim_in, int32_t* const dset_out, const int64_t dim_out, int32_t* const num);
    trexio_exit_code trexio_safe_to_orbital_list_up_dn (const int32_t N_int, const bitfield_t* dset_in, const int64_t dim_in, int32_t* const dset_up_out, const int64_t dim_up_out, int32_t* const dset_dn_out, const int64_t dim_dn_out, int32_t* const num_up, int32_t* const num_dn);
    trexio_exit_code trexio_to_bitfield_list (const int32_t* orb_list, const int32_t occupied_num, bitfield_t* const bit_list, const int32_t N_int);

    trexio_exit_code trexio_to_bitfield_list (const int32_t* orb_list,
                                              const int32_t occupied_num,
                                              bitfield_t* const bit_list,
                                              const int32_t N_int)
    {
      if (orb_list == NULL)  return TREXIO_INVALID_ARG_1;
      if (occupied_num <= 0) return TREXIO_INVALID_ARG_2;
      if (bit_list == NULL)  return TREXIO_INVALID_ARG_3;
      if (N_int <= 0)        return TREXIO_INVALID_ARG_4;

      for (int32_t j = 0 ; j < N_int ; j++) {
          bit_list[j] = (bitfield_t) 0;
      }

      uint32_t nswaps = 0;

      for (int32_t pos = 0 ; pos < occupied_num ; pos++) {
        const uint32_t iorb = ((uint32_t) (orb_list[pos] + 1)) - TREXIO_ORBITAL_SHIFT;

        // Set the bit of to one
        const uint32_t i = (uint32_t) (iorb >> TREXIO_NORB_PER_INT_SHIFT);
        const uint32_t k = (uint32_t) (iorb & (TREXIO_NORB_PER_INT - 1) );
        bitfield_t mask = ((bitfield_t) 1) << k;
        bit_list[i] |= mask;

        // Check for phase changes
        mask = ~(mask - (bitfield_t) 1);
        nswaps += popcnt(mask & bit_list[i]) - 1;
        for (int j=i+1 ; j < N_int ; ++j) {
          if (bit_list[j] != (bitfield_t) 0)
            nswaps += popcnt(bit_list[j]);
        }
      }

      if ( (nswaps & 1) == 0)
        return TREXIO_SUCCESS;
      else
        return TREXIO_PHASE_CHANGE;
    }

    trexio_exit_code trexio_to_orbital_list(const int32_t N_int,
                                            const bitfield_t* d1,
                                            int32_t* const list,
                                            int32_t* const occupied_num)
    {
      if (N_int <= 0) return TREXIO_INVALID_ARG_1;
      if (d1 == NULL) return TREXIO_INVALID_ARG_2;
      if (list == NULL) return TREXIO_INVALID_ARG_3;
      if (occupied_num == NULL) return TREXIO_INVALID_ARG_4;

      int32_t k = 0;
      int32_t shift = TREXIO_ORBITAL_SHIFT;

      for (int32_t i=0 ; i<N_int ; i++)
      {
          bitfield_t tmp = d1[i];

          while (tmp != (bitfield_t) 0)
          {
              const int32_t pos = trailz(tmp);
              if (pos < 0) return TREXIO_FAILURE;

              list[k] = ( (int32_t) pos) + shift - 1;
              tmp ^= ( ((bitfield_t) 1) << pos);
              k++;
          }
          shift += TREXIO_NORB_PER_INT;
      }

      *occupied_num = (int32_t) k;
      return TREXIO_SUCCESS;
    }

    trexio_exit_code trexio_to_orbital_list_up_dn(const int32_t N_int,
                                                  const bitfield_t* d1,
                                                  int32_t* const list_up,
                                                  int32_t* const list_dn,
                                                  int32_t* const occ_num_up,
                                                  int32_t* const occ_num_dn)
    {
      if (N_int <= 0) return TREXIO_INVALID_ARG_1;
      if (d1 == NULL) return TREXIO_INVALID_ARG_2;
      if (list_up == NULL) return TREXIO_INVALID_ARG_3;
      if (list_dn == NULL) return TREXIO_INVALID_ARG_4;
      if (occ_num_up == NULL) return TREXIO_INVALID_ARG_5;
      if (occ_num_dn == NULL) return TREXIO_INVALID_ARG_6;

      trexio_exit_code rc;

