git/odb/pack: introduce v1 index format support

git/odb/pack/index_decode.go

@@ -10,6 +10,26 @@ import (
 const (
 	// V1Width is the total width of the header in V1.
 	V1Width = 0
+
+	// FanoutEntries is the number of entries in the fanout table.
+	FanoutEntries = 256
+	// FanoutEntryWidth is the width of each entry in the fanout table.
+	FanoutEntryWidth = 4
+	// FanoutWidth is the width of the entire fanout table.
+	FanoutWidth = FanoutEntries * FanoutEntryWidth
+
+	// OffsetV1Start is the location of the first object outside of the V1
+	// header.
+	OffsetV1Start = V1Width + FanoutWidth
+
+	// ObjectNameWidth is the width of a SHA1 object name.
+	ObjectNameWidth = 20
+	// ObjectSmallOffsetWidth is the width of the small offset encoded into
+	// each object.
+	ObjectSmallOffsetWidth = 4
+
+	// ObjectEntryV1Width is the width of one contiguous object entry in V1.
+	ObjectEntryV1Width = ObjectNameWidth + ObjectSmallOffsetWidth
 )
 
 var (
@@ -61,10 +81,14 @@ func decodeIndexHeader(r io.ReaderAt) (IndexVersion, error) {
 		}
 
 		version := IndexVersion(binary.BigEndian.Uint32(vb))
+		switch version {
+		case V1:
+			return version, nil
+		}
 
 		return version, &UnsupportedVersionErr{uint32(version)}
 	}
-	return IndexVersion(0), nil
+	return V1, nil
 }
 
 // decodeIndexFanout decodes the fanout table given by "r" and beginning at the

git/odb/pack/index_decode_test.go

@@ -8,6 +8,21 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+func TestDecodeIndexV1(t *testing.T) {
+	idx, err := DecodeIndex(bytes.NewReader(make([]byte, FanoutWidth)))
+
+	assert.NoError(t, err)
+	assert.Equal(t, V1, idx.version)
+	assert.EqualValues(t, 0, idx.Count())
+}
+
+func TestDecodeIndexV1InvalidFanout(t *testing.T) {
+	idx, err := DecodeIndex(bytes.NewReader(make([]byte, FanoutWidth-1)))
+
+	assert.Equal(t, ErrShortFanout, err)
+	assert.Nil(t, idx)
+}
+
 func TestDecodeIndexUnsupportedVersion(t *testing.T) {
 	buf := make([]byte, 0, 4+4)
 	buf = append(buf, 0xff, 0x74, 0x4f, 0x63)

git/odb/pack/index_v1.go

@@ -0,0 +1,52 @@
+package pack
+
+import (
+	"bytes"
+	"encoding/binary"
+)
+
+const (
+	// V1 is an instance of IndexVersion corresponding to the V1 index file
+	// format.
+	V1 IndexVersion = 1
+)
+
+// v1Search implements the IndexVersion.Search method for V1 packfiles.
+func v1Search(idx *Index, name []byte, at int64) (*IndexEntry, int, error) {
+	var sha [20]byte
+	if _, err := idx.readAt(sha[:], v1ShaOffset(at)); err != nil {
+		return nil, 0, err
+	}
+
+	cmp := bytes.Compare(name, sha[:])
+	if cmp != 0 {
+		return nil, cmp, nil
+	}
+
+	var offs [4]byte
+	if _, err := idx.readAt(offs[:], v1EntryOffset(at)); err != nil {
+		return nil, 0, err
+	}
+
+	return &IndexEntry{
+		PackOffset: uint64(binary.BigEndian.Uint32(offs[:])),
+	}, 0, nil
+}
+
+// v1ShaOffset returns the location of the SHA1 of an object given at "at".
+func v1ShaOffset(at int64) int64 {
+	// Skip forward until the desired entry.
+	return v1EntryOffset(at) +
+		// Skip past the 4-byte object offset in the desired entry to
+		// the SHA1.
+		ObjectSmallOffsetWidth
+}
+
+// v1EntryOffset returns the location of the packfile offset for the object
+// given at "at".
+func v1EntryOffset(at int64) int64 {
+	// Skip the L1 fanout table
+	return OffsetV1Start +
+		// Skip the object entries before the one located at "at"
+		(ObjectEntryV1Width * at)
+}

