type mo_class =
  | Core     of int  (* Always doubly occupied *)
  | Inactive of int  (* With 0,1 or 2 holes *)
  | Active   of int  (* With 0,1 or 2 holes or particles *)
  | Virtual  of int  (* With 0,1 or 2 particles *)
  | Deleted  of int  (* Always unoccupied *)

type t = mo_class list


let pp_mo_occ ppf = function
  | Core     i -> Format.fprintf ppf "@[Core %d@]" i
  | Inactive i -> Format.fprintf ppf "@[Inactive %d@]" i
  | Active   i -> Format.fprintf ppf "@[Active %d@]" i
  | Virtual  i -> Format.fprintf ppf "@[Virtual %d@]" i
  | Deleted  i -> Format.fprintf ppf "@[Deleted %d@]" i



let of_list t = t

let core_mos t =
  List.map (fun x ->
      match x with 
      | Core i -> Some i
      | _ -> None) t
  |> Util.list_some

let inactive_mos t =
  List.map (fun x ->
      match x with 
      | Inactive i -> Some i
      | _ -> None ) t
  |> Util.list_some

let active_mos t =
  List.map (fun x ->
      match x with 
      | Active i -> Some i
      | _ -> None ) t
  |> Util.list_some

let virtual_mos t =
  List.map (fun x ->
      match x with 
      | Virtual i -> Some i
      | _ -> None ) t
  |> Util.list_some

let deleted_mos t =
  List.map (fun x ->
      match x with 
      | Deleted i -> Some i
      | _ -> None ) t
  |> Util.list_some


let fci ?(frozen_core=true) mo_basis =
  let mo_num = MOBasis.size mo_basis in
  let ncore  = (Nuclei.small_core @@ Simulation.nuclei @@ MOBasis.simulation mo_basis) / 2 in
  of_list (
    if frozen_core then
      List.concat [
        Util.list_range 1 ncore
        |> List.map (fun i -> Core i) ;
        Util.list_range (ncore+1) mo_num
        |> List.map (fun i -> Active i) 
      ]
    else
      Util.list_range 1 mo_num
      |> List.map (fun i -> Active i)
  )