* JSON
      "ao_2e_int" : {
        "eri_num"            : [ "int", [  ] ]
        "eri"                : [ "float sparse", [ "ao.num", "ao.num", "ao.num", "ao.num" ] ]
    },
    
* Front end

  For example, the integral $\langle ij | kl \rangle = x$ can be
  represented represented as
  - a quartet of integers $(i,j,k,l)$
  - a floating point value $x$

  To store $N$ integrals in the file, we can store
  - An array of quartets of integers
  - An array of values (floats)

  These two arrays have the same size, $N$.

  As the number of integrals to store can be prohibitively large, it
  is important to be able to read/write the integrals in chunks. So we
  need to give two extra parameters to the functions:
  - ~offset~ : the index of the 1st integral we want to read. An
    offset of zero implies to read the first integral
  - ~num~ : the number of integrals to read

  We need to provide one function to read a chunk of indices, and one
  function to read a chunk of values, because some users might want to
  read only the values of the integrals, or only the indices.

  Here is an example for the indices:

#+BEGIN_SRC c
trexio_exit_code
trexio_read_chunk_ao_2e_int_eri_index_32(trexio_t* const file,
                                         const int64_t offset,
                                         const int64_t num,
                                         int32_t* buffer)
{
  if (file  == NULL) return TREXIO_INVALID_ARG_1;
  if (offset   < 0L) return TREXIO_INVALID_ARG_2;
  if (num      < 0L) return TREXIO_INVALID_ARG_3;

  const uint32_t rank = 4;  // To be set by generator : number of indices

  int64_t nmax;             // Max number of integrals
  trexio_exit_code rc;

  rc = trexio_read_ao_2e_int_eri_num(const file, &nmax);
  if (rc != TREXIO_SUCCESS) return rc;

  switch (file->back_end) {

  case TREXIO_TEXT:
    return trexio_text_read_chunk_ao_2e_int_eri_index(file, buffer, offset, num, rank, nmax);
    break;

  case TREXIO_HDF5:
    return trexio_hdf5_read_chunk_ao_2e_int_eri_index(file, buffer, offset, num, rank, nmax);
    break;
/*
  case TREXIO_JSON:
    return trexio_json_read_chunk_ao_2e_int_eri_index(file, buffer, offset, num, rank, nmax);
    break;
,*/
  default:
    return TREXIO_FAILURE;  /* Impossible case */
  }
}
#+END_SRC

For the values, 

#+BEGIN_SRC c
trexio_exit_code
trexio_read_chunk_ao_2e_int_eri_value_64(trexio_t* const file,
                                         const int64_t offset,
                                         const int64_t num,
                                         double* buffer)
{
  if (file  == NULL) return TREXIO_INVALID_ARG_1;
  if (offset   < 0L) return TREXIO_INVALID_ARG_2;
  if (num      < 0L) return TREXIO_INVALID_ARG_3;

  int64_t nmax;             // Max number of integrals
  trexio_exit_code rc;

  rc = trexio_read_ao_2e_int_eri_num(const file, &nmax);
  if (rc != TREXIO_SUCCESS) return rc;

  switch (file->back_end) {

  case TREXIO_TEXT:
    return trexio_text_read_chunk_ao_2e_int_eri_value(file, buffer, offset, num, nmax);
    break;

  case TREXIO_HDF5:
    return trexio_hdf5_read_chunk_ao_2e_int_eri_index(file, buffer, offset, num, nmax);
    break;
/*
  case TREXIO_JSON:
    return trexio_json_read_chunk_ao_2e_int_eri_index(file, buffer, offset, num, nmax);
    break;
,*/
  default:
    return TREXIO_FAILURE;  /* Impossible case */
  }
}
#+END_SRC

* Text back end
  As the size of the dataset should be extensible, the simplest
  solution is to use one file for each sparse data set, and store a
  the name of this file in the group.
  Each integral can be a line in the file:
  i  j  k  l  x
  which can be read with "%10ld %10ld %10ld %10ld %24.16e".
  The offset can be used with ~fseek(69L*offset, SEEK_SET)~
  
* HDF5 Back end

  We need to declare the number of rows of the dataset as
  ~UNLIMITED~. This requires to use the ~Chunked~ storage, and the
  chunks should absolutely not be larger than 1MB.

  To extend the storage, see :
  https://support.hdfgroup.org/HDF5/doc1.6/UG/10_Datasets.html
  (figure 17)

  If the offset+num > nmax, we need to extend the dataset.