Bug fix: minimize the right thing.

[u/mdw/catacomb] / mpbarrett.c

diff --git a/mpbarrett.c b/mpbarrett.c
index e438d02..affbc65 100644 (file)
--- a/mpbarrett.c
+++ b/mpbarrett.c
@@ -1,6 +1,6 @@
 /* -*-c-*-
  *
- * $Id: mpbarrett.c,v 1.1 1999/12/10 23:21:59 mdw Exp $
+ * $Id: mpbarrett.c,v 1.5 2000/07/29 17:04:33 mdw Exp $
  *
  * Barrett modular reduction
  *
@@ -30,6 +30,22 @@
 /*----- Revision history --------------------------------------------------* 
  *
  * $Log: mpbarrett.c,v $
+ * 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.
+ *
+ * Revision 1.4  2000/06/17 11:45:09  mdw
+ * Major memory management overhaul.  Added arena support.  Use the secure
+ * arena for secret integers.  Replace and improve the MP management macros
+ * (e.g., replace MP_MODIFY by MP_DEST).
+ *
+ * Revision 1.3  1999/12/12 15:08:52  mdw
+ * Don't bother shifting %$q$% in @mpbarrett_reduce@, just skip the least
+ * significant digits.
+ *
+ * Revision 1.2  1999/12/11 01:50:56  mdw
+ * Improve initialization slightly.
+ *
  * Revision 1.1  1999/12/10 23:21:59  mdw
  * Barrett reduction support: works with even moduli.
  *
@@ -66,7 +82,9 @@ void mpbarrett_create(mpbarrett *mb, mp *m)
   mp_shrink(m);
   mb->k = MP_LEN(m);
   mb->m = MP_COPY(m);
-  b = mp_lsl(MP_NEW, MP_ONE, 2 * MPW_BITS * mb->k);
+  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;
 }
@@ -109,7 +127,8 @@ mp *mpbarrett_reduce(mpbarrett *mb, mp *d, mp *m)
 
   if (MP_LEN(m) < k) {
     m = MP_COPY(m);
-    MP_DROP(d);
+    if (d)
+      MP_DROP(d);
     return (m);
   }
 
@@ -119,7 +138,12 @@ mp *mpbarrett_reduce(mpbarrett *mb, mp *d, mp *m)
     mp qq;
     mp_build(&qq, m->v + (k - 1), m->vl);
     q = mp_mul(MP_NEW, &qq, mb->mu);
-    q = mp_lsr(q, q, MPW_BITS * (k + 1));
+    if (MP_LEN(q) <= k) {
+      m = MP_COPY(m);
+      if (d)
+       MP_DROP(d);
+      return (m);
+    }
   }
 
   /* --- Second stage --- */
@@ -133,10 +157,9 @@ mp *mpbarrett_reduce(mpbarrett *mb, mp *d, mp *m)
       mvl = m->vl;
     else
       mvl = m->v + k + 1;
-    r = mp_create(k + 1);
-    mpx_umul(r->v, r->vl, q->v, q->vl, mb->m->v, mb->m->vl);
-    r->f = (q->f | mb->m->f) & MP_BURN;
-    MP_MODIFY(d, 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);
     mpx_usub(d->v, d->vl, m->v, mvl, r->v, r->vl);
     d->f = (m->f | r->f) & MP_BURN;
     MP_DROP(r);
@@ -168,31 +191,53 @@ mp *mpbarrett_exp(mpbarrett *mb, mp *d, mp *a, mp *e)
 {
   mpscan sc;
   mp *x = MP_ONE;
-  mp *spare = MP_NEW;
+  mp *spare = (e->f & MP_BURN) ? MP_NEWSEC : MP_NEW;
+  unsigned sq = 0;
 
   a = MP_COPY(a);
-  mp_scan(&sc, e);
-  if (MP_STEP(&sc)) {
-    size_t sq = 0;
+  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 (;;) {
-      mp *dd;
-      if (MP_BIT(&sc)) {
-        while (sq) {
-          dd = mp_sqr(spare, a);
-          dd = mpbarrett_reduce(mb, dd, dd);
-          spare = a; a = dd;
-          sq--;
-        }
-        dd = mp_mul(spare, x, a);
-       dd = mpbarrett_reduce(mb, dd, dd);
-        spare = x; x = dd;
-      }
+      if (!MP_RSTEP(&sc))
+       goto done;
+      if (MP_RBIT(&sc))
+       break;
       sq++;
-      if (!MP_STEP(&sc))
-        break;
     }
   }
 
+  /* --- 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);