git/odb/pack/index_v1_test.go

@@ -0,0 +1,98 @@
+package pack
+
+import (
+	"bytes"
+	"encoding/binary"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+var (
+	V1IndexFanout = make([]uint32, FanoutEntries)
+
+	V1IndexSmallEntry = []byte{
+		0x0, 0x0, 0x0, 0x1,
+
+		0x1, 0x1, 0x1, 0x1, 0x1, 0x1, 0x1, 0x1, 0x1, 0x1,
+		0x1, 0x1, 0x1, 0x1, 0x1, 0x1, 0x1, 0x1, 0x1, 0x1,
+	}
+	V1IndexSmallSha = V1IndexSmallEntry[4:]
+
+	V1IndexMediumEntry = []byte{
+		0x0, 0x0, 0x0, 0x2,
+
+		0x2, 0x2, 0x2, 0x2, 0x2, 0x2, 0x2, 0x2, 0x2, 0x2,
+		0x2, 0x2, 0x2, 0x2, 0x2, 0x2, 0x2, 0x2, 0x2, 0x2,
+	}
+	V1IndexMediumSha = V1IndexMediumEntry[4:]
+
+	V1IndexLargeEntry = []byte{
+		0x0, 0x0, 0x0, 0x3,
+
+		0x3, 0x3, 0x3, 0x3, 0x3, 0x3, 0x3, 0x3, 0x3, 0x3,
+		0x3, 0x3, 0x3, 0x3, 0x3, 0x3, 0x3, 0x3, 0x3, 0x3,
+	}
+	V1IndexLargeSha = V1IndexLargeEntry[4:]
+
+	V1Index = &Index{
+		fanout:  V1IndexFanout,
+		version: V1,
+	}
+)
+
+func TestIndexV1SearchExact(t *testing.T) {
+	e, cmp, err := V1.Search(V1Index, V1IndexMediumSha, 1)
+
+	assert.NoError(t, err)
+	assert.Equal(t, 0, cmp)
+	assert.EqualValues(t, 2, e.PackOffset)
+}
+
+func TestIndexV1SearchSmall(t *testing.T) {
+	e, cmp, err := V1.Search(V1Index, V1IndexMediumSha, 0)
+
+	assert.NoError(t, err)
+	assert.Equal(t, 1, cmp)
+	assert.Nil(t, e)
+}
+
+func TestIndexV1SearchBig(t *testing.T) {
+	e, cmp, err := V1.Search(V1Index, V1IndexMediumSha, 2)
+
+	assert.NoError(t, err)
+	assert.Equal(t, -1, cmp)
+	assert.Nil(t, e)
+}
+
+func TestIndexV1SearchOutOfBounds(t *testing.T) {
+	e, cmp, err := V1.Search(V1Index, V1IndexMediumSha, 10)
+
+	assert.Nil(t, e)
+	assert.Equal(t, 0, cmp)
+	assert.Equal(t, ErrIndexOutOfBounds, err)
+}
+
+func init() {
+	V1IndexFanout[1] = 1
+	V1IndexFanout[2] = 2
+	V1IndexFanout[3] = 3
+
+	for i := 3; i < len(V1IndexFanout); i++ {
+		V1IndexFanout[i] = 3
+	}
+
+	fanout := make([]byte, FanoutWidth)
+	for i, n := range V1IndexFanout {
+		binary.BigEndian.PutUint32(fanout[i*FanoutEntryWidth:], n)
+	}
+
+	buf := make([]byte, 0, OffsetV1Start+(3*ObjectEntryV1Width))
+
+	buf = append(buf, fanout...)
+	buf = append(buf, V1IndexSmallEntry...)
+	buf = append(buf, V1IndexMediumEntry...)
+	buf = append(buf, V1IndexLargeEntry...)
+
+	V1Index.f = bytes.NewReader(buf)
+}

git/odb/pack/index_version.go

@@ -1,6 +1,7 @@
 package pack
 
 import (
+	"errors"
 	"fmt"
 )
 
@@ -16,5 +17,33 @@ const (
 
 // Width returns the width of the header given in the respective version.
 func (v IndexVersion) Width() int64 {
+	switch v {
+	case V1:
+		return V1Width
+	}
 	panic(fmt.Sprintf("git/odb/pack: width unknown for pack version %d", v))
 }
+
+var (
+	// ErrIndexOutOfBounds is an error returned when the object lookup "at"
+	// (see: Search() below) is out of bounds.
+	ErrIndexOutOfBounds = errors.New("git/odb/pack: index is out of bounds")
+)
+
+// Search searches index "idx" for an object given by "name" at location "at".
+//
+// If will return the object if it was found, or a comparison determining
+// whether to search above or below next.
+//
+// Otherwise, it will return an error.
+func (v IndexVersion) Search(idx *Index, name []byte, at int64) (*IndexEntry, int, error) {
+	if at > int64(idx.Count()) {
+		return nil, 0, ErrIndexOutOfBounds
+	}
+
+	switch v {
+	case V1:
+		return v1Search(idx, name, at)
+	}
+	return nil, 0, &UnsupportedVersionErr{Got: uint32(v)}
+}

git/odb/pack/index_version_test.go

@@ -6,6 +6,10 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+func TestIndexVersionWidthV1(t *testing.T) {
+	assert.EqualValues(t, 0, V1.Width())
+}
+
 func TestIndexVersionWidthPanicsOnUnknownVersion(t *testing.T) {
 	v := IndexVersion(5)