let max_index = 1 lsl 14

type index_pair = { first : int ; second : int }


type storage_t = 
  | Dense  of (float, Bigarray.float64_elt, Bigarray.fortran_layout) Bigarray.Array2.t
  | Sparse of (int, float) Hashtbl.t

type t =
  {
    size         : int ;
    four_index   : storage_t ;
  }

let key_of_indices ~r1 ~r2 =
  let { first=i ; second=k } = r1 and { first=j ; second=l } = r2 in
  let f i k = 
    let p, r = 
      if i <= k then i, k else k, i
    in p+ (r*r-r)/2
  in
  let p = f i k and q = f j l in
  f p q


let dense_index i j size =
  (j-1)*size + i


let get_four_index ~r1 ~r2 t = 
  match t.four_index with
  | Dense  a -> (let { first=i ; second=k } = r1 and { first=j ; second=l } = r2 in 
                 let size = t.size in
                 assert ( (i lor j lor k lor l) > 0 );
                 assert ( i <= size && j <= size && k <= size && l <= size );
                 Bigarray.Array2.unsafe_get a (dense_index i j size) (dense_index k l size)
                )
  | Sparse a -> let key = key_of_indices ~r1 ~r2 in
                try Hashtbl.find a key
                with Not_found -> 0.


let set_four_index ~r1 ~r2 ~value t = 
  match t.four_index with
  | Dense  a -> (let { first=i ; second=k } = r1 and { first=j ; second=l } = r2 in
                 let size = t.size in
                 assert ( (i lor j lor k lor l) > 0 );
                 assert ( i <= size && j <= size && k <= size && l <= size);
                 let ij = (dense_index i j size)
                 and kl = (dense_index k l size)
                 and il = (dense_index i l size)
                 and kj = (dense_index k j size)
                 and ji = (dense_index j i size)
                 and lk = (dense_index l k size)
                 and li = (dense_index l i size)
                 and jk = (dense_index j k size)
                 in
                 let open Bigarray.Array2 in
                   unsafe_set a ij kl value;
                   unsafe_set a kj il value;
                   unsafe_set a il kj value;
                   unsafe_set a kl ij value;
                   unsafe_set a ji lk value;
                   unsafe_set a li jk value;
                   unsafe_set a jk li value;
                   unsafe_set a lk ji value
                )

  | Sparse a -> let key = key_of_indices ~r1 ~r2 in
                Hashtbl.replace a key value
    

let increment_four_index ~r1 ~r2 ~value t = 
  match t.four_index with
  | Dense  a -> (let { first=i ; second=k } = r1 and { first=j ; second=l } = r2 in
                 let size = t.size in
                 assert ( (i lor j lor k lor l) > 0 );
                 assert ( i <= size && j <= size && k <= size && l <= size);
                 let ij = (dense_index i j size)
                 and kl = (dense_index k l size)
                 and il = (dense_index i l size)
                 and kj = (dense_index k j size)
                 and ji = (dense_index j i size)
                 and lk = (dense_index l k size)
                 and li = (dense_index l i size)
                 and jk = (dense_index j k size)
                 in
                 let open Bigarray.Array2 in
                   unsafe_set a ij kl (value +. unsafe_get a ij kl) ;
                   unsafe_set a kj il (value +. unsafe_get a kj il) ;
                   unsafe_set a il kj (value +. unsafe_get a il kj) ;
                   unsafe_set a kl ij (value +. unsafe_get a kl ij) ;
                   unsafe_set a ji lk (value +. unsafe_get a ji lk) ;
                   unsafe_set a li jk (value +. unsafe_get a li jk) ;
                   unsafe_set a jk li (value +. unsafe_get a jk li) ;
                   unsafe_set a lk ji (value +. unsafe_get a lk ji) 
                )

  | Sparse a -> let key = key_of_indices ~r1 ~r2 in
                let old_value =
                  try Hashtbl.find a key
                  with Not_found -> 0.
                in
                Hashtbl.replace a key (old_value +. value)

let get ~r1 ~r2 a = 
  get_four_index ~r1 ~r2 a

let set ~r1 ~r2 ~value = 
  match classify_float value with
  | FP_normal -> set_four_index ~r1 ~r2 ~value
  | FP_zero    
  | FP_subnormal -> fun _ -> ()
  | FP_infinite
  | FP_nan ->
    let msg =
      Printf.sprintf "FourIdxStorage.ml : set : r1 = (%d,%d) ; r2 = (%d,%d)"
        r1.first r1.second r2.first r2.second
    in
    raise (Invalid_argument msg)

let increment ~r1 ~r2 = 
  increment_four_index ~r1 ~r2 

let create ~size sparsity =
  assert (size < max_index);
  let four_index = 
    match sparsity with
    | `Dense ->
      let result = 
        Bigarray.Array2.create Float64 Bigarray.fortran_layout (size*size) (size*size)
      in
      Bigarray.Array2.fill result 0.;
      Dense result

    | `Sparse -> 
      let result = Hashtbl.create (size*size+13) in
      Sparse result
  in
  { size ; four_index }


let size t = t.size

(** TODO : remove epsilons *)


let get_chem t i j k l = get ~r1:{ first=i ; second=j } ~r2:{ first=k ; second=l } t
let get_phys t i j k l = get ~r1:{ first=i ; second=k } ~r2:{ first=j ; second=l } t

let set_chem t i j k l value = set ~r1:{ first=i ; second=j } ~r2:{ first=k ; second=l } ~value t
let set_phys t i j k l value = set ~r1:{ first=i ; second=k } ~r2:{ first=j ; second=l } ~value t 


(** Write all integrals to a file with the <ij|kl> convention *)
let to_file ?(cutoff=Constants.epsilon) ~filename data =
  let oc = open_out filename in
  for l_c=1 to size data do
    for k_c=1 to l_c do
      for j_c=1 to l_c do
        for i_c=1 to k_c do
          let value = get_phys data i_c j_c k_c l_c in
           if (abs_float value > cutoff) then
            Printf.fprintf oc " %5d %5d %5d %5d%20.15f\n" i_c j_c k_c l_c value;
        done;
      done;
    done;
  done;
  close_out oc