git.distorted.org.uk Git - u/mdw/catacomb/blob - math/mpbarrett.c

   1 /* -*-c-*-
   2  *
   3  * Barrett modular reduction
   4  *
   5  * (c) 1999 Straylight/Edgeware
   6  */
   7
   8 /*----- Licensing notice --------------------------------------------------*
   9  *
  10  * This file is part of Catacomb.
  11  *
  12  * Catacomb is free software; you can redistribute it and/or modify
  13  * it under the terms of the GNU Library General Public License as
  14  * published by the Free Software Foundation; either version 2 of the
  15  * License, or (at your option) any later version.
  16  *
  17  * Catacomb is distributed in the hope that it will be useful,
  18  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  19  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  20  * GNU Library General Public License for more details.
  21  *
  22  * You should have received a copy of the GNU Library General Public
  23  * License along with Catacomb; if not, write to the Free
  24  * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
  25  * MA 02111-1307, USA.
  26  */
  27
  28 /*----- Header files ------------------------------------------------------*/
  29
  30 #include "mp.h"
  31 #include "mpbarrett.h"
  32
  33 /*----- Main code ---------------------------------------------------------*/
  34
  35 /* --- @mpbarrett_create@ --- *
  36  *
  37  * Arguments:   @mpbarrett *mb@ = pointer to Barrett reduction context
  38  *              @mp *m@ = modulus to work to
  39  *
  40  *
  41  * Returns:     Zero on success, nonzero on error.
  42  *
  43  * Use:         Initializes a Barrett reduction context ready for use.
  44  */
  45
  46 int mpbarrett_create(mpbarrett *mb, mp *m)
  47 {
  48   mp *b;
  49
  50   /* --- Validate the arguments --- */
  51
  52   if (!MP_POSP(m))
  53     return (-1);
  54
  55   /* --- Compute %$\mu$% --- */
  56
  57   mp_shrink(m);
  58   mb->k = MP_LEN(m);
  59   mb->m = MP_COPY(m);
  60   b = mp_new(2 * mb->k + 1, 0);
  61   MPX_ZERO(b->v, b->vl - 1);
  62   b->vl[-1] = 1;
  63   mp_div(&b, 0, b, m);
  64   mb->mu = b;
  65   return (0);
  66 }
  67
  68 /* --- @mpbarrett_destroy@ --- *
  69  *
  70  * Arguments:   @mpbarrett *mb@ = pointer to Barrett reduction context
  71  *
  72  * Returns:     ---
  73  *
  74  * Use:         Destroys a Barrett reduction context releasing any resources
  75  *              claimed.
  76  */
  77
  78 void mpbarrett_destroy(mpbarrett *mb)
  79 {
  80   mp_drop(mb->m);
  81   mp_drop(mb->mu);
  82 }
  83
  84 /* --- @mpbarrett_reduce@ --- *
  85  *
  86  * Arguments:   @mpbarrett *mb@ = pointer to Barrett reduction context
  87  *              @mp *d@ = destination for result
  88  *              @mp *m@ = number to reduce
  89  *
  90  * Returns:     The residue of @m@ modulo the number in the reduction
  91  *              context.
  92  *
  93  * Use:         Performs an efficient modular reduction.
  94  */
  95
  96 mp *mpbarrett_reduce(mpbarrett *mb, mp *d, mp *m)
  97 {
  98   mp *q;
  99   size_t k = mb->k;
 100
 101   /* --- Special case if @m@ is too small --- */
 102
 103   if (MP_LEN(m) < k) {
 104     m = MP_COPY(m);
 105     if (d)
 106       MP_DROP(d);
 107     return (m);
 108   }
 109
 110   /* --- First stage --- */
 111
 112   {
 113     mp qq;
 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) {
 117       m = MP_COPY(m);
 118       if (d)
 119         MP_DROP(d);
 120       MP_DROP(q);
 121       return (m);
 122     }
 123   }
 124
 125   /* --- Second stage --- */
 126
 127   {
 128     mp *r;
 129     mpw *mvl;
 130
 131     MP_COPY(m);
 132     if (MP_LEN(m) <= k + 1)
 133       mvl = m->vl;
 134     else
 135       mvl = m->v + 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);
 141     MP_DROP(r);
 142     MP_DROP(q);
 143     MP_DROP(m);
 144   }
 145
 146   /* --- Final stage --- */
 147
 148   MP_SHRINK(d);
 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);
 151
 152   /* --- Fix up the sign --- */
 153
 154   if (d->f & MP_NEG) {
 155     mpx_usub(d->v, d->vl, mb->m->v, mb->m->vl, d->v, d->vl);
 156     d->f &= ~MP_NEG;
 157   }
 158
 159   MP_SHRINK(d);
 160   return (d);
 161 }
 162
 163 /*----- Test rig ----------------------------------------------------------*/
 164
 165 #ifdef TEST_RIG
 166
 167 static int vmod(dstr *v)
 168 {
 169   mp *x = *(mp **)v[0].buf;
 170   mp *n = *(mp **)v[1].buf;
 171   mp *r = *(mp **)v[2].buf;
 172   mp *s;
 173   mpbarrett mb;
 174   int ok = 1;
 175
 176   mpbarrett_create(&mb, n);
 177   s = mpbarrett_reduce(&mb, MP_NEW, x);
 178
 179   if (!MP_EQ(s, r)) {
 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);
 185     ok = 0;
 186   }
 187
 188   mpbarrett_destroy(&mb);
 189   mp_drop(x);
 190   mp_drop(n);
 191   mp_drop(r);
 192   mp_drop(s);
 193   assert(mparena_count(MPARENA_GLOBAL) == 0);
 194   return (ok);
 195 }
 196
 197 static test_chunk tests[] = {
 198   { "mpbarrett-reduce", vmod, { &type_mp, &type_mp, &type_mp, 0 } },
 199   { 0, 0, { 0 } }
 200 };
 201
 202 int main(int argc, char *argv[])
 203 {
 204   sub_init();
 205   test_run(argc, argv, tests, SRCDIR "/t/mpbarrett");
 206   return (0);
 207 }
 208
 209 #endif
 210
 211 /*----- That's all, folks -------------------------------------------------*/