type t =
{
  elec_num : int;
  mo_basis : MOBasis.t;
  mo_class : MOClass.t;
  spindets : Spindeterminant.t array;
}


let fci_of_mo_basis ?(frozen_core=true) mo_basis elec_num =
  let mo_num = MOBasis.size mo_basis in
  let ncore  = (Nuclei.small_core @@ Simulation.nuclei @@ MOBasis.simulation mo_basis) / 2 in
  let mo_class =
    MOClass.of_list @@
    if frozen_core then
      List.concat [
        Util.list_range 1 ncore
        |> List.map (fun i -> MOClass.Core i) ;
        Util.list_range (ncore+1) mo_num
        |> List.map (fun i -> MOClass.Active i) 
      ]
    else
      Util.list_range 1 mo_num
      |> List.map (fun i -> MOClass.Active i)
  in
  let m l = 
    List.fold_left (fun accu i -> let j = i-1 in Z.(logor accu (shift_left one j)) 
      ) Z.zero l
  in
  let occ_mask    = m (MOClass.core_mos    mo_class)
  and active_mask = m (MOClass.active_mos  mo_class)
  in
  let neg_active_mask = Z.neg active_mask in
Format.printf "%a\n" Util.pp_bitstring occ_mask;
Format.printf "%a\n" Util.pp_bitstring active_mask;
Format.printf "%a\n" Util.pp_bitstring neg_active_mask;
  let spindets = 
    Util.bit_permtutations elec_num mo_num 
    |> List.filter (fun b -> Z.logand neg_active_mask b = occ_mask)
    |> List.map (fun b -> Spindeterminant.of_bitstring b)
    |> Array.of_list
  in
  { elec_num ; mo_basis ; mo_class ; spindets }


let pp_spindet_space ppf t =
  Format.fprintf ppf "@[<v>[";
  Array.iteri (fun i d -> Format.fprintf ppf "@[<h>@[%d@]@;@[%a@]@]@," i Spindeterminant.pp_spindet d) t.spindets;
  Format.fprintf ppf "]@]"