mdw@git.distorted.org.uk Git - storin/blob - arith24.c

   1 /* -*-c-*-
   2  *
   3  * $Id$
   4  *
   5  * Arithmetic mod %$2^{24}$%
   6  *
   7  * (c) 2000 Mark Wooding
   8  */
   9
  10 /*----- Licensing notice --------------------------------------------------*
  11  *
  12  * Copyright (c) 2000 Mark Wooding
  13  * All rights reserved.
  14  *
  15  * Redistribution and use in source and binary forms, with or without
  16  * modification, are permitted provided that the following conditions are
  17  * met:
  18  *
  19  * 1. Redistributions of source code must retain the above copyright
  20  *    notice, this list of conditions and the following disclaimer.
  21  *
  22  * 2, Redistributions in binary form must reproduce the above copyright
  23  *    notice, this list of conditions and the following disclaimer in the
  24  *    documentation and/or other materials provided with the distribution.
  25  *
  26  * 3. The name of the authors may not be used to endorse or promote
  27  *    products derived from this software without specific prior written
  28  *    permission.
  29  *
  30  * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
  31  * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
  32  * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN
  33  * NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
  34  * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
  35  * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
  36  * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  37  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
  38  * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
  39  * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  40  * POSSIBILITY OF SUCH DAMAGE.
  41  *
  42  * Instead of accepting the above terms, you may redistribute and/or modify
  43  * this software under the terms of either the GNU General Public License,
  44  * or the GNU Library General Public License, published by the Free
  45  * Software Foundation; either version 2 of the License, or (at your
  46  * option) any later version.
  47  */
  48
  49 /*----- Revision history --------------------------------------------------*
  50  *
  51  * $Log: arith24.c,v $
  52  * Revision 1.2  2000/07/02 15:21:20  mdw
  53  * Fix licence text.
  54  *
  55  * Revision 1.1  2000/05/21 11:28:30  mdw
  56  * Initial check-in.
  57  *
  58  */
  59
  60 /*----- Header files ------------------------------------------------------*/
  61
  62 #include "arith24.h"
  63 #include "bits.h"
  64
  65 /*----- Main code ---------------------------------------------------------*/
  66
  67 /* --- @inv24@ --- *
  68  *
  69  * Arguments:   @uint24 x@ = a number to invert
  70  *
  71  * Returns:     The multiplicative inverse %$x^{-1}$% of %$x$% (mod
  72  *              %$2^{24}$%, if %$x$% is odd, or zero if %$x$% is even (and
  73  *              hence uninvertible).
  74  *
  75  * Use:         Computes multiplicative inverses mod %$2^{24}$%.
  76  */
  77
  78 uint24 inv24(uint24 x)
  79 {
  80   uint32 m = MASK24 + 1;
  81   long a = 1, b = 0;
  82   uint32 n = x;
  83
  84   for (;;) {
  85     uint32 q, r;
  86     long t;
  87     if (!(r = m % n))
  88       break;
  89     q = m / n;
  90     m = n; n = r;
  91     t = a; a = b - q * a; b = t;
  92   }
  93   if (n != 1)
  94     return (0);
  95   return (U24(a));
  96 }
  97
  98 /*----- That's all, folks -------------------------------------------------*/