packfile: create packfile.Index and reuse it

There was an internal type (i.e. storage/filesystem.idx) to
use as in-memory index for packfiles. This was not convenient
to reuse in the packfile.

This commit creates a new representation (format/packfile.Index)
that can be converted to and from idxfile.Idxfile.

A packfile.Index now contains the functionality that was scattered
on storage/filesystem.idx and packfile.Decoder's internals.

storage/filesystem now reuses packfile.Index instances and this
also results in higher cache hit ratios when resolving deltas.

5 files changed, 13 insertions, 11 deletions

diff --git a/plumbing/format/idxfile/decoder.go b/plumbing/format/idxfile/decoder.go
index fea5f0b..4243f76 100644
--- a/plumbing/format/idxfile/decoder.go
+++ b/plumbing/format/idxfile/decoder.go
@@ -104,7 +104,7 @@ func readObjectNames(idx *Idxfile, r io.Reader) error {
 			return err
 		}
 
-		idx.Entries = append(idx.Entries, Entry{Hash: ref})
+		idx.Entries = append(idx.Entries, &Entry{Hash: ref})
 	}
 
 	return nil
diff --git a/plumbing/format/idxfile/decoder_test.go b/plumbing/format/idxfile/decoder_test.go
index 609f4e3..991232d 100644
--- a/plumbing/format/idxfile/decoder_test.go
+++ b/plumbing/format/idxfile/decoder_test.go
@@ -1,4 +1,4 @@
-package idxfile
+package idxfile_test
 
 import (
 	"bytes"
@@ -6,6 +6,7 @@ import (
 	"testing"
 
 	"github.com/src-d/go-git-fixtures"
+	. "gopkg.in/src-d/go-git.v4/plumbing/format/idxfile"
 	"gopkg.in/src-d/go-git.v4/plumbing/format/packfile"
 	"gopkg.in/src-d/go-git.v4/storage/memory"
 
@@ -48,12 +49,8 @@ func (s *IdxfileSuite) TestDecodeCRCs(c *C) {
 	_, err = pd.Decode()
 	c.Assert(err, IsNil)
 
-	i := &Idxfile{Version: VersionSupported}
-
-	offsets := pd.Offsets()
-	for h, crc := range pd.CRCs() {
-		i.Add(h, uint64(offsets[h]), crc)
-	}
+	i := pd.Index().ToIdxFile()
+	i.Version = VersionSupported
 
 	buf := bytes.NewBuffer(nil)
 	e := NewEncoder(buf)
diff --git a/plumbing/format/idxfile/encoder.go b/plumbing/format/idxfile/encoder.go
index 71e1b3f..d8f4d94 100644
--- a/plumbing/format/idxfile/encoder.go
+++ b/plumbing/format/idxfile/encoder.go
@@ -124,7 +124,7 @@ func (e *Encoder) encodeChecksums(idx *Idxfile) (int, error) {
 }
 
 // EntryList implements sort.Interface allowing sorting in increasing order.
-type EntryList []Entry
+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() }
diff --git a/plumbing/format/idxfile/encoder_test.go b/plumbing/format/idxfile/encoder_test.go
index 1fc4e9c..d566b0d 100644
--- a/plumbing/format/idxfile/encoder_test.go
+++ b/plumbing/format/idxfile/encoder_test.go
@@ -1,4 +1,4 @@
-package idxfile
+package idxfile_test
 
 import (
 	"bytes"
@@ -6,6 +6,7 @@ import (
 
 	"github.com/src-d/go-git-fixtures"
 	"gopkg.in/src-d/go-git.v4/plumbing"
+	. "gopkg.in/src-d/go-git.v4/plumbing/format/idxfile"
 
 	. "gopkg.in/check.v1"
 )
diff --git a/plumbing/format/idxfile/idxfile.go b/plumbing/format/idxfile/idxfile.go
index 5a718f3..b9bb1c2 100644
--- a/plumbing/format/idxfile/idxfile.go
+++ b/plumbing/format/idxfile/idxfile.go
@@ -21,6 +21,10 @@ type Idxfile struct {
 	IdxChecksum      [20]byte
 }
 
+func NewIdxfile() *Idxfile {
+	return &Idxfile{}
+}
+
 // Entry is the in memory representation of an object entry in the idx file.
 type Entry struct {
 	Hash   plumbing.Hash
@@ -30,7 +34,7 @@ type Entry struct {
 
 // Add adds a new Entry with the given values to the Idxfile.
 func (idx *Idxfile) Add(h plumbing.Hash, offset uint64, crc32 uint32) {
-	idx.Entries = append(idx.Entries, Entry{
+	idx.Entries = append(idx.Entries, &Entry{
 		Hash:   h,
 		Offset: offset,
 		CRC32:  crc32,