1 files changed, 338 insertions, 0 deletions
diff --git a/plumbing/format/commitgraph/v2/file.go b/plumbing/format/commitgraph/v2/file.go
new file mode 100644
index 0000000..69e0250
--- /dev/null
+++ b/plumbing/format/commitgraph/v2/file.go
@@ -0,0 +1,338 @@
+package v2
+
+import (
+	"bytes"
+	"crypto"
+	encbin "encoding/binary"
+	"errors"
+	"io"
+	"time"
+
+	"github.com/go-git/go-git/v5/plumbing"
+	"github.com/go-git/go-git/v5/plumbing/hash"
+	"github.com/go-git/go-git/v5/utils/binary"
+)
+
+var (
+	// ErrUnsupportedVersion is returned by OpenFileIndex when the commit graph
+	// file version is not supported.
+	ErrUnsupportedVersion = errors.New("unsupported version")
+	// ErrUnsupportedHash is returned by OpenFileIndex when the commit graph
+	// hash function is not supported. Currently only SHA-1 is defined and
+	// supported.
+	ErrUnsupportedHash = errors.New("unsupported hash algorithm")
+	// ErrMalformedCommitGraphFile is returned by OpenFileIndex when the commit
+	// graph file is corrupted.
+	ErrMalformedCommitGraphFile = errors.New("malformed commit graph file")
+
+	commitFileSignature = []byte{'C', 'G', 'P', 'H'}
+
+	parentNone        = uint32(0x70000000)
+	parentOctopusUsed = uint32(0x80000000)
+	parentOctopusMask = uint32(0x7fffffff)
+	parentLast        = uint32(0x80000000)
+)
+
+const (
+	commitDataSize = 16
+)
+
+type fileIndex struct {
+	reader  ReaderAtCloser
+	fanout  [256]uint32
+	offsets [9]int64
+	parent  Index
+}
+
+// ReaderAtCloser is an interface that combines io.ReaderAt and io.Closer.
+type ReaderAtCloser interface {
+	io.ReaderAt
+	io.Closer
+}
+
+// OpenFileIndex opens a serialized commit graph file in the format described at
+// https://github.com/git/git/blob/master/Documentation/technical/commit-graph-format.txt
+func OpenFileIndex(reader ReaderAtCloser) (Index, error) {
+	return OpenFileIndexWithParent(reader, nil)
+}
+
+// OpenFileIndexWithParent opens a serialized commit graph file in the format described at
+// https://github.com/git/git/blob/master/Documentation/technical/commit-graph-format.txt
+func OpenFileIndexWithParent(reader ReaderAtCloser, parent Index) (Index, error) {
+	if reader == nil {
+		return nil, io.ErrUnexpectedEOF
+	}
+	fi := &fileIndex{reader: reader, parent: parent}
+
+	if err := fi.verifyFileHeader(); err != nil {
+		return nil, err
+	}
+	if err := fi.readChunkHeaders(); err != nil {
+		return nil, err
+	}
+	if err := fi.readFanout(); err != nil {
+		return nil, err
+	}
+
+	return fi, nil
+}
+
+// Close closes the underlying reader and the parent index if it exists.
+func (fi *fileIndex) Close() (err error) {
+	if fi.parent != nil {
+		defer func() {
+			parentErr := fi.parent.Close()
+			// only report the error from the parent if there is no error from the reader
+			if err == nil {
+				err = parentErr
+			}
+		}()
+	}
+	err = fi.reader.Close()
+	return
+}
+
+func (fi *fileIndex) verifyFileHeader() error {
+	// Verify file signature
+	signature := make([]byte, 4)
+	if _, err := fi.reader.ReadAt(signature, 0); err != nil {
+		return err
+	}
+	if !bytes.Equal(signature, commitFileSignature) {
+		return ErrMalformedCommitGraphFile
+	}
+
+	// Read and verify the file header
+	header := make([]byte, 4)
+	if _, err := fi.reader.ReadAt(header, 4); err != nil {
+		return err
+	}
+	if header[0] != 1 {
+		return ErrUnsupportedVersion
+	}
+	if !(hash.CryptoType == crypto.SHA1 && header[1] == 1) &&
+		!(hash.CryptoType == crypto.SHA256 && header[1] == 2) {
+		// Unknown hash type / unsupported hash type
+		return ErrUnsupportedHash
+	}
+
+	return nil
+}
+
+func (fi *fileIndex) readChunkHeaders() error {
+	chunkID := make([]byte, 4)
+	for i := 0; ; i++ {
+		chunkHeader := io.NewSectionReader(fi.reader, 8+(int64(i)*12), 12)
+		if _, err := io.ReadAtLeast(chunkHeader, chunkID, 4); err != nil {
+			return err
+		}
+		chunkOffset, err := binary.ReadUint64(chunkHeader)
+		if err != nil {
+			return err
+		}
+
+		chunkType, ok := ChunkTypeFromBytes(chunkID)
+		if !ok {
+			continue
+		}
+		if chunkType == ZeroChunk || int(chunkType) >= len(fi.offsets) {
+			break
+		}
+		fi.offsets[chunkType] = int64(chunkOffset)
+	}
+
+	if fi.offsets[OIDFanoutChunk] <= 0 || fi.offsets[OIDLookupChunk] <= 0 || fi.offsets[CommitDataChunk] <= 0 {
+		return ErrMalformedCommitGraphFile
+	}
+
+	return nil
+}
+
+func (fi *fileIndex) readFanout() error {
+	fanoutReader := io.NewSectionReader(fi.reader, fi.offsets[OIDFanoutChunk], 256*4)
+	for i := 0; i < 256; i++ {
+		fanoutValue, err := binary.ReadUint32(fanoutReader)
+		if err != nil {
+			return err
+		}
+		if fanoutValue > 0x7fffffff {
+			return ErrMalformedCommitGraphFile
+		}
+		fi.fanout[i] = fanoutValue
+	}
+	return nil
+}
+
+// GetIndexByHash looks up the provided hash in the commit-graph fanout and returns the index of the commit data for the given hash.
