source/libs/gmp/gmp-src/mpz/invert.c

   1 /* mpz_invert (inv, x, n).  Find multiplicative inverse of X in Z(N).
   2    If X has an inverse, return non-zero and store inverse in INVERSE,
   3    otherwise, return 0 and put garbage in INVERSE.
   4
   5 Copyright 1996-2001, 2005, 2012, 2014 Free Software 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 either:
  11
  12   * the GNU Lesser General Public License as published by the Free
  13     Software Foundation; either version 3 of the License, or (at your
  14     option) any later version.
  15
  16 or
  17
  18   * the GNU General Public License as published by the Free Software
  19     Foundation; either version 2 of the License, or (at your option) any
  20     later version.
  21
  22 or both in parallel, as here.
  23
  24 The GNU MP Library is distributed in the hope that it will be useful, but
  25 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  26 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  27 for more details.
  28
  29 You should have received copies of the GNU General Public License and the
  30 GNU Lesser General Public License along with the GNU MP Library.  If not,
  31 see https://www.gnu.org/licenses/.  */
  32
  33 #include "gmp.h"
  34 #include "gmp-impl.h"
  35
  36 int
  37 mpz_invert (mpz_ptr inverse, mpz_srcptr x, mpz_srcptr n)
  38 {
  39   mpz_t gcd, tmp;
  40   mp_size_t xsize, nsize, size;
  41   TMP_DECL;
  42
  43   xsize = ABSIZ (x);
  44   nsize = ABSIZ (n);
  45
  46   size = MAX (xsize, nsize) + 1;
  47   TMP_MARK;
  48
  49   MPZ_TMP_INIT (gcd, size);
  50   MPZ_TMP_INIT (tmp, size);
  51   mpz_gcdext (gcd, tmp, (mpz_ptr) 0, x, n);
  52
  53   /* If no inverse existed, return with an indication of that.  */
  54   if (!MPZ_EQUAL_1_P (gcd))
  55     {
  56       TMP_FREE;
  57       return 0;
  58     }
  59
  60   /* Make sure we return a positive inverse.  */
  61   if (SIZ (tmp) < 0)
  62     {
  63       if (SIZ (n) < 0)
  64         mpz_sub (inverse, tmp, n);
  65       else
  66         mpz_add (inverse, tmp, n);
  67     }
  68   else
  69     mpz_set (inverse, tmp);
  70
  71   TMP_FREE;
  72   return 1;
  73 }