missing_letters.ml

open Core

let all_letters =
  let all = "abcdefghijklmnopqrstuvwxyz" in
  List.init (String.length all) ~f:(String.get all)

let missing_letters (letters : char list) =
  let start_pos, end_pos =
    ( List.findi all_letters ~f:(fun _ letter ->
          Char.compare letter (List.hd_exn letters) = 0)
      |> Option.value_exn |> fst,
      List.findi all_letters ~f:(fun _ letter ->
          Char.compare letter (List.last_exn letters) = 0)
      |> Option.value_exn |> fst )
  in
  let slice = List.slice all_letters start_pos end_pos in
  List.filter slice ~f:(fun letter ->
      not
        (List.find letters ~f:(fun l -> Char.compare l letter = 0)
        |> Option.is_some))

let%test_module "missing_letters" =
  (module struct
    let%test_unit "test 1" =
      [%test_eq: char list]
        (missing_letters [ 'a'; 'b'; 'c'; 'd'; 'f' ])
        [ 'e' ]

    let%test_unit "test 2" =
      [%test_eq: char list]
        (missing_letters
           [
             'a';
             'b';
             'c';
             'd';
             'e';
             'h';
             'i';
             'j';
             'k';
             'l';
             'm';
             'n';
             'o';
             'p';
             'q';
             'r';
             's';
             't';
             'u';
             'w';
             'x';
             'y';
             'z';
           ])
        [ 'f'; 'g'; 'v' ]
  end)