plumbing/packfile: PACK encoder (#131)

* plumbing/packfile: PACK encoder

- Added simple PACK encoder, deltas not supported by now

* Requested changes

* Requested changes

* Requested changes

1 files changed, 116 insertions, 0 deletions

diff --git a/plumbing/format/packfile/encoder.go b/plumbing/format/packfile/encoder.go
new file mode 100644
index 0000000..1404dbe
--- /dev/null
+++ b/plumbing/format/packfile/encoder.go
@@ -0,0 +1,116 @@
+package packfile
+
+import (
+	"compress/zlib"
+	"crypto/sha1"
+	"fmt"
+	"io"
+
+	"gopkg.in/src-d/go-git.v4/plumbing"
+	"gopkg.in/src-d/go-git.v4/plumbing/storer"
+	"gopkg.in/src-d/go-git.v4/utils/binary"
+)
+
+// Encoder gets the data from the storage and write it into the writer in PACK
+// format
+type Encoder struct {
+	storage storer.ObjectStorer
+	w       io.Writer
+	zw      *zlib.Writer
+	hasher  plumbing.Hasher
+}
+
+// NewEncoder creates a new packfile encoder using a specific Writer and
+// ObjectStorer
+func NewEncoder(w io.Writer, s storer.ObjectStorer) *Encoder {
+	h := plumbing.Hasher{
+		Hash: sha1.New(),
+	}
+	mw := io.MultiWriter(w, h)
+	zw := zlib.NewWriter(mw)
+	return &Encoder{
+		storage: s,
+		w:       mw,
+		zw:      zw,
+		hasher:  h,
+	}
+}
+
+// Encode creates a packfile containing all the objects referenced in hashes
+// and writes it to the writer in the Encoder.
+func (e *Encoder) Encode(hashes []plumbing.Hash) (plumbing.Hash, error) {
+	if err := e.head(len(hashes)); err != nil {
+		return plumbing.ZeroHash, err
+	}
+
+	for _, h := range hashes {
+		o, err := e.storage.Object(plumbing.AnyObject, h)
+		if err != nil {
+			return plumbing.ZeroHash, err
+		}
+
+		if err := e.entry(o); err != nil {
+			return plumbing.ZeroHash, err
+		}
+	}
+
+	return e.footer()
+}
+
+func (e *Encoder) head(numEntries int) error {
+	return binary.Write(
+		e.w,
+		signature,
+		int32(VersionSupported),
+		int32(numEntries),
+	)
+}
+
+func (e *Encoder) entry(o plumbing.Object) error {
+	t := o.Type()
+	if t == plumbing.OFSDeltaObject || t == plumbing.REFDeltaObject {
+		// TODO implements delta objects
+		return fmt.Errorf("delta object not supported: %v", t)
+	}
+
+	if err := e.entryHead(t, o.Size()); err != nil {
+		return err
+	}
+
+	e.zw.Reset(e.w)
+	or, err := o.Reader()
+	if err != nil {
+		return err
+	}
+	_, err = io.Copy(e.zw, or)
+	if err != nil {
+		return err
+	}
+
+	return e.zw.Close()
+}
+
+func (e *Encoder) entryHead(typeNum plumbing.ObjectType, size int64) error {
+	t := int64(typeNum)
+	header := []byte{}
+	c := (t << firstLengthBits) | (size & maskFirstLength)
+	size >>= firstLengthBits
+	for {
+		if size == 0 {
+			break
+		}
+		header = append(header, byte(c|maskContinue))
+		c = size & int64(maskLength)
+		size >>= lengthBits
+	}
+
+	header = append(header, byte(c))
+	_, err := e.w.Write(header)
+
+	return err
+}
+
+func (e *Encoder) footer() (plumbing.Hash, error) {
+	h := e.hasher.Sum()
+	return h, binary.Write(e.w, h)
+}