      /* First process up-spin electrons */
      rc = trexio_to_orbital_list(N_int, &d1[0], list_up, occ_num_up);
      if (rc != TREXIO_SUCCESS) return rc;

      /* Now process down-spin electrons */
      rc = trexio_to_orbital_list(N_int, &d1[N_int], list_dn, occ_num_dn);
      if (rc != TREXIO_SUCCESS) return rc;

      return TREXIO_SUCCESS;
    }

    trexio_exit_code
    trexio_safe_to_orbital_list (const int32_t N_int,
                                 const bitfield_t* dset_in,
                                 const int64_t dim_in,
                                 int32_t* const dset_out,
                                 const int64_t dim_out,
                                 int32_t* const num)
    {
      return trexio_to_orbital_list(N_int, dset_in, dset_out, num);
    }

    trexio_exit_code
    trexio_safe_to_orbital_list_up_dn (const int32_t N_int,
                                       const bitfield_t* dset_in,
                                       const int64_t dim_in,
                                       int32_t* const dset_up_out,
                                       const int64_t dim_up_out,
                                       int32_t* const dset_dn_out,
                                       const int64_t dim_dn_out,
                                       int32_t* const num_up,
                                       int32_t* const num_dn)
    {
      return trexio_to_orbital_list_up_dn(N_int, dset_in, dset_up_out, dset_dn_out, num_up, num_dn);
    }

    /* Popcount and trailz */
    #if TREXIO_INT_SIZE == 64

      extern int __builtin_popcountll (unsigned long long x_0);
      #define popcnt(X) __builtin_popcountll((unsigned long long) X)

      extern int __builtin_ctzll (unsigned long long x_0);
      #define trailz(X) __builtin_ctzll((unsigned long long) X)

    #elif TREXIO_INT_SIZE == 32

      extern int __builtin_popcountl (unsigned long x_0);
      #define popcnt(X) __builtin_popcountl((unsigned long) X)

      extern int __builtin_ctzl(unsigned long x_0);
      #define trailz(X) __builtin_ctzl((unsigned long) X)

    #elif TREXIO_INT_SIZE == 16

      extern int __builtin_popcount (unsigned int x_0);
      #define popcnt(X) __builtin_popcount((unsigned int) X)

      extern int __builtin_ctz (unsigned int x_0);
      #define trailz(X) __builtin_ctz((unsigned int) X)

    #else

     #error("Invalid TREXIO_INT_SIZE")

    #endif

    trexio_exit_code
    trexio_info (void)
    {
      printf("TREXIO_PACKAGE_VERSION : %s\n", TREXIO_PACKAGE_VERSION);

    #ifdef TREXIO_GIT_HASH
      printf("TREXIO_GIT_HASH        : %s\n", TREXIO_GIT_HASH);
    #else
      printf("GIT_HASH is stored in the config.h file, which is missing.");
    #endif

    #ifdef HAVE_HDF5
      printf("HAVE_HDF5              : true\n");
      printf("%s\n", H5_VERS_INFO);
    #else
      printf("HAVE_HDF5              : false\n");
      printf("TREXIO configured without the HDF5 library\n");
    #endif

      return TREXIO_SUCCESS;
    }

    trexio_exit_code
    trexio_mark_safety (trexio_t* const file, const int32_t safety_flag)
    {

     if (file == NULL) return TREXIO_INVALID_ARG_1;
     /* 1 for true ; 0 for false */
     if (safety_flag != 0 && safety_flag != 1) return TREXIO_INVALID_ARG_2;
     /* Cannot mark the file in safe mode */
     if (file->mode != 'u') return TREXIO_FAILURE;

     return trexio_write_metadata_unsafe(file, safety_flag);
    }

### 5.2 Fortran

    interface
       integer function trexio_info () bind(C)
         use, intrinsic :: iso_c_binding
       end function trexio_info
    end interface

    interface
       integer(trexio_exit_code) function trexio_to_bitfield_list_c(list, occupied_num, det_list, N_int) &
       bind(C, name="trexio_to_bitfield_list")
         use, intrinsic :: iso_c_binding
         import
         integer(c_int32_t), intent(in)        :: list(*)
         integer(c_int32_t), intent(in), value :: occupied_num
         integer(c_int64_t), intent(inout)     :: det_list(*)
         integer(c_int32_t), intent(in), value :: N_int
       end function trexio_to_bitfield_list_c
    end interface

