mpir/fft/revbin.c

/*=============================================================================

    This file is part of FLINT.

    FLINT is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    FLINT is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with FLINT; if not, write to the Free Software
    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA

=============================================================================*/
/******************************************************************************

    Copyright (C) 2009 William Hart

******************************************************************************/

#include <mpir.h>

const mp_limb_t revtab0[1] = { 0 };
const mp_limb_t revtab1[2] = { 0, 1 };
const mp_limb_t revtab2[4] = { 0, 2, 1, 3 };
const mp_limb_t revtab3[8] = { 0, 4, 2, 6, 1, 5, 3, 7 };
const mp_limb_t revtab4[16] = { 0, 8, 4, 12, 2, 10, 6, 14, 1, 9, 5, 13, 3, 11, 7, 15 };

const mp_limb_t * revtab[5] = { revtab0, revtab1, revtab2, revtab3, revtab4 };

/*
   computes the reverse binary of a binary number of the given number of bits
 */
mp_limb_t n_revbin(mp_limb_t in, mp_limb_t bits)
{
    mp_limb_t out = 0, i;
    
    if (bits <= 4)
        return revtab[bits][in];

    for (i = 0; i < bits; i++)
    {   
       out <<= 1;
       out += (in & 1);
       in >>= 1;
    }

    return out;
}