(**
 * Programming Pearls Column 2 Exercise B
 *
 * In-place routines to reverse and rotate arrays.
 *
 * Author: Michael DiBernardo
 *)

(** Reverse an array in-place from start_index to end_index inclusive. *)
let rec reverse_array arr start_index end_index =
  if start_index >= end_index then
    ignore ()
  else 
    let first_elem = arr.(start_index) and
        last_elem = arr.(end_index) in (
      arr.(end_index) <- first_elem;
      arr.(start_index) <- last_elem;
      (reverse_array arr (start_index + 1) (end_index - 1))
    )

(** Rotate an array in-place by 'amount' positions to the left. *)
let rotate_left arr amount =
  let i = (amount mod (Array.length arr)) and
      last_index = ((Array.length arr) - 1) in (
    reverse_array arr 0 (i - 1);
    reverse_array arr i last_index;
    reverse_array arr 0 last_index;
  )