xdiff: give up scanning similar lines early

In a corner case of large files whose lines do not match uniquely, the
loop to eliminate a line that matches multiple locations adjacent to a run
of lines that do not uniquely match wasted too much cycles.  Fix this by
giving up early after scanning 100 lines in both direction.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/xdiff/xprepare.c b/xdiff/xprepare.c
index e87ab57c652b56b1a684e2a0a56885c1d1b27ef7..a43aa72cd088af07b13a6bc8de304ecc178047e5 100644 (file)
--- a/xdiff/xprepare.c
+++ b/xdiff/xprepare.c
@@ -23,10 +23,9 @@
 #include "xinclude.h"
 
 
-
 #define XDL_KPDIS_RUN 4
 #define XDL_MAX_EQLIMIT 1024
-
+#define XDL_SIMSCAN_WINDOW 100
 
 
 typedef struct s_xdlclass {
@@ -312,6 +311,18 @@ void xdl_free_env(xdfenv_t *xe) {
 static int xdl_clean_mmatch(char const *dis, long i, long s, long e) {
        long r, rdis0, rpdis0, rdis1, rpdis1;
 
+       /*
+        * Limits the window the is examined during the similar-lines
+        * scan. The loops below stops when dis[i - r] == 1 (line that
+        * has no match), but there are corner cases where the loop
+        * proceed all the way to the extremities by causing huge
+        * performance penalties in case of big files.
+        */
+       if (i - s > XDL_SIMSCAN_WINDOW)
+               s = i - XDL_SIMSCAN_WINDOW;
+       if (e - i > XDL_SIMSCAN_WINDOW)
+               e = i + XDL_SIMSCAN_WINDOW;
+
        /*
         * Scans the lines before 'i' to find a run of lines that either
         * have no match (dis[j] == 0) or have multiple matches (dis[j] > 1).