Use a second, less quick upper bound based on character occurrence counts.

diff --git a/ChangeLog b/ChangeLog
index eae0145..375d43f 100644 (file)
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2008-09-15  Ralf Wildenhues  <Ralf.Wildenhues@gmx.de>
+
+       * lib/fstrcmp.c (fstrcmp_bounded): Use a second, less quick upper bound
+       based on character occurrence counts.
+
 2008-09-15  Eric Blake  <ebb9@byu.net>
 
        tests: avoid some compiler warnings

diff --git a/lib/fstrcmp.c b/lib/fstrcmp.c
index cd0d7b7..796c5e8 100644 (file)
--- a/lib/fstrcmp.c
+++ b/lib/fstrcmp.c
@@ -141,6 +141,55 @@ fstrcmp_bounded (const char *string1, const char *string2, double lower_bound)
       if (upper_bound < lower_bound)
        /* Return an arbitrary value < LOWER_BOUND.  */
        return 0.0;
+
+#if CHAR_BIT <= 8
+      /* When X and Y are both small, avoid the overhead of setting up an
+        array of size 256.  */
+      if (xvec_length + yvec_length >= 20)
+       {
+         /* Compute a less quick upper bound.
+            Each edit is an insertion or deletion of a character, hence
+            modifies the occurrence count of a character by 1 and leaves the
+            other occurrence counts unchanged.
+            Therefore, when starting from a sequence X and ending at a
+            sequence Y, and denoting the occurrence count of C in X with
+            OCC (X, C), with N edits,
+              sum_C | OCC (X, C) - OCC (Y, C) | <= N.
+            (Proof by induction over N.)
+            So, at the end, we will have
+              edit_count >= sum_C | OCC (X, C) - OCC (Y, C) |,
+            and hence
+              result
+                = (xvec_length + yvec_length - edit_count)
+                  / (xvec_length + yvec_length)
+                <= (xvec_length + yvec_length - sum_C | OCC(X,C) - OCC(Y,C) |)
+                   / (xvec_length + yvec_length).
+          */
+         int occ_diff[UCHAR_MAX + 1]; /* array C -> OCC(X,C) - OCC(Y,C) */
+         int sum;
+
+         /* Determine the occurrence counts in X.  */
+         memset (occ_diff, 0, sizeof (occ_diff));
+         for (i = xvec_length - 1; i >= 0; i--)
+           occ_diff[(unsigned char) string1[i]]++;
+         /* Subtract the occurrence counts in Y.  */
+         for (i = yvec_length - 1; i >= 0; i--)
+           occ_diff[(unsigned char) string2[i]]--;
+         /* Sum up the absolute values.  */
+         sum = 0;
+         for (i = 0; i <= UCHAR_MAX; i++)
+           {
+             int d = occ_diff[i];
+             sum += (d >= 0 ? d : -d);
+           }
+
+         upper_bound = 1.0 - (double) sum / (xvec_length + yvec_length);
+
+         if (upper_bound < lower_bound)
+           /* Return an arbitrary value < LOWER_BOUND.  */
+           return 0.0;
+       }
+#endif
     }
 
   /* set the info for each string.  */