open Bigarray

let rec is_prime n i primes =
  let limit = sqrt (float_of_int n) in
  let p = primes.{i} in
  if n mod p = 0 then false
  else if float_of_int p > limit then true
  else is_prime n (i + 1) primes

let primes lim =
  let res = Array1.create int c_layout (lim / 2) in
  let rec primes' n i = match n with
    | x when x = lim + 1 -> res
    | x -> if is_prime x 0 res then begin
             res.{i} <- x;
             primes' (n + 1) (i + 1)
           end else
             primes' (n + 1) i
  in res.{0} <-2;
     primes' 3 1

let _ = primes (int_of_string Sys.argv.(1))