X-Git-Url: https://git.distorted.org.uk/u/mdw/catacomb/blobdiff_plain/a86e33af1ea11503350b37dbf64a7518c27b583e..674cd11ec63b56561980249cb19a0db54bacfa86:/mpx-kmul.c

diff --git a/mpx-kmul.c b/mpx-kmul.c
index d97700f..d53622b 100644
--- a/mpx-kmul.c
+++ b/mpx-kmul.c
@@ -1,6 +1,6 @@
 /* -*-c-*-
  *
- * $Id: mpx-kmul.c,v 1.1 1999/12/10 23:23:51 mdw Exp $
+ * $Id: mpx-kmul.c,v 1.3 1999/12/13 15:35:01 mdw Exp $
  *
  * Karatsuba's multiplication algorithm
  *
@@ -30,6 +30,12 @@
 /*----- Revision history --------------------------------------------------* 
  *
  * $Log: mpx-kmul.c,v $
+ * Revision 1.3  1999/12/13 15:35:01  mdw
+ * Simplify and improve.
+ *
+ * Revision 1.2  1999/12/11 10:58:02  mdw
+ * Remove tweakable comments.
+ *
  * Revision 1.1  1999/12/10 23:23:51  mdw
  * Karatsuba-Ofman multiplication algorithm.
  *
@@ -37,19 +43,13 @@
 
 /*----- Header files ------------------------------------------------------*/
 
+#include <assert.h>
 #include <stdio.h>
 
 #include "mpx.h"
 
 /*----- Tweakables --------------------------------------------------------*/
 
-/* --- @KARATSUBA_CUTOFF@ --- *
- *
- * If either of the arguments to @mpx_kmul@ contains this number of words or
- * fewer, the job is dumped out to @mpx_umul@ instead.  Reduce the size when
- * testing, to ensure better coverage.
- */
-
 #ifdef TEST_RIG
 #  undef KARATSUBA_CUTOFF
 #  define KARATSUBA_CUTOFF 2
@@ -144,10 +144,6 @@
  *		The destination must be twice as large as the larger
  *		argument.  The scratch space must be twice as large as the
  *		larger argument, plus the magic number @KARATSUBA_SLOP@.
- *		(Actually, a number of words proportional to the depth of
- *		recursion, but since recusion is strongly bounded by memory,
- *		I can replace it with a constant as long as it's `big
- *		enough'.)
  */
 
 void mpx_kmul(mpw *dv, mpw *dvl,
@@ -209,64 +205,51 @@ void mpx_kmul(mpw *dv, mpw *dvl,
       avm = avl;
   }
 
-  /* --- Sort out the middle term --- *
-   *
-   * I'm going to keep track of the carry by hand rather than pass it down to
-   * the next level, because it means multiplication by one or zero, which I
-   * can do easily myself.
-   */
+  assert(((void)"Destination too small for Karatsuba multiply",
+	  dvl - dv >= 4 * m));
+  assert(((void)"Not enough workspace for Karatsuba multiply",
+	  svl - sv >= 4 * m));
+
+  /* --- Sort out the middle term --- */
 
   {
-    unsigned f = 0;
-    enum {
-      carry_a = 1,
-      carry_b = 2
-    };
-
-    mpw *bsv = sv + m, *ssv = bsv + m;
-    mpw *rdv = dv + m, *rdvl = rdv + 2 * m;
-
-    UADD2(sv, bsv + 1, av, avm, avm, avl);
-    if (*bsv)
-      f |= carry_a;
-    UADD2(bsv, ssv + 1, bv, bvm, bvm, bvl);
-    if (*ssv)
-      f |= carry_b;
-    MPX_ZERO(dv, rdv);
+    mpw *bsv = sv + m + 1, *ssv = bsv + m + 1;
+    mpw *rdv = dv + m, *rdvl = rdv + 2 * (m + 2);
+
+    UADD2(sv, bsv, av, avm, avm, avl);
+    UADD2(bsv, ssv, bv, bvm, bvm, bvl);
     if (m > KARATSUBA_CUTOFF)
       mpx_kmul(rdv, rdvl, sv, bsv, bsv, ssv, ssv, svl);
     else
       mpx_umul(rdv, rdvl, sv, bsv, bsv, ssv);
-    MPX_ZERO(rdvl, dvl);
-    rdv += m; rdvl += m;
-    if (f & carry_b)
-      UADD(rdv, sv, bsv);
-    if (f & carry_a)
-      UADD(rdv, bsv, ssv);
-    if (!(~f & (carry_a | carry_b)))
-      MPX_UADDN(rdv + m, rdvl, 1);
   }
 
   /* --- Sort out the other two terms --- */
 
   {
-    mpw *ssv = sv + 2 * m;
+    mpw *svm = sv + m, *svn = svm + m, *ssv = svn + 4;
     mpw *tdv = dv + m;
     mpw *rdv = tdv + m;
 
-    if (m > KARATSUBA_CUTOFF)
-      mpx_kmul(sv, ssv, avm, avl, bvm, bvl, ssv, svl);
-    else
-      mpx_umul(sv, ssv, avm, avl, bvm, bvl);
-    UADD(rdv, sv, ssv);
-    USUB(tdv, sv, ssv);
-    
+    if (avl == avm || bvl == bvm)
+      MPX_ZERO(rdv + m + 1, dvl);
+    else {
+      if (m > KARATSUBA_CUTOFF)
+	mpx_kmul(sv, ssv, avm, avl, bvm, bvl, ssv, svl);
+      else
+	mpx_umul(sv, ssv, avm, avl, bvm, bvl);
+      MPX_COPY(rdv + m + 1, dvl, svm + 1, svn);
+      UADD(rdv, sv, svm + 1);
+      USUB(tdv, sv, svn);
+    }
+
     if (m > KARATSUBA_CUTOFF)
       mpx_kmul(sv, ssv, av, avm, bv, bvm, ssv, svl);
     else
       mpx_umul(sv, ssv, av, avm, bv, bvm);
-    USUB(tdv, sv, ssv);
-    UADD(dv, sv, ssv);
+    MPX_COPY(dv, tdv, sv, svm);
+    USUB(tdv, sv, svn);
+    UADD(tdv, svm, svn);
   }
 }