1 files changed, 93 insertions, 0 deletions
diff --git a/count-delta.c b/count-delta.c
new file mode 100644
index 0000000..dd81e92
--- /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;
+}