(** Storage for four-index data (integrals, density matrices, ...).

There are two kinds of ordering of indices:

- Physicist's : {% $\langle i j | k l \rangle$ %}
- Chemist's   : {% $(ij|kl)$ %}

*)

type t

val create : size:int -> [< `Dense | `Sparse ] -> t
(** If [`Dense] is chosen, internally the data is stored as a 4-dimensional
    [Bigarray]. Else, it is stored as a hash table.
*)

(** {2 Accessors} *)
val get_chem : t -> int -> int -> int -> int -> float
val get_phys : t -> int -> int -> int -> int -> float
val set_chem : t -> int -> int -> int -> int -> float -> unit
val set_phys : t -> int -> int -> int -> int -> float -> unit

(** {2 I/O} *)
val to_file : ?cutoff:float -> filename:string -> t -> unit
(** Write the data to file, using the physicist's ordering. *)