1 files changed, 33 insertions, 1 deletions

diff --git a/xdiff-interface.c b/xdiff-interface.c
index 69a022c134..f2cd488de0 100644
--- a/xdiff-interface.c
+++ b/xdiff-interface.c
@@ -103,9 +103,41 @@ int xdiff_outf(void *priv_, mmbuffer_t *mb, int nbuf)
 	return 0;
 }
 
+/*
+ * Trim down common substring at the end of the buffers,
+ * but leave at least ctx lines at the end.
+ */
+static void trim_common_tail(mmfile_t *a, mmfile_t *b, int ctx)
+{
+	const int blk = 1024;
+	long trimmed = 0, recovered = 0;
+	int i;
+	char *ap = a->ptr + a->size;
+	char *bp = b->ptr + b->size;
+	long smaller = (a->size < b->size) ? a->size : b->size;
+
+	while (blk + trimmed <= smaller && !memcmp(ap - blk, bp - blk, blk)) {
+		trimmed += blk;
+		ap -= blk;
+		bp -= blk;
+	}
+
+	for (i = 0, recovered = 0; recovered < trimmed && i <= ctx; i++) {
+		while (recovered < trimmed && ap[recovered] != '\n')
+			recovered++;
+	}
+	a->size -= (trimmed - recovered);
+	b->size -= (trimmed - recovered);
+}
+
 int xdi_diff(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp, xdemitconf_t const *xecfg, xdemitcb_t *xecb)
 {
-	return xdl_diff(mf1, mf2, xpp, xecfg, xecb);
+	mmfile_t a = *mf1;
+	mmfile_t b = *mf2;
+
+	trim_common_tail(&a, &b, xecfg->ctxlen);
+
+	return xdl_diff(&a, &b, xpp, xecfg, xecb);
 }
 
 int read_mmfile(mmfile_t *ptr, const char *filename)