(************************************************************
   useSequence.ml	Created      : Wed Feb 27 01:16:20 2002
  			Last modified: Thu Feb 28 00:08:34 2002
  Compile: make #
************************************************************)
open LibSequence

let rec filter_sequence f stream =
  let hd = (head stream) in
  if (f hd) then cons hd
      (ref (Lazy.Delayed (fun () -> (filter_sequence f (tail stream)))))
  else filter_sequence f (tail stream);;

let rec integers_from_n n =
  cons n (ref (Lazy.Delayed (fun () -> integers_from_n (n + 1))))
let integers = integers_from_n 0
let odds = filter_sequence (fun x -> x mod 2 = 0) integers
(* エラトネスのふるい *)
let rec seive n numbers = 
  let tmp = filter_sequence (fun x -> x mod n <> 0) numbers in
  let hd = head tmp in
  cons hd (ref (Lazy.Delayed(fun () -> seive hd (tail tmp))));;
let from_2 = integers_from_n 2
let primes = seive 2 from_2      

let print_usage () = 
  print_string "prime [nth]\n"
  
let _ = 
  let argv = Sys.argv in
  let argc = Array.length Sys.argv in
  if argc <> 2 then 
    begin print_usage (); exit 1; end
  else
    print_int (nth ((int_of_string Sys.argv.(1)) -1) primes)