X-Git-Url: https://git.distorted.org.uk/u/mdw/catacomb/blobdiff_plain/ba6e6b64033b1f9de49feccb5c9cd438354481f7..0f00dc4c8eb47e67bc0f148c2dd109f73a451e0a:/math/mpbarrett.c

diff --git a/math/mpbarrett.c b/math/mpbarrett.c
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+/* -*-c-*-
+ *
+ * Barrett modular reduction
+ *
+ * (c) 1999 Straylight/Edgeware
+ */
+
+/*----- Licensing notice --------------------------------------------------*
+ *
+ * This file is part of Catacomb.
+ *
+ * Catacomb is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Library General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * Catacomb 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 Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Library General Public
+ * License along with Catacomb; if not, write to the Free
+ * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
+ * MA 02111-1307, USA.
+ */
+
+/*----- Header files ------------------------------------------------------*/
+
+#include "mp.h"
+#include "mpbarrett.h"
+
+/*----- Main code ---------------------------------------------------------*/
+
+/* --- @mpbarrett_create@ --- *
+ *
+ * Arguments:	@mpbarrett *mb@ = pointer to Barrett reduction context
+ *		@mp *m@ = modulus to work to
+ *
+ *
+ * Returns:	Zero on success, nonzero on error.
+ *
+ * Use:		Initializes a Barrett reduction context ready for use.
+ */
+
+int mpbarrett_create(mpbarrett *mb, mp *m)
+{
+  mp *b;
+
+  /* --- Validate the arguments --- */
+
+  if (!MP_POSP(m))
+    return (-1);
+
+  /* --- Compute %$\mu$% --- */
+
+  mp_shrink(m);
+  mb->k = MP_LEN(m);
+  mb->m = MP_COPY(m);
+  b = mp_new(2 * mb->k + 1, 0);
+  MPX_ZERO(b->v, b->vl - 1);
+  b->vl[-1] = 1;
+  mp_div(&b, 0, b, m);
+  mb->mu = b;
+  return (0);
+}
+
+/* --- @mpbarrett_destroy@ --- *
+ *
+ * Arguments:	@mpbarrett *mb@ = pointer to Barrett reduction context
+ *
+ * Returns:	---
+ *
+ * Use:		Destroys a Barrett reduction context releasing any resources
+ *		claimed.
+ */
+
+void mpbarrett_destroy(mpbarrett *mb)
+{
+  mp_drop(mb->m);
+  mp_drop(mb->mu);
+}
+
+/* --- @mpbarrett_reduce@ --- *
+ *
+ * Arguments:	@mpbarrett *mb@ = pointer to Barrett reduction context
+ *		@mp *d@ = destination for result
+ *		@mp *m@ = number to reduce
+ *
+ * Returns:	The residue of @m@ modulo the number in the reduction
+ *		context.
+ *
+ * Use:		Performs an efficient modular reduction.
+ */
+
+mp *mpbarrett_reduce(mpbarrett *mb, mp *d, mp *m)
+{
+  mp *q;
+  size_t k = mb->k;
+
+  /* --- Special case if @m@ is too small --- */
+
+  if (MP_LEN(m) < k) {
+    m = MP_COPY(m);
+    if (d)
+      MP_DROP(d);
+    return (m);
+  }
+
+  /* --- First stage --- */
+
+  {
+    mp qq;
+    mp_build(&qq, m->v + (k - 1), m->vl);
+    q = mp_mul(MP_NEW, &qq, mb->mu);
+    if (MP_LEN(q) <= k) {
+      m = MP_COPY(m);
+      if (d)
+	MP_DROP(d);
+      MP_DROP(q);
+      return (m);
+    }
+  }
+
+  /* --- Second stage --- */
+
+  {
+    mp *r;
+    mpw *mvl;
+
+    MP_COPY(m);
+    if (MP_LEN(m) <= k + 1)
+      mvl = m->vl;
+    else
+      mvl = m->v + k + 1;
+    r = mp_new(k + 1, (q->f | mb->m->f) & MP_BURN);
+    mpx_umul(r->v, r->vl, q->v + k + 1, q->vl, mb->m->v, mb->m->vl);
+    MP_DEST(d, k + 1, r->f | MP_UNDEF);
+    mpx_usub(d->v, d->vl, m->v, mvl, r->v, r->vl);
+    d->f = (m->f | r->f) & (MP_BURN | MP_NEG);
+    MP_DROP(r);
+    MP_DROP(q);
+    MP_DROP(m);
+  }
+
+  /* --- Final stage --- */
+
+  MP_SHRINK(d);
+  while (MPX_UCMP(d->v, d->vl, >=, mb->m->v, mb->m->vl))
+    mpx_usub(d->v, d->vl, d->v, d->vl, mb->m->v, mb->m->vl);
+
+  /* --- Fix up the sign --- */
+
+  if (d->f & MP_NEG) {
+    mpx_usub(d->v, d->vl, mb->m->v, mb->m->vl, d->v, d->vl);
+    d->f &= ~MP_NEG;
+  }
+
+  MP_SHRINK(d);
+  return (d);
+}
+
+/*----- Test rig ----------------------------------------------------------*/
+
+#ifdef TEST_RIG
+
+static int vmod(dstr *v)
+{
+  mp *x = *(mp **)v[0].buf;
+  mp *n = *(mp **)v[1].buf;
+  mp *r = *(mp **)v[2].buf;
+  mp *s;
+  mpbarrett mb;
+  int ok = 1;
+
+  mpbarrett_create(&mb, n);
+  s = mpbarrett_reduce(&mb, MP_NEW, x);
+
+  if (!MP_EQ(s, r)) {
+    fputs("\n*** barrett reduction failure\n", stderr);
+    fputs("x = ", stderr); mp_writefile(x, stderr, 10); fputc('\n', stderr);
+    fputs("n = ", stderr); mp_writefile(n, stderr, 10); fputc('\n', stderr);
+    fputs("r = ", stderr); mp_writefile(r, stderr, 10); fputc('\n', stderr);
+    fputs("s = ", stderr); mp_writefile(s, stderr, 10); fputc('\n', stderr);
+    ok = 0;
+  }
+
+  mpbarrett_destroy(&mb);
+  mp_drop(x);
+  mp_drop(n);
+  mp_drop(r);
+  mp_drop(s);
+  assert(mparena_count(MPARENA_GLOBAL) == 0);
+  return (ok);
+}
+
+static test_chunk tests[] = {
+  { "mpbarrett-reduce", vmod, { &type_mp, &type_mp, &type_mp, 0 } },
+  { 0, 0, { 0 } }
+};
+
+int main(int argc, char *argv[])
+{
+  sub_init();
+  test_run(argc, argv, tests, SRCDIR "/t/mpbarrett");
+  return (0);
+}
+
+#endif
+
+/*----- That's all, folks -------------------------------------------------*/