1 files changed, 93 insertions, 0 deletions
diff --git a/packfile/delta.go b/packfile/delta.go
new file mode 100644
index 0000000..86b556f
--- /dev/null
+++ b/packfile/delta.go
@@ -0,0 +1,93 @@
+package packfile
+
+const delta_size_min = 4
+
+func deltaHeaderSize(b []byte) (uint, []byte) {
+	var size, j uint
+	var cmd byte
+	for {
+		cmd = b[j]
+		size |= (uint(cmd) & 0x7f) << (j * 7)
+		j++
+		if uint(cmd)&0xb80 == 0 || j == uint(len(b)) {
+			break
+		}
+	}
+	return size, b[j:]
+}
+
+func PatchDelta(src, delta []byte) []byte {
+	if len(delta) < delta_size_min {
+		return nil
+	}
+	size, delta := deltaHeaderSize(delta)
+	if size != uint(len(src)) {
+		return nil
+	}
+	size, delta = deltaHeaderSize(delta)
+	origSize := size
+
+	dest := make([]byte, 0)
+
+	// var offset uint
+	var cmd byte
+	for {
+		cmd = delta[0]
+		delta = delta[1:]
+		if (cmd & 0x80) != 0 {
+			var cp_off, cp_size uint
+			if (cmd & 0x01) != 0 {
+				cp_off = uint(delta[0])
+				delta = delta[1:]
+			}
+			if (cmd & 0x02) != 0 {
+				cp_off |= uint(delta[0]) << 8
+				delta = delta[1:]
+			}
+			if (cmd & 0x04) != 0 {
+				cp_off |= uint(delta[0]) << 16
+				delta = delta[1:]
+			}
+			if (cmd & 0x08) != 0 {
+				cp_off |= uint(delta[0]) << 24
+				delta = delta[1:]
+			}
+
+			if (cmd & 0x10) != 0 {
+				cp_size = uint(delta[0])
+				delta = delta[1:]
+			}
+			if (cmd & 0x20) != 0 {
+				cp_size |= uint(delta[0]) << 8
+				delta = delta[1:]
+			}
+			if (cmd & 0x40) != 0 {
+				cp_size |= uint(delta[0]) << 16
+				delta = delta[1:]
+			}
+			if cp_size == 0 {
+				cp_size = 0x10000
+			}
+			if cp_off+cp_size < cp_off ||
+				cp_off+cp_size > uint(len(src)) ||
+				cp_size > origSize {
+				break
+			}
+			dest = append(dest, src[cp_off:cp_off+cp_size]...)
+			size -= cp_size
+		} else if cmd != 0 {
+			if uint(cmd) > origSize {
+				break
+			}
+			dest = append(dest, delta[0:uint(cmd)]...)
+			size -= uint(cmd)
+			delta = delta[uint(cmd):]
+		} else {
+			return nil
+		}
+		if size <= 0 {
+			break
+		}
+	}
+	return dest
+}