invert.c
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上传日期:2022-08-06
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- /* mpz_invert (inv, x, n). Find multiplicative inverse of X in Z(N).
- If X has an inverse, return non-zero and store inverse in INVERSE,
- otherwise, return 0 and put garbage in INVERSE.
- Copyright 1996, 1997, 1998, 1999, 2000, 2001, 2005 Free Software Foundation,
- Inc.
- This file is part of the GNU MP Library.
- The GNU MP Library is free software; you can redistribute it and/or modify
- it under the terms of the GNU Lesser General Public License as published by
- the Free Software Foundation; either version 3 of the License, or (at your
- option) any later version.
- The GNU MP Library 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 Lesser General Public
- License for more details.
- You should have received a copy of the GNU Lesser General Public License
- along with the GNU MP Library. If not, see http://www.gnu.org/licenses/. */
- #include "gmp.h"
- #include "gmp-impl.h"
- int
- mpz_invert (mpz_ptr inverse, mpz_srcptr x, mpz_srcptr n)
- {
- mpz_t gcd, tmp;
- mp_size_t xsize, nsize, size;
- TMP_DECL;
- xsize = SIZ (x);
- nsize = SIZ (n);
- xsize = ABS (xsize);
- nsize = ABS (nsize);
- size = MAX (xsize, nsize) + 1;
- /* No inverse exists if the leftside operand is 0. Likewise, no
- inverse exists if the mod operand is 1. */
- if (xsize == 0 || (nsize == 1 && (PTR (n))[0] == 1))
- return 0;
- TMP_MARK;
- MPZ_TMP_INIT (gcd, size);
- MPZ_TMP_INIT (tmp, size);
- mpz_gcdext (gcd, tmp, (mpz_ptr) 0, x, n);
- /* If no inverse existed, return with an indication of that. */
- if (SIZ (gcd) != 1 || PTR(gcd)[0] != 1)
- {
- TMP_FREE;
- return 0;
- }
- /* Make sure we return a positive inverse. */
- if (SIZ (tmp) < 0)
- {
- if (SIZ (n) < 0)
- mpz_sub (inverse, tmp, n);
- else
- mpz_add (inverse, tmp, n);
- }
- else
- mpz_set (inverse, tmp);
- TMP_FREE;
- return 1;
- }