packfile: cache undeltified objects to improve decode performance (#218)

* Simple object cache that keeps in memory the last undeltified objects.
  When no more objects can be kept into memory, the oldest one is deleted (FIFO).
  This speeds up packfile operations preventing redundant seeks and decodes.

1 files changed, 33 insertions, 8 deletions

diff --git a/plumbing/format/packfile/decoder.go b/plumbing/format/packfile/decoder.go
index 70d41ef..5793fdd 100644
--- a/plumbing/format/packfile/decoder.go
+++ b/plumbing/format/packfile/decoder.go
@@ -3,6 +3,7 @@ package packfile
 import (
 	"bytes"
 
+	"gopkg.in/src-d/go-git.v4/cache"
 	"gopkg.in/src-d/go-git.v4/plumbing"
 	"gopkg.in/src-d/go-git.v4/plumbing/storer"
 )
@@ -62,6 +63,8 @@ type Decoder struct {
 
 	offsetToType map[int64]plumbing.ObjectType
 	decoderType  plumbing.ObjectType
+
+	cache cache.Object
 }
 
 // NewDecoder returns a new Decoder that decodes a Packfile using the given
@@ -105,6 +108,8 @@ func NewDecoderForType(s *Scanner, o storer.EncodedObjectStorer,
 
 		offsetToType: make(map[int64]plumbing.ObjectType, 0),
 		decoderType:  t,
+
+		cache: cache.NewObjectFIFO(cache.MaxSize),
 	}, nil
 }
 
@@ -341,13 +346,20 @@ func (d *Decoder) fillREFDeltaObjectContent(obj plumbing.EncodedObject, ref plum
 		return 0, err
 	}
 
-	base, err := d.recallByHash(ref)
-	if err != nil {
-		return 0, err
+	base := d.cache.Get(ref)
+
+	if base == nil {
+		base, err = d.recallByHash(ref)
+		if err != nil {
+			return 0, err
+		}
 	}
 
 	obj.SetType(base.Type())
-	return crc, ApplyDelta(obj, base, buf.Bytes())
+	err = ApplyDelta(obj, base, buf.Bytes())
+	d.cache.Add(obj)
+
+	return crc, err
 }
 
 func (d *Decoder) fillOFSDeltaObjectContent(obj plumbing.EncodedObject, offset int64) (uint32, error) {
@@ -357,13 +369,24 @@ func (d *Decoder) fillOFSDeltaObjectContent(obj plumbing.EncodedObject, offset i
 		return 0, err
 	}
 
-	base, err := d.recallByOffset(offset)
-	if err != nil {
-		return 0, err
+	h := d.offsetToHash[offset]
+	var base plumbing.EncodedObject
+	if h != plumbing.ZeroHash {
+		base = d.cache.Get(h)
+	}
+
+	if base == nil {
+		base, err = d.recallByOffset(offset)
+		if err != nil {
+			return 0, err
+		}
 	}
 
 	obj.SetType(base.Type())
-	return crc, ApplyDelta(obj, base, buf.Bytes())
+	err = ApplyDelta(obj, base, buf.Bytes())
+	d.cache.Add(obj)
+
+	return crc, err
 }
 
 func (d *Decoder) setOffset(h plumbing.Hash, offset int64) {
@@ -434,5 +457,7 @@ func (d *Decoder) CRCs() map[plumbing.Hash]uint32 {
 // Close close the Scanner, usually this mean that the whole reader is read and
 // discarded
 func (d *Decoder) Close() error {
+	d.cache.Clear()
+
 	return d.s.Close()
 }