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Date: Fri, 27 May 2011 19:15:11 +0200
From: Xavier Leroy <Xavier.Leroy@inria.fr>
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References: <707567.2845.qm@web111505.mail.gq1.yahoo.com> <833816.58675.qm@web111502.mail.gq1.yahoo.com> <slrnitv9jo.ks.sylvain@gallu.homelinux.org>
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Subject: Re: [Caml-list] Re: Binary logarithm of a power of 2

On 05/27/2011 03:26 PM, Sylvain Le Gall wrote:

> Friday afternoon exercice ;-)

Same here :-)

> Here is my code:

[ Recursive & array-based implementation in Caml ]

Simpler and maybe faster:

let logbin (n: int32) =
  let n = ref (Int32.add n 0l) and i = ref 0 in
  if !n >= 0x10000l then (n := Int32.shift_right !n 16; i := 16);
  if !n >= 0x100l then (n := Int32.shift_right !n 8; i := !i + 8);
  if !n >= 0x10l then (n := Int32.shift_right !n 4; i := !i + 4);
  if !n >= 0x4l then (n := Int32.shift_right !n 2; i := !i + 2);
  if !n >= 0x2l then !i + 1 else !i

Have fun,

- Xavier