From 30cbe7a7d521a9c1806caba10fba69f7335a9757 Mon Sep 17 00:00:00 2001
From: mdw <mdw>
Date: Sun, 8 Oct 2000 12:03:44 +0000
Subject: [PATCH] (mpbarrett_reduce): Cope with negative numbers.

---
 mpbarrett.c | 21 +++++++++++++++------
 mpbarrett.h |  8 +++++---
 2 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/mpbarrett.c b/mpbarrett.c
index affbc65..0af753e 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 1.6 2000/10/08 12:03:44 mdw Exp $
  *
  * Barrett modular reduction
  *
@@ -30,6 +30,9 @@
 /*----- Revision history --------------------------------------------------* 
  *
  * $Log: mpbarrett.c,v $
+ * 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 +117,7 @@ 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 +163,7 @@ 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);
@@ -173,6 +175,13 @@ 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);
 
+  /* --- 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);
 }
@@ -262,7 +271,7 @@ 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);
@@ -294,7 +303,7 @@ static int vexp(dstr *v)
 
   mr = mpbarrett_exp(&mb, MP_NEW, a, b);
 
-  if (MP_CMP(mr, !=, r)) {
+  if (!MP_EQ(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);
diff --git a/mpbarrett.h b/mpbarrett.h
index d9c02ad..aa1971e 100644
--- a/mpbarrett.h
+++ b/mpbarrett.h
@@ -1,6 +1,6 @@
 /* -*-c-*-
  *
- * $Id: mpbarrett.h,v 1.1 1999/12/10 23:22:00 mdw Exp $
+ * $Id: mpbarrett.h,v 1.2 2000/10/08 12:03:44 mdw Exp $
  *
  * Barrett modular reduction
  *
@@ -30,6 +30,9 @@
 /*----- Revision history --------------------------------------------------* 
  *
  * $Log: mpbarrett.h,v $
+ * Revision 1.2  2000/10/08 12:03:44  mdw
+ * (mpbarrett_reduce): Cope with negative numbers.
+ *
  * Revision 1.1  1999/12/10 23:22:00  mdw
  * Barrett reduction support: works with even moduli.
  *
@@ -109,8 +112,7 @@ extern 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.
  */
 
 extern mp *mpbarrett_reduce(mpbarrett */*mb*/, mp */*d*/, mp */*m*/);
-- 
2.11.0