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, 1993-1997, 2000, 2001, 2005, 2011, 2012, 2015 Free
5 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 <stdio.h> /* for NULL */
34 #include "gmp-impl.h"
35
36 void
mpz_gcdext(mpz_ptr g,mpz_ptr s,mpz_ptr t,mpz_srcptr a,mpz_srcptr b)37 mpz_gcdext (mpz_ptr g, mpz_ptr s, mpz_ptr t, mpz_srcptr a, mpz_srcptr b)
38 {
39 mp_size_t asize, bsize;
40 mp_ptr tmp_ap, tmp_bp;
41 mp_size_t gsize, ssize, tmp_ssize;
42 mp_ptr gp, tmp_gp, tmp_sp;
43 TMP_DECL;
44
45 /* mpn_gcdext requires that Usize >= Vsize. Therefore, we often
46 have to swap U and V. The computed cofactor will be the
47 "smallest" one, which is faster to produce. The wanted one will
48 be computed here; this is needed anyway when both are requested. */
49
50 asize = ABSIZ (a);
51 bsize = ABSIZ (b);
52
53 ASSERT (s != NULL);
54
55 if (asize < bsize)
56 {
57 MPZ_SRCPTR_SWAP (a, b);
58 MP_SIZE_T_SWAP (asize, bsize);
59 MPZ_PTR_SWAP (s, t);
60 }
61
62 if (bsize == 0)
63 {
64 /* g = |a|, s = sgn(a), t = 0. */
65 ssize = SIZ (a) >= 0 ? (asize != 0) : -1;
66
67 if (g != NULL)
68 {
69 /* If g == a, then ALLOC(g) == ALLOC(a) >= asize, i.e.
70 the next MPZ_NEWALLOC returns the old PTR(a) .*/
71 gp = MPZ_NEWALLOC (g, asize);
72 MPN_COPY (gp, PTR (a), asize);
73 SIZ (g) = asize;
74 }
75 if (t != NULL)
76 SIZ (t) = 0;
77 if (s != NULL)
78 {
79 SIZ (s) = ssize;
80 MPZ_NEWALLOC (s, 1)[0] = 1;
81 }
82 return;
83 }
84
85 TMP_MARK;
86
87 TMP_ALLOC_LIMBS_2 (tmp_gp, bsize, tmp_sp, asize + bsize + bsize + 1);
88 tmp_bp = tmp_sp + bsize + 1;
89 tmp_ap = tmp_bp + bsize;
90 MPN_COPY (tmp_ap, PTR (a), asize);
91 MPN_COPY (tmp_bp, PTR (b), bsize);
92
93 gsize = mpn_gcdext (tmp_gp, tmp_sp, &tmp_ssize, tmp_ap, asize, tmp_bp, bsize);
94
95 ssize = ABS (tmp_ssize);
96 tmp_ssize = SIZ (a) >= 0 ? tmp_ssize : -tmp_ssize;
97
98 if (t != NULL)
99 {
100 mpz_t x;
101 mpz_t gtmp, stmp;
102
103 PTR (gtmp) = tmp_gp;
104 SIZ (gtmp) = gsize;
105
106 PTR (stmp) = tmp_sp;
107 SIZ (stmp) = tmp_ssize;
108
109 ASSERT (ssize <= bsize); /* ssize*2 + asize + 1 <= asize + bsize*2 + 1 */
110 PTR (x) = tmp_sp + ssize;
111 ALLOC (x) = ssize + asize + 1;
112
113 mpz_mul (x, stmp, a);
114 mpz_sub (x, gtmp, x);
115 mpz_divexact (t, x, b);
116 }
117
118 if (s != NULL)
119 {
120 mp_ptr sp;
121
122 sp = MPZ_NEWALLOC (s, ssize);
123 MPN_COPY (sp, tmp_sp, ssize);
124 SIZ (s) = tmp_ssize;
125 }
126
127 if (g != NULL)
128 {
129 gp = MPZ_NEWALLOC (g, gsize);
130 MPN_COPY (gp, tmp_gp, gsize);
131 SIZ (g) = gsize;
132 }
133
134 TMP_FREE;
135 }
136