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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>
This commit is contained in:
Davide Libenzi 2008-11-07 21:24:33 -08:00 committed by Junio C Hamano
parent 0fd9d7e66d
commit 733070bea9
1 changed files with 13 additions and 2 deletions
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15
xdiff/xprepare.c
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@ -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).