diff --git a/Makefile b/Makefile
index b957cec1a0..92d0e87535 100644
--- a/Makefile
+++ b/Makefile
@@ -46,8 +46,9 @@ LIB_H=cache.h object.h blob.h tree.h commit.h tag.h delta.h
 LIB_H += strbuf.h
 LIB_OBJS += strbuf.o
 
-LIB_H += diff.h
-LIB_OBJS += diff.o diffcore-rename.o diffcore-pickaxe.o diffcore-pathspec.o
+LIB_H += diff.h count-delta.h
+LIB_OBJS += diff.o diffcore-rename.o diffcore-pickaxe.o diffcore-pathspec.o \
+	count-delta.o
 
 LIB_OBJS += gitenv.o
 
diff --git a/count-delta.c b/count-delta.c
new file mode 100644
index 0000000000..dd81e92965
--- /dev/null
+++ b/count-delta.c
@@ -0,0 +1,93 @@
+/*
+ * Copyright (C) 2005 Junio C Hamano
+ * The delta-parsing part is almost straight copy of patch-delta.c
+ * which is (C) 2005 Nicolas Pitre <nico@cam.org>.
+ */
+#include <stdlib.h>
+#include <string.h>
+#include <limits.h>
+#include "delta.h"
+#include "count-delta.h"
+
+static unsigned long get_hdr_size(const unsigned char **datap)
+{
+	const unsigned char *data = *datap;
+	unsigned long size;
+	unsigned char cmd;
+	int i;
+	size = i = 0;
+	cmd = *data++;
+	while (cmd) {
+		if (cmd & 1)
+			size |= *data++ << i;
+		i += 8;
+		cmd >>= 1;
+	}
+	*datap = data;
+	return size;
+}
+
+/*
+ * NOTE.  We do not _interpret_ delta fully.  As an approximation, we
+ * just count the number of bytes that are copied from the source, and
+ * the number of literal data bytes that are inserted.  Number of
+ * bytes that are _not_ copied from the source is deletion, and number
+ * of inserted literal bytes are addition, so sum of them is what we
+ * return.  xdelta can express an edit that copies data inside of the
+ * destination which originally came from the source.  We do not count
+ * that in the following routine, so we are undercounting the source
+ * material that remains in the final output that way.
+ */
+unsigned long count_delta(void *delta_buf, unsigned long delta_size)
+{
+	unsigned long copied_from_source, added_literal;
+	const unsigned char *data, *top;
+	unsigned char cmd;
+	unsigned long src_size, dst_size, out;
+
+	/* the smallest delta size possible is 6 bytes */
+	if (delta_size < 6)
+		return UINT_MAX;
+
+	data = delta_buf;
+	top = delta_buf + delta_size;
+
+	src_size = get_hdr_size(&data);
+	dst_size = get_hdr_size(&data);
+
+	added_literal = copied_from_source = out = 0;
+	while (data < top) {
+		cmd = *data++;
+		if (cmd & 0x80) {
+			unsigned long cp_off = 0, cp_size = 0;
+			if (cmd & 0x01) cp_off = *data++;
+			if (cmd & 0x02) cp_off |= (*data++ << 8);
+			if (cmd & 0x04) cp_off |= (*data++ << 16);
+			if (cmd & 0x08) cp_off |= (*data++ << 24);
+			if (cmd & 0x10) cp_size = *data++;
+			if (cmd & 0x20) cp_size |= (*data++ << 8);
+			if (cp_size == 0) cp_size = 0x10000;
+
+			if (cmd & 0x40)
+				/* copy from dst */
+				;
+			else
+				copied_from_source += cp_size;
+			out += cp_size;
+		} else {
+			/* write literal into dst */
+			added_literal += cmd;
+			out += cmd;
+			data += cmd;
+		}
+	}
+
+	/* sanity check */
+	if (data != top || out != dst_size)
+		return UINT_MAX;
+
+	/* delete size is what was _not_ copied from source.
+	 * edit size is that and literal additions.
+	 */
+	return (src_size - copied_from_source) + added_literal;
+}
diff --git a/count-delta.h b/count-delta.h
new file mode 100644
index 0000000000..4e6b584f43
--- /dev/null
+++ b/count-delta.h
@@ -0,0 +1,9 @@
+/*
+ * Copyright (C) 2005 Junio C Hamano
+ */
+#ifndef COUNT_DELTA_H
+#define COUNT_DELTA_H
+
+unsigned long count_delta(void *, unsigned long);
+
+#endif
diff --git a/diffcore-rename.c b/diffcore-rename.c
index 34e83dac8d..07782f4b7b 100644
--- a/diffcore-rename.c
+++ b/diffcore-rename.c
@@ -5,6 +5,7 @@
 #include "diff.h"
 #include "diffcore.h"
 #include "delta.h"
+#include "count-delta.h"
 
 /* Table of rename/copy destinations */
 
@@ -158,13 +159,18 @@ static int estimate_similarity(struct diff_filespec *src,
 	delta = diff_delta(src->data, src->size,
 			   dst->data, dst->size,
 			   &delta_size);
-	/*
-	 * We currently punt here, but we may later end up parsing the
-	 * delta to really assess the extent of damage.  A big consecutive
-	 * remove would produce small delta_size that affects quite a
-	 * big portion of the file.
+
+	/* A delta that has a lot of literal additions would have
+	 * big delta_size no matter what else it does.
 	 */
+	if (minimum_score < MAX_SCORE * delta_size / base_size)
+		return 0;
+
+	/* Estimate the edit size by interpreting delta. */
+	delta_size = count_delta(delta, delta_size);
 	free(delta);
+	if (delta_size == UINT_MAX)
+		return 0;
 
 	/*
 	 * Now we will give some score to it.  100% edit gets 0 points