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

1 files changed, 47 insertions, 0 deletions

diff --git a/plumbing/format/packfile/object_pack.go b/plumbing/format/packfile/object_pack.go
new file mode 100644
index 0000000..dc5a3c7
--- /dev/null
+++ b/plumbing/format/packfile/object_pack.go
@@ -0,0 +1,47 @@
+package packfile
+
+import "gopkg.in/src-d/go-git.v4/plumbing"
+
+// ObjectToPack is a representation of an object that is going to be into a
+// pack file.
+type ObjectToPack struct {
+	// The main object to pack, it could be any object, including deltas
+	Object plumbing.Object
+	// Base is the object that a delta is based on (it could be also another delta).
+	// If the main object is not a delta, Base will be null
+	Base *ObjectToPack
+	// Original is the object that we can generate applying the delta to
+	// Base, or the same object as Object in the case of a non-delta object.
+	Original plumbing.Object
+	// Depth is the amount of deltas needed to resolve to obtain Original
+	// (delta based on delta based on ...)
+	Depth int
+}
+
+// newObjectToPack creates a correct ObjectToPack based on a non-delta object
+func newObjectToPack(o plumbing.Object) *ObjectToPack {
+	return &ObjectToPack{
+		Object:   o,
+		Original: o,
+	}
+}
+
+// newDeltaObjectToPack creates a correct ObjectToPack for a delta object, based on
+// his base (could be another delta), the delta target (in this case called original),
+// and the delta Object itself
+func newDeltaObjectToPack(base *ObjectToPack, original, delta plumbing.Object) *ObjectToPack {
+	return &ObjectToPack{
+		Object:   delta,
+		Base:     base,
+		Original: original,
+		Depth:    base.Depth + 1,
+	}
+}
+
+func (o *ObjectToPack) IsDelta() bool {
+	if o.Base != nil {
+		return true
+	}
+
+	return false
+}