2 files changed, 45 insertions, 22 deletions

diff --git a/plumbing/format/idxfile/idxfile.go b/plumbing/format/idxfile/idxfile.go
index 5fed278..26cca59 100644
--- a/plumbing/format/idxfile/idxfile.go
+++ b/plumbing/format/idxfile/idxfile.go
@@ -55,7 +55,8 @@ type MemoryIndex struct {
 	PackfileChecksum [20]byte
 	IdxChecksum      [20]byte
 
-	offsetHash map[int64]plumbing.Hash
+	offsetHash       map[int64]plumbing.Hash
+	offsetHashIsFull bool
 }
 
 var _ Index = (*MemoryIndex)(nil)
@@ -121,7 +122,17 @@ func (idx *MemoryIndex) FindOffset(h plumbing.Hash) (int64, error) {
 		return 0, plumbing.ErrObjectNotFound
 	}
 
-	return idx.getOffset(k, i)
+	offset, err := idx.getOffset(k, i)
+
+	if !idx.offsetHashIsFull {
+		// Save the offset for reverse lookup
+		if idx.offsetHash == nil {
+			idx.offsetHash = make(map[int64]plumbing.Hash)
+		}
+		idx.offsetHash[offset] = h
+	}
+
+	return offset, err
 }
 
 const isO64Mask = uint64(1) << 31
@@ -167,14 +178,24 @@ func (idx *MemoryIndex) getCRC32(firstLevel, secondLevel int) (uint32, error) {
 
 // FindHash implements the Index interface.
 func (idx *MemoryIndex) FindHash(o int64) (plumbing.Hash, error) {
+	var hash plumbing.Hash
+	var ok bool
+
+	if idx.offsetHash != nil {
+		if hash, ok = idx.offsetHash[o]; ok {
+			return hash, nil
+		}
+	}
+
 	// Lazily generate the reverse offset/hash map if required.
-	if idx.offsetHash == nil {
+	if !idx.offsetHashIsFull || idx.offsetHash == nil {
 		if err := idx.genOffsetHash(); err != nil {
 			return plumbing.ZeroHash, err
 		}
+
+		hash, ok = idx.offsetHash[o]
 	}
 
-	hash, ok := idx.offsetHash[o]
 	if !ok {
 		return plumbing.ZeroHash, plumbing.ErrObjectNotFound
 	}
@@ -190,6 +211,7 @@ func (idx *MemoryIndex) genOffsetHash() error {
 	}
 
 	idx.offsetHash = make(map[int64]plumbing.Hash, count)
+	idx.offsetHashIsFull = true
 
 	iter, err := idx.Entries()
 	if err != nil {
diff --git a/plumbing/format/index/decoder.go b/plumbing/format/index/decoder.go
index ac57d08..98f92fd 100644
--- a/plumbing/format/index/decoder.go
+++ b/plumbing/format/index/decoder.go
@@ -1,6 +1,7 @@
 package index
 
 import (
+	"bufio"
 	"bytes"
 	"crypto/sha1"
 	"errors"
@@ -42,14 +43,17 @@ type Decoder struct {
 	r         io.Reader
 	hash      hash.Hash
 	lastEntry *Entry
+
+	extReader *bufio.Reader
 }
 
 // NewDecoder returns a new decoder that reads from r.
 func NewDecoder(r io.Reader) *Decoder {
 	h := sha1.New()
 	return &Decoder{
-		r:    io.TeeReader(r, h),
-		hash: h,
+		r:         io.TeeReader(r, h),
+		hash:      h,
+		extReader: bufio.NewReader(nil),
 	}
 }
 
@@ -184,11 +188,9 @@ func (d *Decoder) doReadEntryNameV4() (string, error) {
 
 func (d *Decoder) doReadEntryName(len uint16) (string, error) {
 	name := make([]byte, len)
-	if err := binary.Read(d.r, &name); err != nil {
-		return "", err
-	}
+	_, err := io.ReadFull(d.r, name[:])
 
-	return string(name), nil
+	return string(name), err
 }
 
 // Index entries are padded out to the next 8 byte alignment
@@ -279,20 +281,21 @@ func (d *Decoder) readExtension(idx *Index, header []byte) error {
 	return nil
 }
 
-func (d *Decoder) getExtensionReader() (io.Reader, error) {
+func (d *Decoder) getExtensionReader() (*bufio.Reader, error) {
 	len, err := binary.ReadUint32(d.r)
 	if err != nil {
 		return nil, err
 	}
 
-	return &io.LimitedReader{R: d.r, N: int64(len)}, nil
+	d.extReader.Reset(&io.LimitedReader{R: d.r, N: int64(len)})
+	return d.extReader, nil
 }
 
 func (d *Decoder) readChecksum(expected []byte, alreadyRead [4]byte) error {
 	var h plumbing.Hash
 	copy(h[:4], alreadyRead[:])
 
-	if err := binary.Read(d.r, h[4:]); err != nil {
+	if _, err := io.ReadFull(d.r, h[4:]); err != nil {
 		return err
 	}
 
@@ -326,7 +329,7 @@ func validateHeader(r io.Reader) (version uint32, err error) {
 }
 
 type treeExtensionDecoder struct {
-	r io.Reader
+	r *bufio.Reader
 }
 
 func (d *treeExtensionDecoder) Decode(t *Tree) error {
@@ -386,16 +389,13 @@ func (d *treeExtensionDecoder) readEntry() (*TreeEntry, error) {
 	}
 
 	e.Trees = i
-
-	if err := binary.Read(d.r, &e.Hash); err != nil {
-		return nil, err
-	}
+	_, err = io.ReadFull(d.r, e.Hash[:])
 
 	return e, nil
 }
 
 type resolveUndoDecoder struct {
-	r io.Reader
+	r *bufio.Reader
 }
 
 func (d *resolveUndoDecoder) Decode(ru *ResolveUndo) error {
@@ -433,7 +433,7 @@ func (d *resolveUndoDecoder) readEntry() (*ResolveUndoEntry, error) {
 
 	for s := range e.Stages {
 		var hash plumbing.Hash
-		if err := binary.Read(d.r, hash[:]); err != nil {
+		if _, err := io.ReadFull(d.r, hash[:]); err != nil {
 			return nil, err
 		}
 
@@ -462,7 +462,7 @@ func (d *resolveUndoDecoder) readStage(e *ResolveUndoEntry, s Stage) error {
 }
 
 type endOfIndexEntryDecoder struct {
-	r io.Reader
+	r *bufio.Reader
 }
 
 func (d *endOfIndexEntryDecoder) Decode(e *EndOfIndexEntry) error {
@@ -472,5 +472,6 @@ func (d *endOfIndexEntryDecoder) Decode(e *EndOfIndexEntry) error {
 		return err
 	}
 
-	return binary.Read(d.r, &e.Hash)
+	_, err = io.ReadFull(d.r, e.Hash[:])
+	return err
 }