1 files changed, 160 insertions, 0 deletions
diff --git a/formats/idxfile/decoder.go b/formats/idxfile/decoder.go
new file mode 100644
index 0000000..72a9338
--- /dev/null
+++ b/formats/idxfile/decoder.go
@@ -0,0 +1,160 @@
+package idxfile
+
+import (
+	"bytes"
+	"encoding/binary"
+	"errors"
+	"io"
+
+	"gopkg.in/src-d/go-git.v3/core"
+)
+
+var (
+	// ErrUnsupportedVersion is returned by Decode when the idx file version
+	// is not supported.
+	ErrUnsupportedVersion = errors.New("Unsuported version")
+	// ErrMalformedIdxFile is returned by Decode when the idx file is corrupted.
+	ErrMalformedIdxFile = errors.New("Malformed IDX file")
+)
+
+// A Decoder reads and decodes idx files from an input stream.
+type Decoder struct {
+	io.Reader
+}
+
+// NewDecoder returns a new decoder that reads from r.
+func NewDecoder(r io.Reader) *Decoder {
+	return &Decoder{r}
+}
+
+// Decode reads the whole idx object from its input and stores it in the
+// value pointed to by idx.
+func (d *Decoder) Decode(idx *Idxfile) error {
+	if err := validateHeader(d); err != nil {
+		return err
+	}
+
+	flow := []func(*Idxfile, io.Reader) error{
+		readVersion,
+		readFanout,
+		readObjectNames,
+		readCRC32,
+		readOffsets,
+		readChecksums,
+	}
+
+	for _, f := range flow {
+		if err := f(idx, d); err != nil {
+			return err
+		}
+	}
+
+	if !idx.isValid() {
+		return ErrMalformedIdxFile
+	}
+
+	return nil
+}
+
+func validateHeader(r io.Reader) error {
+	var h = make([]byte, 4)
+	if _, err := r.Read(h); err != nil {
+		return err
+	}
+
+	if !bytes.Equal(h, idxHeader) {
+		return ErrMalformedIdxFile
+	}
+
+	return nil
+}
+
+func readVersion(idx *Idxfile, r io.Reader) error {
+	v, err := readInt32(r)
+	if err != nil {
+		return err
+	}
+
+	if v > VersionSupported {
+		return ErrUnsupportedVersion
+	}
+
+	idx.Version = v
+
+	return nil
+}
+
+func readFanout(idx *Idxfile, r io.Reader) error {
+	var err error
+
+	for i := 0; i < 255; i++ {
+		idx.Fanout[i], err = readInt32(r)
+		if err != nil {
+			return err
+		}
+	}
+
+	idx.ObjectCount, err = readInt32(r)
+
+	return err
+}
+
+func readObjectNames(idx *Idxfile, r io.Reader) error {
+	c := int(idx.ObjectCount)
+	for i := 0; i < c; i++ {
+		var ref core.Hash
+		if _, err := r.Read(ref[:]); err != nil {
+			return err
+		}
+
+		idx.Entries = append(idx.Entries, Entry{Hash: ref})
+	}
+
+	return nil
+}
+
+func readCRC32(idx *Idxfile, r io.Reader) error {
+	c := int(idx.ObjectCount)
+	for i := 0; i < c; i++ {
+		if _, err := r.Read(idx.Entries[i].CRC32[:]); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+func readOffsets(idx *Idxfile, r io.Reader) error {
+	c := int(idx.ObjectCount)
+	for i := 0; i < c; i++ {
+		o, err := readInt32(r)
+		if err != nil {
+			return err
+		}
+
+		idx.Entries[i].Offset = uint64(o)
+	}
+
+	return nil
+}
+
+func readChecksums(idx *Idxfile, r io.Reader) error {
+	if _, err := r.Read(idx.PackfileChecksum[:]); err != nil {
+		return err
+	}
+
+	if _, err := r.Read(idx.IdxChecksum[:]); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func readInt32(r io.Reader) (uint32, error) {
+	var v uint32
+	if err := binary.Read(r, binary.BigEndian, &v); err != nil {
+		return 0, err
+	}
+
+	return v, nil
+}