Implement efficient reduction for pleasant-looking primes.

[u/mdw/catacomb] / f-niceprime.c

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+/* -*-c-*-
+ *
+ * $Id: f-niceprime.c,v 1.1 2004/03/27 00:04:46 mdw Exp $
+ *
+ * Prime fields with efficient reduction for special-form primes
+ *
+ * (c) 2004 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.
+ */
+
+/*----- Revision history --------------------------------------------------* 
+ *
+ * $Log: f-niceprime.c,v $
+ * Revision 1.1  2004/03/27 00:04:46  mdw
+ * Implement efficient reduction for pleasant-looking primes.
+ *
+ */
+
+/*----- Header files ------------------------------------------------------*/
+
+#include <mLib/sub.h>
+
+#include "field.h"
+#include "mpreduce.h"
+#include "mprand.h"
+
+/*----- Data structures ---------------------------------------------------*/
+
+typedef struct fctx {
+  field f;
+  mpreduce r;
+} fctx;
+
+/*----- Main code ---------------------------------------------------------*/
+
+/* --- Field operations --- */
+
+static void fdestroy(field *ff)
+{
+  fctx *f = (fctx *)ff;
+  mpreduce_destroy(&f->r);
+  DESTROY(f);
+}
+
+static mp *frand(field *ff, mp *d, grand *r)
+{
+  fctx *f = (fctx *)ff;
+  return (mprand_range(d, f->r.p, r, 0));
+}
+
+static int fzerop(field *ff, mp *x)
+{
+  return (!MP_LEN(x));
+}
+
+static mp *fneg(field *ff, mp *d, mp *x)
+{
+  fctx *f = (fctx *)ff;
+  return (mp_sub(d, f->r.p, x));
+}
+
+static mp *fadd(field *ff, mp *d, mp *x, mp *y)
+{
+  fctx *f = (fctx *)ff;
+  d = mp_add(d, x, y);
+  if (d->f & MP_NEG)
+    d = mp_add(d, d, f->r.p);
+  else if (MP_CMP(d, >, f->r.p))
+    d = mp_sub(d, d, f->r.p);
+  return (d);
+}
+
+static mp *fsub(field *ff, mp *d, mp *x, mp *y)
+{
+  fctx *f = (fctx *)ff;
+  d = mp_sub(d, x, y);
+  if (d->f & MP_NEG)
+    d = mp_add(d, d, f->r.p);
+  else if (MP_CMP(d, >, f->r.p))
+    d = mp_sub(d, d, f->r.p);
+  return (d);
+}
+
+static mp *fmul(field *ff, mp *d, mp *x, mp *y)
+{
+  fctx *f = (fctx *)ff;
+  d = mp_mul(d, x, y);
+  return (mpreduce_do(&f->r, d, d));
+}
+
+static mp *fsqr(field *ff, mp *d, mp *x)
+{
+  fctx *f = (fctx *)ff;
+  d = mp_sqr(d, x);
+  return (mpreduce_do(&f->r, d, d));
+}
+
+static mp *finv(field *ff, mp *d, mp *x)
+{
+  fctx *f = (fctx *)ff;
+  mp_gcd(0, 0, &d, f->r.p, x);
+  return (d);
+}
+
+static mp *freduce(field *ff, mp *d, mp *x)
+{
+  fctx *f = (fctx *)ff;
+  return (mpreduce_do(&f->r, d, x));
+}
+
+static mp *fsqrt(field *ff, mp *d, mp *x)
+{
+  fctx *f = (fctx *)ff;
+  return (mp_modsqrt(d, x, f->r.p));
+}
+
+static mp *fdbl(field *ff, mp *d, mp *x)
+{
+  fctx *f = (fctx *)ff;
+  d = mp_lsl(d, x, 1);
+  if (MP_CMP(d, >, f->r.p))
+    d = mp_sub(d, d, f->r.p);
+  return (d);
+}
+
+static mp *ftpl(field *ff, mp *d, mp *x)
+{
+  fctx *f = (fctx *)ff;
+  MP_DEST(d, MP_LEN(x) + 1, x->f);
+  MPX_UMULN(d->v, d->vl, x->v, x->vl, 3);
+  while (MP_CMP(d, >, f->r.p))
+    d = mp_sub(d, d, f->r.p);
+  return (d);
+}
+
+static mp *fqdl(field *ff, mp *d, mp *x)
+{
+  fctx *f = (fctx *)ff;
+  d = mp_lsl(d, x, 2);
+  while (MP_CMP(d, >, f->r.p))
+    d = mp_sub(d, d, f->r.p);
+  return (d);
+}
+
+static mp *fhlv(field *ff, mp *d, mp *x)
+{
+  fctx *f = (fctx *)ff;
+  if (!MP_LEN(x)) {
+    MP_COPY(x);
+    MP_DROP(d);
+    return (x);
+  }
+  if (x->v[0] & 1) {
+    d = mp_add(d, x, f->r.p);
+    x = d;
+  }
+  return (mp_lsr(d, x, 1));
+}
+
+/* --- Field operations table --- */
+
+static field_ops fops = {
+  FTY_PRIME, "niceprime",
+  fdestroy, frand,
+  freduce, field_id,
+  fzerop, fneg, fadd, fsub, fmul, fsqr, finv, freduce, fsqrt,
+  0,
+  fdbl, ftpl, fqdl, fhlv
+};
+
+/* --- @field_niceprime@ --- *
+ *
+ * Arguments:  @mp *p@ = the characteristic of the field
+ *
+ * Returns:    A pointer to the field.
+ *
+ * Use:                Creates a field structure for a prime field of size %$p$%,
+ *             using efficient reduction for nice primes.
+ */
+
+field *field_niceprime(mp *p)
+{
+  fctx *f = CREATE(fctx);
+  f->f.ops = &fops;
+  f->f.zero = MP_ZERO;
+  f->f.one = MP_ONE;
+  mpreduce_create(&f->r, p);
+  return (&f->f);
+}
+
+/*----- That's all, folks -------------------------------------------------*/