X-Git-Url: https://git.distorted.org.uk/u/mdw/catacomb/blobdiff_plain/419d33ed2f44bd5f00b0c78a81aaa1abc37f2056..34e4f738bcba58e6d8c4cabbb0b3232a65b42a9d:/mpbarrett.c

diff --git a/mpbarrett.c b/mpbarrett.c
index affbc65..934097d 100644
--- a/mpbarrett.c
+++ b/mpbarrett.c
@@ -1,6 +1,6 @@
 /* -*-c-*-
  *
- * $Id: mpbarrett.c,v 1.5 2000/07/29 17:04:33 mdw Exp $
+ * $Id: mpbarrett.c,v 1.9 2004/04/01 12:50:09 mdw Exp $
  *
  * Barrett modular reduction
  *
@@ -30,6 +30,22 @@
 /*----- Revision history --------------------------------------------------* 
  *
  * $Log: mpbarrett.c,v $
+ * Revision 1.9  2004/04/01 12:50:09  mdw
+ * Add cyclic group abstraction, with test code.  Separate off exponentation
+ * functions for better static linking.  Fix a buttload of bugs on the way.
+ * Generally ensure that negative exponents do inversion correctly.  Add
+ * table of standard prime-field subgroups.  (Binary field subgroups are
+ * currently unimplemented but easy to add if anyone ever finds a good one.)
+ *
+ * Revision 1.8  2001/06/16 13:00:20  mdw
+ * Use the generic exponentiation functions.
+ *
+ * Revision 1.7  2001/04/19 18:25:26  mdw
+ * Use sliding-window exponentiation.
+ *
+ * Revision 1.6  2000/10/08 12:03:44  mdw
+ * (mpbarrett_reduce): Cope with negative numbers.
+ *
  * Revision 1.5  2000/07/29 17:04:33  mdw
  * Change to use left-to-right bitwise exponentiation.  This will improve
  * performance when the base is small.
@@ -114,8 +130,7 @@ void mpbarrett_destroy(mpbarrett *mb)
  * Returns:	The residue of @m@ modulo the number in the reduction
  *		context.
  *
- * Use:		Performs an efficient modular reduction.  The argument is
- *		assumed to be positive.
+ * Use:		Performs an efficient modular reduction.
  */
 
 mp *mpbarrett_reduce(mpbarrett *mb, mp *d, mp *m)
@@ -161,7 +176,7 @@ mp *mpbarrett_reduce(mpbarrett *mb, mp *d, mp *m)
     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);
     mpx_usub(d->v, d->vl, m->v, mvl, r->v, r->vl);
-    d->f = (m->f | r->f) & MP_BURN;
+    d->f = (m->f | r->f) & (MP_BURN | MP_NEG);
     MP_DROP(r);
     MP_DROP(q);
     MP_DROP(m);
@@ -173,77 +188,15 @@ mp *mpbarrett_reduce(mpbarrett *mb, mp *d, mp *m)
   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);
 
-  MP_SHRINK(d);
-  return (d);
-}
+  /* --- Fix up the sign --- */
 
-/* --- @mpbarrett_exp@ --- *
- *
- * Arguments:	@mpbarrett *mb@ = pointer to Barrett reduction context
- *              @mp *d@ = fake destination
- *              @mp *a@ = base
- *              @mp *e@ = exponent
- *
- * Returns:     Result, %$a^e \bmod m$%.
- */
-
-mp *mpbarrett_exp(mpbarrett *mb, mp *d, mp *a, mp *e)
-{
-  mpscan sc;
-  mp *x = MP_ONE;
-  mp *spare = (e->f & MP_BURN) ? MP_NEWSEC : MP_NEW;
-  unsigned sq = 0;
-
-  a = MP_COPY(a);
-  mp_rscan(&sc, e);
-  if (!MP_RSTEP(&sc))
-    goto exit;
-  while (!MP_RBIT(&sc))
-    MP_RSTEP(&sc);
-
-  /* --- Do the main body of the work --- */
-
-  for (;;) {
-    sq++;
-    while (sq) {
-      mp *y;
-      y = mp_sqr(spare, x);
-      y = mpbarrett_reduce(mb, y, y);
-      spare = x; x = y;
-      sq--;
-    }
-    {
-      mp *y = mp_mul(spare, x, a);
-      y = mpbarrett_reduce(mb, y, y);
-      spare = x; x = y;
-    }
-    for (;;) {
-      if (!MP_RSTEP(&sc))
-	goto done;
-      if (MP_RBIT(&sc))
-	break;
-      sq++;
-    }
+  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;
   }
 
-  /* --- Do a final round of squaring --- */
-
-done:
-  while (sq) {
-    mp *y;
-    y = mp_sqr(spare, x);
-    y = mpbarrett_reduce(mb, y, y);
-    spare = x; x = y;
-    sq--;
-  }  
-
-exit:
-  MP_DROP(a);
-  if (spare != MP_NEW)
-    MP_DROP(spare);
-  if (d != MP_NEW)
-    MP_DROP(d);
-  return (x);
+  MP_SHRINK(d);
+  return (d);
 }
 
 /*----- Test rig ----------------------------------------------------------*/
@@ -262,7 +215,7 @@ static int vmod(dstr *v)
   mpbarrett_create(&mb, n);
   s = mpbarrett_reduce(&mb, MP_NEW, x);
 
-  if (MP_CMP(s, !=, r)) {
+  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);
@@ -280,44 +233,8 @@ static int vmod(dstr *v)
   return (ok);
 }
 
-static int vexp(dstr *v)
-{
-  mp *m = *(mp **)v[0].buf;
-  mp *a = *(mp **)v[1].buf;
-  mp *b = *(mp **)v[2].buf;
-  mp *r = *(mp **)v[3].buf;
-  mp *mr;
-  int ok = 1;
-
-  mpbarrett mb;
-  mpbarrett_create(&mb, m);
-
-  mr = mpbarrett_exp(&mb, MP_NEW, a, b);
-
-  if (MP_CMP(mr, !=, r)) {
-    fputs("\n*** barrett modexp failed", stderr);
-    fputs("\n m = ", stderr); mp_writefile(m, stderr, 10);
-    fputs("\n a = ", stderr); mp_writefile(a, stderr, 10);
-    fputs("\n e = ", stderr); mp_writefile(b, stderr, 10);
-    fputs("\n r = ", stderr); mp_writefile(r, stderr, 10);
-    fputs("\nmr = ", stderr); mp_writefile(mr, stderr, 10);
-    fputc('\n', stderr);
-    ok = 0;
-  }
-
-  mp_drop(m);
-  mp_drop(a);
-  mp_drop(b);
-  mp_drop(r);
-  mp_drop(mr);
-  mpbarrett_destroy(&mb);
-  assert(mparena_count(MPARENA_GLOBAL) == 0);
-  return ok;
-}
-
 static test_chunk tests[] = {
   { "mpbarrett-reduce", vmod, { &type_mp, &type_mp, &type_mp, 0 } },
-  { "mpbarrett-exp", vexp, { &type_mp, &type_mp, &type_mp, &type_mp, 0 } },
   { 0, 0, { 0 } }
 };