1 files changed, 131 insertions, 0 deletions
diff --git a/plumbing/format/idxfile/encoder.go b/plumbing/format/idxfile/encoder.go
new file mode 100644
index 0000000..164414a
--- /dev/null
+++ b/plumbing/format/idxfile/encoder.go
@@ -0,0 +1,131 @@
+package idxfile
+
+import (
+	"crypto/sha1"
+	"hash"
+	"io"
+	"sort"
+
+	"gopkg.in/src-d/go-git.v4/utils/binary"
+)
+
+// An Encoder writes idx files to an output stream.
+type Encoder struct {
+	io.Writer
+	hash hash.Hash
+}
+
+// NewEncoder returns a new encoder that writes to w.
+func NewEncoder(w io.Writer) *Encoder {
+	h := sha1.New()
+	mw := io.MultiWriter(w, h)
+	return &Encoder{mw, h}
+}
+
+// Encode writes the idx in an idx file format to the stream of the encoder.
+func (e *Encoder) Encode(idx *Idxfile) (int, error) {
+	idx.Entries.Sort()
+
+	flow := []func(*Idxfile) (int, error){
+		e.encodeHeader,
+		e.encodeFanout,
+		e.encodeHashes,
+		e.encodeCRC32,
+		e.encodeOffsets,
+		e.encodeChecksums,
+	}
+
+	sz := 0
+	for _, f := range flow {
+		i, err := f(idx)
+		sz += i
+
+		if err != nil {
+			return sz, err
+		}
+	}
+
+	return sz, nil
+}
+
+func (e *Encoder) encodeHeader(idx *Idxfile) (int, error) {
+	c, err := e.Write(idxHeader)
+	if err != nil {
+		return c, err
+	}
+
+	return c + 4, binary.WriteUint32(e, idx.Version)
+}
+
+func (e *Encoder) encodeFanout(idx *Idxfile) (int, error) {
+	fanout := idx.calculateFanout()
+	for _, c := range fanout {
+		if err := binary.WriteUint32(e, c); err != nil {
+			return 0, err
+		}
+	}
+
+	return 1024, nil
+}
+
+func (e *Encoder) encodeHashes(idx *Idxfile) (int, error) {
+	sz := 0
+	for _, ent := range idx.Entries {
+		i, err := e.Write(ent.Hash[:])
+		sz += i
+
+		if err != nil {
+			return sz, err
+		}
+	}
+
+	return sz, nil
+}
+
+func (e *Encoder) encodeCRC32(idx *Idxfile) (int, error) {
+	sz := 0
+	for _, ent := range idx.Entries {
+		err := binary.Write(e, ent.CRC32)
+		sz += 4
+
+		if err != nil {
+			return sz, err
+		}
+	}
+
+	return sz, nil
+}
+
+func (e *Encoder) encodeOffsets(idx *Idxfile) (int, error) {
+	sz := 0
+	for _, ent := range idx.Entries {
+		if err := binary.WriteUint32(e, uint32(ent.Offset)); err != nil {
+			return sz, err
+		}
+
+		sz += 4
+
+	}
+
+	return sz, nil
+}
+
+func (e *Encoder) encodeChecksums(idx *Idxfile) (int, error) {
+	if _, err := e.Write(idx.PackfileChecksum[:]); err != nil {
+		return 0, err
+	}
+
+	copy(idx.IdxChecksum[:], e.hash.Sum(nil)[:20])
+	if _, err := e.Write(idx.IdxChecksum[:]); err != nil {
+		return 0, err
+	}
+
+	return 40, nil
+}
+
+type EntryList []Entry
+
+func (p EntryList) Len() int           { return len(p) }
+func (p EntryList) Less(i, j int) bool { return p[i].Hash.String() < p[j].Hash.String() }
+func (p EntryList) Swap(i, j int)      { p[i], p[j] = p[j], p[i] }
+func (p EntryList) Sort()              { sort.Sort(p) }