format/packfile: implement delta encoding (#172)

* format/packfile: implement delta encoding

- Added all the logic to the encoder to be able to encode ref-delta and offset-delta objects
- Created plumbing.ObjectToPack to handle deltas and standard objects when we are writting them into a packfile
- Added specific encoder delta tests, one standard object and one delta, and one standard object and two deltas

* Requested changes.

* Requested changes

2 files changed, 30 insertions, 0 deletions

diff --git a/utils/binary/write.go b/utils/binary/write.go
index 3ea1d91..2ec3581 100644
--- a/utils/binary/write.go
+++ b/utils/binary/write.go
@@ -17,6 +17,20 @@ func Write(w io.Writer, data ...interface{}) error {
 	return nil
 }
 
+func WriteVariableWidthInt(w io.Writer, n int64) error {
+	buf := []byte{byte(n & 0x7f)}
+	n >>= 7
+	for n != 0 {
+		n--
+		buf = append([]byte{0x80 | (byte(n & 0x7f))}, buf...)
+		n >>= 7
+	}
+
+	_, err := w.Write(buf)
+
+	return err
+}
+
 // WriteUint32 writes the binary representation of a uint32 into w, in BigEndian
 // order
 func WriteUint32(w io.Writer, value uint32) error {
diff --git a/utils/binary/writer_test.go b/utils/binary/writer_test.go
index 88140a1..1380280 100644
--- a/utils/binary/writer_test.go
+++ b/utils/binary/writer_test.go
@@ -41,3 +41,19 @@ func (s *BinarySuite) TestWriteUint16(c *C) {
 	c.Assert(err, IsNil)
 	c.Assert(buf, DeepEquals, expected)
 }
+
+func (s *BinarySuite) TestWriteVariableWidthInt(c *C) {
+	buf := bytes.NewBuffer(nil)
+
+	err := WriteVariableWidthInt(buf, 366)
+	c.Assert(err, IsNil)
+	c.Assert(buf.Bytes(), DeepEquals, []byte{129, 110})
+}
+
+func (s *BinarySuite) TestWriteVariableWidthIntShort(c *C) {
+	buf := bytes.NewBuffer(nil)
+
+	err := WriteVariableWidthInt(buf, 19)
+	c.Assert(err, IsNil)
+	c.Assert(buf.Bytes(), DeepEquals, []byte{19})
+}