Project Euler #28

let make_spiral n =
  if n mod 2 = 0 then
    raise (Invalid_argument "Size should be odd");
  let res = Array.make_matrix n n 0 in
  let last_value = n * n in
  let current_value = ref 1 in
  let i = ref (n / 2) in
  let j = ref (n / 2) in
  let shift = ref 1 in
  let delta = ref 1 in
  res.(!i).(!j) <- !current_value;
  while !current_value < last_value do
    for k = 1 to !shift do
      if !current_value < last_value then
        begin
          j := !j + !delta;
          current_value := !current_value + 1;
          res.(!i).(!j) <- !current_value;
        end;
    done;
    for k = 1 to !shift do
      if !current_value < last_value then
        begin
          i := !i + !delta;
          current_value := !current_value + 1;
          res.(!i).(!j) <- !current_value;
        end;
    done;
    delta := - !delta;
    shift := !shift + 1;
  done;
  res;;

let sum_diagonals arr = 
  let size = Array.length arr in
  let sum = ref 0 in
  for i = 0 to size - 1 do
    sum := !sum + arr.(i).(i) + arr.(i).(size - 1 - i)
  done;
  !sum - 1;;
    
Printf.printf "%d\n" (sum_diagonals (make_spiral 1001));;

Very bad OCaml ;)