X-Git-Url: https://git.distorted.org.uk/u/mdw/catacomb/blobdiff_plain/1ba69a1e60cc3e3b8e1ddff0689ae537e306299b..962dd3329d51f1d18313a17eb0cb4695ee4421a0:/mpbarrett.c

diff --git a/mpbarrett.c b/mpbarrett.c
index 889b268..b38aa91 100644
--- a/mpbarrett.c
+++ b/mpbarrett.c
@@ -1,6 +1,6 @@
 /* -*-c-*-
  *
- * $Id: mpbarrett.c,v 1.7 2001/04/19 18:25:26 mdw Exp $
+ * $Id: mpbarrett.c,v 1.8 2001/06/16 13:00:20 mdw Exp $
  *
  * Barrett modular reduction
  *
@@ -30,6 +30,9 @@
 /*----- Revision history --------------------------------------------------* 
  *
  * $Log: mpbarrett.c,v $
+ * 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.
  *
@@ -61,6 +64,7 @@
 
 #include "mp.h"
 #include "mpbarrett.h"
+#include "mpbarrett-exp.h"
 
 /*----- Main code ---------------------------------------------------------*/
 
@@ -199,189 +203,18 @@ mp *mpbarrett_reduce(mpbarrett *mb, mp *d, mp *m)
  * Returns:     Result, %$a^e \bmod m$%.
  */
 
-#define WINSZ 5
-#define TABSZ (1 << (WINSZ - 1))
-
-#define THRESH (((MPW_BITS / WINSZ) << 2) + 1)
-
-static mp *exp_simple(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++;
-    }
-  }
-
-  /* --- 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 *mpbarrett_exp(mpbarrett *mb, mp *d, mp *a, mp *e)
 {
-  mp **tab;
-  mp *a2;
-  mp *spare = (e->f & MP_BURN) ? MP_NEWSEC : MP_NEW;
   mp *x = MP_ONE;
-  unsigned i, sq = 0;
-  mpscan sc;
-
-  /* --- Do we bother? --- */
+  mp *spare = (e->f & MP_BURN) ? MP_NEWSEC : MP_NEW;
 
   MP_SHRINK(e);
-  if (MP_LEN(e) == 0)
-    goto exit;
-  if (MP_LEN(e) < THRESH) {
-    x->ref--;
-    return (exp_simple(mb, d, a, e));
-  }
-
-  /* --- Do the precomputation --- */
-
-  a2 = mp_sqr(MP_NEW, a);
-  a2 = mpbarrett_reduce(mb, a2, a2);
-  tab = xmalloc(TABSZ * sizeof(mp *));
-  tab[0] = MP_COPY(a);
-  for (i = 1; i < TABSZ; i++) {
-    mp *x = mp_mul(MP_NEW, tab[i - 1], a2);
-    tab[i] = mpbarrett_reduce(mb, x, x);
-  }
-  mp_drop(a2);
-  mp_rscan(&sc, e);
-  
-  /* --- Skip top-end zero bits --- *
-   *
-   * If the initial step worked, there must be a set bit somewhere, so keep
-   * stepping until I find it.
-   */
-
-  MP_RSTEP(&sc);
-  while (!MP_RBIT(&sc))
-    MP_RSTEP(&sc);
-
-  /* --- Now for the main work --- */
-
-  for (;;) {
-    unsigned l = 0;
-    unsigned z = 0;
-
-    /* --- The next bit is set, so read a window index --- *
-     *
-     * Reset @i@ to zero and increment @sq@.  Then, until either I read
-     * @WINSZ@ bits or I run out of bits, scan in a bit: if it's clear, bump
-     * the @z@ counter; if it's set, push a set bit into @i@, shift it over
-     * by @z@ bits, bump @sq@ by @z + 1@ and clear @z@.  By the end of this
-     * palaver, @i@ is an index to the precomputed value in @tab@.
-     */
-
-    i = 0;
-    sq++;
-    for (;;) {
-      l++;
-      if (l >= WINSZ || !MP_RSTEP(&sc))
-	break;
-      if (!MP_RBIT(&sc))
-	z++;
-      else {
-	i = ((i << 1) | 1) << z;
-	sq += z + 1;
-	z = 0;
-      }
-    }
-
-    /* --- Do the squaring --- *
-     *
-     * Remember that @sq@ carries over from the zero-skipping stuff below.
-     */
-
-    while (sq) {
-      mp *y;
-      y = mp_sqr(spare, x);
-      y = mpbarrett_reduce(mb, y, y);
-      spare = x; x = y;
-      sq--;
-    }
-
-    /* --- Do the multiply --- */
-
-    { mp *y = mp_mul(spare, x, tab[i]); spare = x;
-      x = mpbarrett_reduce(mb, y, y); }
-
-    /* --- Now grind along through the rest of the bits --- */
-
-    sq = z;
-    for (;;) {
-      if (!MP_RSTEP(&sc))
-	goto done;
-      if (MP_RBIT(&sc))
-	break;
-      sq++;
-    }
-  }
-
-  /* --- 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--;
-  }
-
-  /* --- Done --- */
-
-  for (i = 0; i < TABSZ; i++)
-    mp_drop(tab[i]);
-  xfree(tab);
-exit:
+  if (!MP_LEN(e))
+    ;
+  else if (MP_LEN(e) < EXP_THRESH)
+    EXP_SIMPLE(x, a, e);
+  else
+    EXP_WINDOW(x, a, e);
   mp_drop(d);
   mp_drop(spare);
   return (x);