    interface
       integer(trexio_exit_code) function trexio_to_orbital_list_c(N_int, d1, list, occupied_num) &
       bind(C, name="trexio_to_orbital_list")
         use, intrinsic :: iso_c_binding
         import
         integer(c_int32_t), intent(in), value :: N_int
         integer(c_int64_t), intent(in)        :: d1(*)
         integer(c_int32_t), intent(inout)     :: list(*)
         integer(c_int32_t), intent(inout)     :: occupied_num
       end function trexio_to_orbital_list_c
    end interface

    interface
       integer(trexio_exit_code) function trexio_to_orbital_list_up_dn_c(N_int, d1, list_up, list_dn, occ_num_up, occ_num_dn) &
       bind(C, name="trexio_to_orbital_list_up_dn")
         use, intrinsic :: iso_c_binding
         import
         integer(c_int32_t), intent(in), value :: N_int
         integer(c_int64_t), intent(in)        :: d1(*)
         integer(c_int32_t), intent(inout)     :: list_up(*)
         integer(c_int32_t), intent(inout)     :: list_dn(*)
         integer(c_int32_t), intent(inout)     :: occ_num_up
         integer(c_int32_t), intent(inout)     :: occ_num_dn
       end function trexio_to_orbital_list_up_dn_c
    end interface

### 5.3 Python

    def info():
        """Print the info about the installed TREXIO library."""

        rc = pytr.trexio_info()
        if rc != TREXIO_SUCCESS:
            raise Error(rc)

    def to_bitfield_list(n_int: int, orbitals: list) -> list:
        """Convert a list of occupied orbitals into a bitfield determinant.

        Input:
            ~orbitals~ - list of occupied orbital indices fields (integers)
            ~n_int~ - number of bitfields per determinant of a given spin

        Returns:
            ~bitfield_list~: list

        Raises:
            - Exception from AssertionError if TREXIO return code ~rc~ is different from TREXIO_SUCCESS and prints the error message using trexio_string_of_error.
            - Exception from some other error (e.g. RuntimeError).
        """

        rc, bitfield_list = pytr.trexio_to_bitfield_list(orbitals, n_int)
        if rc != TREXIO_SUCCESS:
            raise Error(rc)
        if len(bitfield_list) != n_int:
            raise Exception("Inconsistent size of the bitfield_list.")

        return bitfield_list

    def to_orbital_list(n_int: int, determinant: list) -> list:
        """Convert a given determinant into a list of occupied orbitals.

        Input:
            ~determinant~ - list of bit fields (integers)
            ~n_int~ - number of bit fields per determinant of a given spin

        Returns:
            ~orbital_list~: list

        Raises:
            - Exception from AssertionError if TREXIO return code ~rc~ is different from TREXIO_SUCCESS and prints the error message using trexio_string_of_error.
            - Exception from some other error (e.g. RuntimeError).
        """

        # max possible size of the orbital list per spin component (upper limit on the number of MOs)
        size_max = n_int * 64

        rc, orbital_list, occ_num = pytr.trexio_safe_to_orbital_list(n_int, determinant, size_max)
        if rc != TREXIO_SUCCESS:
            raise Error(rc)
        if len(orbital_list) < occ_num:
            raise Exception("Inconsistent size of the orbital_list.")

        return orbital_list[0:occ_num]

    def to_orbital_list_up_dn(n_int: int, determinant: list) -> tuple:
        """Convert a given determinant into two lists of occupied orbitals.

        Input:
            ~determinant~ - list of bit fields (integers)
            ~n_int~ - number of bit fields per determinant of a given spin

        Returns:
            result: tuple with the following items:
              ~orbital_list_up~: list of orbitals occupied by up-spin electrons
              ~orbital_list_dn~: list of orbitals occupied by down-spin electrons

        Raises:
            - Exception from AssertionError if TREXIO return code ~rc~ is different from TREXIO_SUCCESS and prints the error message using trexio_string_of_error.
            - Exception from some other error (e.g. RuntimeError).
        """

