Wagstaff primes: Difference between revisions

Content added Content deleted

Inline

@@ Line 307: / Line 307: @@
 =={{header|OCaml}}==
-Using the function <code>seq_primes</code> from [[Extensible prime generator#OCaml]]:
 <syntaxhighlight lang="ocaml">let is_prime n =
-  let not_divisible x = n mod x <> 0 in
+  let rec test x =
-  seq_primes |> Seq.take_while (fun x -> x * x <= n) |> Seq.for_all not_divisible
+    let q = n / x in x > q || x * q <> n && n mod (x + 2) <> 0 && test (x + 6)
+  in if n < 5 then n lor 1 = 3 else n land 1 <> 0 && n mod 3 <> 0 && test 5
-let wagstaff n =
+let is_wagstaff n =
   let w = succ (1 lsl n) / 3 in
-  if n <> 2 && is_prime w then Some (n, w) else None
+  if is_prime n && is_prime w then Some (n, w) else None
 let () =
   let show (p, w) = Printf.printf "%u -> %u%!\n" p w in
-  seq_primes |> Seq.filter_map wagstaff |> Seq.take 11 |> Seq.iter show</syntaxhighlight>
+  Seq.(ints 3 |> filter_map is_wagstaff |> take 11 |> iter show)</syntaxhighlight>
-...checking for primality in a safe way. So, even when compiled with ''ocamlopt'', it takes more than three minutes to finish with the 11th.
 {{out}}
 <pre>