Heap-sort in OCaml

heap_sort.ml

let swap a i j =
  let t = a.(i) in
  a.(i) <- a.(j);
  a.(j) <- t;;

let printArray arr =
  Printf.printf "[";
  for idx = 0 to Array.length arr - 1 do
    if idx > 0 then Printf.printf ", ";
    Printf.printf "%d" @@ Array.get arr idx
  done;
  Printf.printf "]\n";;

let rec shift_down a i n =
  let left  = i * 2 + 1 in
  let right = i * 2 + 2 in
  let max_idx_left = if left < n && a.(left) > a.(i) then left else i in
  let max_idx = if right < n && a.(right) > a.(max_idx_left) then right else max_idx_left in
  if max_idx != i then
    begin
      swap a max_idx i;
      shift_down a max_idx n
    end;;

let build_heap a n =
  for i = (n - 1) / 2 downto 0 do
    shift_down a i n
  done;;

let heap_sort a n =
  build_heap a n;
  for i = n - 1 downto 1 do
    swap a 0 i;
    shift_down a 0 i;    
  done;;

let () =
  let x = [|5;4;6;2;1;6|] in
  printArray x;
  heap_sort x 6;
  printArray x;;