        # max possible size of the orbital list per spin component (upper limit on the number of MOs)
        size_max = n_int * 64

        rc, orbital_list_up, orbital_list_dn, occ_num_up, occ_num_dn = pytr.trexio_safe_to_orbital_list_up_dn(n_int, determinant, size_max, size_max)
        if rc != TREXIO_SUCCESS:
            raise Error(rc)
        if len(orbital_list_up) < occ_num_up:
            raise Exception("Inconsistent size of the orbital_list for up-spin electrons.")
        if len(orbital_list_dn) < occ_num_dn:
            raise Exception("Inconsistent size of the orbital_list for down-spin electrons.")

        return (orbital_list_up[0:occ_num_up], orbital_list_dn[0:occ_num_dn])

## 6 Fortran helper/wrapper functions

The function below adapts the original C-based `trexio_open` for Fortran. This is needed due to the fact that strings in C are terminated by `NULL` character `\0` unlike strings in Fortran. Note, that Fortran interface calls the main `TREXIO` API, which is written in C.

    contains
       integer(trexio_t) function trexio_open (filename, mode, back_end, rc_open)
         use, intrinsic :: iso_c_binding, only : c_null_char
         implicit none
         character(len=*), intent(in)                    :: filename
         character, intent(in), value                    :: mode
         integer(trexio_back_end_t), intent(in), value   :: back_end
         integer(trexio_exit_code), intent(out)          :: rc_open
         character(len=len_trim(filename)+1)             :: filename_c
         integer(trexio_exit_code) :: rc

         filename_c = trim(filename) // c_null_char
         trexio_open = trexio_open_c(filename_c, mode, back_end, rc_open)
         if (trexio_open == 0_8 .or. rc_open /= TREXIO_SUCCESS) then
           return
         endif
         rc = trexio_set_one_based(trexio_open)
         if (rc /= TREXIO_SUCCESS) then
            rc = trexio_close(trexio_open)
            trexio_open = 0_8
         endif
       end function trexio_open

The function below adapts the original C-based `trexio_inquire` for Fortran. This is needed due to the same reasons as for `trexio_open` function. Note, that Fortran interface calls the main `TREXIO` API, which is written in C.

    integer(trexio_exit_code) function trexio_inquire (filename)
      use, intrinsic :: iso_c_binding
      implicit none
      character(len=*), intent(in)        :: filename
      character(len=len_trim(filename)+1) :: filename_c

      filename_c = trim(filename) // c_null_char
      trexio_inquire = trexio_inquire_c(filename_c)
    end function trexio_inquire

Similarly, the following function adapts `trexio_cp`.

    integer(trexio_exit_code) function trexio_cp (source, destination)
      use, intrinsic :: iso_c_binding
      implicit none
      character(len=*), intent(in)           :: source
      character(len=*), intent(in)           :: destination
      character(len=len_trim(source)+1)      :: source_c
      character(len=len_trim(destination)+1) :: destination_c

      source_c = trim(source) // c_null_char
      destination_c = trim(destination) // c_null_char
      trexio_cp = trexio_cp_c(source_c, destination_c)
    end function trexio_cp

The subroutines below wrap the `to_orbital_list` functions to shift the MO indices by 1 since in Fortran arrays are 1-based and C/Python they are 0-based.

    integer(trexio_exit_code) function trexio_to_bitfield_list(list, occupied_num, det_list, N_int)
      use, intrinsic :: iso_c_binding
      implicit none

      integer(c_int32_t), intent(in)        :: list(*)
      integer(c_int32_t), intent(in), value :: occupied_num
      integer(c_int64_t), intent(out)       :: det_list(*)
      integer(c_int32_t), intent(in), value :: N_int
      integer(c_int32_t)                    :: list_0based(occupied_num)

      integer :: i
      do i = 1,occupied_num
        list_0based(i) = list(i) - 1
      enddo

      trexio_to_bitfield_list = trexio_to_bitfield_list_c(list_0based, occupied_num, det_list, N_int)
      if (trexio_to_bitfield_list /= TREXIO_SUCCESS) then
        return
      endif

    end function trexio_to_bitfield_list

    integer(trexio_exit_code) function trexio_to_orbital_list(N_int, d1, list, occupied_num)
      use, intrinsic :: iso_c_binding
      implicit none

      integer(c_int32_t), intent(in), value :: N_int
      integer(c_int64_t), intent(in)        :: d1(*)
      integer(c_int32_t), intent(out)       :: list(*)
      integer(c_int32_t), intent(out)       :: occupied_num

