3 * Barrett modular reduction
5 * (c) 1999 Straylight/Edgeware
8 /*----- Licensing notice --------------------------------------------------*
10 * This file is part of Catacomb.
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20 * GNU Library General Public License for more details.
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28 /*----- Header files ------------------------------------------------------*/
31 #include "mpbarrett.h"
33 /*----- Main code ---------------------------------------------------------*/
35 /* --- @mpbarrett_create@ --- *
37 * Arguments: @mpbarrett *mb@ = pointer to Barrett reduction context
38 * @mp *m@ = modulus to work to
41 * Returns: Zero on success, nonzero on error.
43 * Use: Initializes a Barrett reduction context ready for use.
46 int mpbarrett_create(mpbarrett *mb, mp *m)
50 /* --- Validate the arguments --- */
55 /* --- Compute %$\mu$% --- */
60 b = mp_new(2 * mb->k + 1, 0);
61 MPX_ZERO(b->v, b->vl - 1);
68 /* --- @mpbarrett_destroy@ --- *
70 * Arguments: @mpbarrett *mb@ = pointer to Barrett reduction context
74 * Use: Destroys a Barrett reduction context releasing any resources
78 void mpbarrett_destroy(mpbarrett *mb)
84 /* --- @mpbarrett_reduce@ --- *
86 * Arguments: @mpbarrett *mb@ = pointer to Barrett reduction context
87 * @mp *d@ = destination for result
88 * @mp *m@ = number to reduce
90 * Returns: The residue of @m@ modulo the number in the reduction
93 * Use: Performs an efficient modular reduction.
96 mp *mpbarrett_reduce(mpbarrett *mb, mp *d, mp *m)
101 /* --- Special case if @m@ is too small --- */
110 /* --- First stage --- */
114 mp_build(&qq, m->v + (k - 1), m->vl);
115 q = mp_mul(MP_NEW, &qq, mb->mu);
116 if (MP_LEN(q) <= k) {
125 /* --- Second stage --- */
132 if (MP_LEN(m) <= k + 1)
136 r = mp_new(k + 1, (q->f | mb->m->f) & MP_BURN);
137 mpx_umul(r->v, r->vl, q->v + k + 1, q->vl, mb->m->v, mb->m->vl);
138 MP_DEST(d, k + 1, r->f | MP_UNDEF);
139 mpx_usub(d->v, d->vl, m->v, mvl, r->v, r->vl);
140 d->f = (m->f | r->f) & (MP_BURN | MP_NEG);
146 /* --- Final stage --- */
149 while (MPX_UCMP(d->v, d->vl, >=, mb->m->v, mb->m->vl))
150 mpx_usub(d->v, d->vl, d->v, d->vl, mb->m->v, mb->m->vl);
152 /* --- Fix up the sign --- */
155 mpx_usub(d->v, d->vl, mb->m->v, mb->m->vl, d->v, d->vl);
163 /*----- Test rig ----------------------------------------------------------*/
167 static int vmod(dstr *v)
169 mp *x = *(mp **)v[0].buf;
170 mp *n = *(mp **)v[1].buf;
171 mp *r = *(mp **)v[2].buf;
176 mpbarrett_create(&mb, n);
177 s = mpbarrett_reduce(&mb, MP_NEW, x);
180 fputs("\n*** barrett reduction failure\n", stderr);
181 fputs("x = ", stderr); mp_writefile(x, stderr, 10); fputc('\n', stderr);
182 fputs("n = ", stderr); mp_writefile(n, stderr, 10); fputc('\n', stderr);
183 fputs("r = ", stderr); mp_writefile(r, stderr, 10); fputc('\n', stderr);
184 fputs("s = ", stderr); mp_writefile(s, stderr, 10); fputc('\n', stderr);
188 mpbarrett_destroy(&mb);
193 assert(mparena_count(MPARENA_GLOBAL) == 0);
197 static test_chunk tests[] = {
198 { "mpbarrett-reduce", vmod, { &type_mp, &type_mp, &type_mp, 0 } },
202 int main(int argc, char *argv[])
205 test_run(argc, argv, tests, SRCDIR "/t/mpbarrett");
211 /*----- That's all, folks -------------------------------------------------*/