Source file utils.ml

let remove ~idx l =
  let rec aux ~left ~right ~i =
    match right with
    | _ :: tl when i = idx -> List.rev_append left tl
    | hd :: tl -> aux ~left:(hd :: left) ~right:tl ~i:(i + 1)
    | [] -> l
  in
  aux ~left:[] ~right:l ~i:0

let rec permutations = function
  | [] -> [ [] ]
  | [ elm ] -> [ [ elm ] ]
  | l ->
      List.mapi
        (fun idx elm ->
          List.map (List.cons elm) (permutations @@ remove ~idx l))
        l
      |> List.flatten