      integer :: i

      trexio_to_orbital_list = trexio_to_orbital_list_c(N_int, d1, list, occupied_num)
      if (trexio_to_orbital_list /= TREXIO_SUCCESS) then
        return
      endif

      do i = 1,occupied_num
        list(i) = list(i) + 1
      enddo
    end function trexio_to_orbital_list

    integer(trexio_exit_code) function trexio_to_orbital_list_up_dn(N_int, d1, list_up, list_dn, occ_num_up, occ_num_dn)
      use, intrinsic :: iso_c_binding
      implicit none
      integer(c_int32_t), intent(in), value :: N_int
      integer(c_int64_t), intent(in)        :: d1(*)
      integer(c_int32_t), intent(out)       :: list_up(*)
      integer(c_int32_t), intent(out)       :: list_dn(*)
      integer(c_int32_t), intent(out)       :: occ_num_up
      integer(c_int32_t), intent(out)       :: occ_num_dn

      integer :: i

      trexio_to_orbital_list_up_dn = trexio_to_orbital_list_up_dn_c(N_int, d1, list_up, list_dn, occ_num_up, occ_num_dn)
      if (trexio_to_orbital_list_up_dn /= TREXIO_SUCCESS) then
        return
      endif

      do i = 1,occ_num_up
        list_up(i) = list_up(i) + 1
      enddo
      do i = 1,occ_num_dn
        list_dn(i) = list_dn(i) + 1
      enddo
    end function trexio_to_orbital_list_up_dn

The subroutine below transforms an array of Fortran strings into one big string using `TREXIO_DELIM` symbol as a delimeter and adds `NULL` character in the end in order to properly pass the desired string to C API. This is needed due to the fact that strings in C are terminated by `NULL` character `\0`.

    subroutine trexio_strarray2str(str_array, max_num_str, max_len_str, str_res)
      use, intrinsic :: iso_c_binding, only : c_null_char
      implicit none

      integer(c_int64_t), intent(in), value   :: max_num_str  ! number of elements in strign array
      integer, intent(in), value   :: max_len_str  ! maximum length of a string in an array
      character(len=*), intent(in)  :: str_array(*)
      character(len=:), allocatable, intent(out) :: str_res
      integer(c_int64_t) :: i

      str_res = ''
      do i = 1, max_num_str
        str_res = str_res // trim(str_array(i)) // TREXIO_DELIM
      enddo
      str_res = str_res // c_null_char

    end subroutine trexio_strarray2str

The subroutine below does the reverse tranformation from one big string with delimeters into an array of Fortran strings.

    subroutine trexio_str2strarray(str_flat, max_num_str, max_len_str, str_array)
      implicit none

      integer(c_int64_t), intent(in), value   :: max_num_str  ! number of elements in strign array
      integer, intent(in), value              :: max_len_str  ! maximum length of a string in an array
      character(kind=c_char), intent(in)      :: str_flat(*)
      character(len=*), intent(inout)         :: str_array(*)

      character(len=max_len_str)  :: tmp_str
      integer(c_int64_t) :: i, j, k, ind, len_flat

      len_flat = (max_len_str+1)*max_num_str + 1

      ind=1
      do i=1,max_num_str
        k = 1
        tmp_str=''
        do j=ind,len_flat
          if (str_flat(j) == TREXIO_DELIM) then
            ind=j+1
            exit
          endif
          tmp_str(k:k) = str_flat(j)
          k = k + 1
        enddo
        str_array(i)=tmp_str
      enddo

    end subroutine trexio_str2strarray

The subroutine is a Fortran analogue of `assert` in C. It check that the the return code of the TREXIO API call is equal to a given return code. It can optionally print a success message if the two code are identical, i.e. if the `assert` statement pass.

    subroutine trexio_assert(trexio_rc, check_rc, success_message)
      implicit none

      integer, intent(in), value :: trexio_rc
      integer, intent(in), value :: check_rc
      character(len=*), intent(in), optional  :: success_message

      character*(128) :: str

      if (trexio_rc == check_rc) then
        if (present(success_message)) write(*,*) success_message
      else
        call trexio_string_of_error(trexio_rc, str)
        print *, trim(str)
        stop 1
      endif

    end subroutine trexio_assert

Author: TREX-CoE

Created: 2023-04-26 Wed 08:13

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