contrib/gmp/mpz/gcdext.c

   1 /* mpz_gcdext(g, s, t, a, b) -- Set G to gcd(a, b), and S and T such that
   2    g = as + bt.
   3
   4 Copyright 1991, 1993, 1994, 1995, 1996, 1997, 2000, 2001, 2005 Free Software
   5 Foundation, Inc.
   6
   7 This file is part of the GNU MP Library.
   8
   9 The GNU MP Library is free software; you can redistribute it and/or modify
  10 it under the terms of the GNU Lesser General Public License as published by
  11 the Free Software Foundation; either version 3 of the License, or (at your
  12 option) any later version.
  13
  14 The GNU MP Library is distributed in the hope that it will be useful, but
  15 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  16 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
  17 License for more details.
  18
  19 You should have received a copy of the GNU Lesser General Public License
  20 along with the GNU MP Library.  If not, see http://www.gnu.org/licenses/.  */
  21
  22 #include <stdio.h> /* for NULL */
  23 #include "gmp.h"
  24 #include "gmp-impl.h"
  25
  26 void
  27 mpz_gcdext (mpz_ptr g, mpz_ptr s, mpz_ptr t, mpz_srcptr a, mpz_srcptr b)
  28 {
  29   mp_size_t asize, bsize, usize, vsize;
  30   mp_srcptr ap, bp;
  31   mp_ptr up, vp;
  32   mp_size_t gsize, ssize, tmp_ssize;
  33   mp_ptr gp, sp, tmp_gp, tmp_sp;
  34   mpz_srcptr u, v;
  35   mpz_ptr ss, tt;
  36   __mpz_struct stmp, gtmp;
  37   TMP_DECL;
  38
  39   TMP_MARK;
  40
  41   /* mpn_gcdext requires that U >= V.  Therefore, we often have to swap U and
  42      V.  This in turn leads to a lot of complications.  The computed cofactor
  43      will be the wrong one, so we have to fix that up at the end.  */
  44
  45   asize = ABS (SIZ (a));
  46   bsize = ABS (SIZ (b));
  47   ap = PTR (a);
  48   bp = PTR (b);
  49   if (asize > bsize || (asize == bsize && mpn_cmp (ap, bp, asize) > 0))
  50     {
  51       usize = asize;
  52       vsize = bsize;
  53       up = TMP_ALLOC_LIMBS (usize + 1);
  54       vp = TMP_ALLOC_LIMBS (vsize + 1);
  55       MPN_COPY (up, ap, usize);
  56       MPN_COPY (vp, bp, vsize);
  57       u = a;
  58       v = b;
  59       ss = s;
  60       tt = t;
  61     }
  62   else
  63     {
  64       usize = bsize;
  65       vsize = asize;
  66       up = TMP_ALLOC_LIMBS (usize + 1);
  67       vp = TMP_ALLOC_LIMBS (vsize + 1);
  68       MPN_COPY (up, bp, usize);
  69       MPN_COPY (vp, ap, vsize);
  70       u = b;
  71       v = a;
  72       ss = t;
  73       tt = s;
  74     }
  75
  76   tmp_gp = TMP_ALLOC_LIMBS (usize + 1);
  77   tmp_sp = TMP_ALLOC_LIMBS (usize + 1);
  78
  79   if (vsize == 0)
  80     {
  81       tmp_sp[0] = 1;
  82       tmp_ssize = 1;
  83       MPN_COPY (tmp_gp, up, usize);
  84       gsize = usize;
  85     }
  86   else
  87     gsize = mpn_gcdext (tmp_gp, tmp_sp, &tmp_ssize, up, usize, vp, vsize);
  88   ssize = ABS (tmp_ssize);
  89
  90   PTR (&gtmp) = tmp_gp;
  91   SIZ (&gtmp) = gsize;
  92
  93   PTR (&stmp) = tmp_sp;
  94   SIZ (&stmp) = (tmp_ssize ^ SIZ (u)) >= 0 ? ssize : -ssize;
  95
  96   if (tt != NULL)
  97     {
  98       if (SIZ (v) == 0)
  99         SIZ (tt) = 0;
 100       else
 101         {
 102           mpz_t x;
 103           MPZ_TMP_INIT (x, ssize + usize + 1);
 104           mpz_mul (x, &stmp, u);
 105           mpz_sub (x, &gtmp, x);
 106           mpz_tdiv_q (tt, x, v);
 107         }
 108     }
 109
 110   if (ss != NULL)
 111     {
 112       if (ALLOC (ss) < ssize)
 113         _mpz_realloc (ss, ssize);
 114       sp = PTR (ss);
 115       MPN_COPY (sp, tmp_sp, ssize);
 116       SIZ (ss) = SIZ (&stmp);
 117     }
 118
 119   if (ALLOC (g) < gsize)
 120     _mpz_realloc (g, gsize);
 121   gp = PTR (g);
 122   MPN_COPY (gp, tmp_gp, gsize);
 123   SIZ (g) = gsize;
 124
 125   TMP_FREE;
 126 }