+func (fi *fileIndex) GetIndexByHash(h plumbing.Hash) (uint32, error) {
+	var oid plumbing.Hash
+
+	// Find the hash in the oid lookup table
+	var low uint32
+	if h[0] == 0 {
+		low = 0
+	} else {
+		low = fi.fanout[h[0]-1]
+	}
+	high := fi.fanout[h[0]]
+	for low < high {
+		mid := (low + high) >> 1
+		offset := fi.offsets[OIDLookupChunk] + int64(mid)*hash.Size
+		if _, err := fi.reader.ReadAt(oid[:], offset); err != nil {
+			return 0, err
+		}
+		cmp := bytes.Compare(h[:], oid[:])
+		if cmp < 0 {
+			high = mid
+		} else if cmp == 0 {
+			return mid, nil
+		} else {
+			low = mid + 1
+		}
+	}
+
+	if fi.parent != nil {
+		idx, err := fi.parent.GetIndexByHash(h)
+		if err != nil {
+			return 0, err
+		}
+		return idx + fi.fanout[0xff], nil
+	}
+
+	return 0, plumbing.ErrObjectNotFound
+}
+
+// GetCommitDataByIndex returns the commit data for the given index in the commit-graph.
+func (fi *fileIndex) GetCommitDataByIndex(idx uint32) (*CommitData, error) {
+	if idx >= fi.fanout[0xff] {
+		if fi.parent != nil {
+			data, err := fi.parent.GetCommitDataByIndex(idx - fi.fanout[0xff])
+			if err != nil {
+				return nil, err
+			}
+			for i := range data.ParentIndexes {
+				data.ParentIndexes[i] += fi.fanout[0xff]
+			}
+			return data, nil
+		}
+
+		return nil, plumbing.ErrObjectNotFound
+	}
+
+	offset := fi.offsets[CommitDataChunk] + int64(idx)*(hash.Size+commitDataSize)
+	commitDataReader := io.NewSectionReader(fi.reader, offset, hash.Size+commitDataSize)
+
+	treeHash, err := binary.ReadHash(commitDataReader)
+	if err != nil {
+		return nil, err
+	}
+	parent1, err := binary.ReadUint32(commitDataReader)
+	if err != nil {
+		return nil, err
+	}
+	parent2, err := binary.ReadUint32(commitDataReader)
+	if err != nil {
+		return nil, err
+	}
+	genAndTime, err := binary.ReadUint64(commitDataReader)
+	if err != nil {
+		return nil, err
+	}
+
+	var parentIndexes []uint32
+	if parent2&parentOctopusUsed == parentOctopusUsed {
+		// Octopus merge
+		parentIndexes = []uint32{parent1 & parentOctopusMask}
+		offset := fi.offsets[ExtraEdgeListChunk] + 4*int64(parent2&parentOctopusMask)
+		buf := make([]byte, 4)
+		for {
+			_, err := fi.reader.ReadAt(buf, offset)
+			if err != nil {
+				return nil, err
+			}
+
+			parent := encbin.BigEndian.Uint32(buf)
+			offset += 4
+			parentIndexes = append(parentIndexes, parent&parentOctopusMask)
+			if parent&parentLast == parentLast {
+				break
+			}
+		}
+	} else if parent2 != parentNone {
+		parentIndexes = []uint32{parent1 & parentOctopusMask, parent2 & parentOctopusMask}
+	} else if parent1 != parentNone {
+		parentIndexes = []uint32{parent1 & parentOctopusMask}
+	}
+
+	parentHashes, err := fi.getHashesFromIndexes(parentIndexes)
+	if err != nil {
+		return nil, err
+	}
+
+	return &CommitData{
+		TreeHash:      treeHash,
+		ParentIndexes: parentIndexes,
+		ParentHashes:  parentHashes,
+		Generation:    genAndTime >> 34,
+		When:          time.Unix(int64(genAndTime&0x3FFFFFFFF), 0),
+	}, nil
+}
+
+// GetHashByIndex looks up the hash for the given index in the commit-graph.
+func (fi *fileIndex) GetHashByIndex(idx uint32) (found plumbing.Hash, err error) {
+	if idx >= fi.fanout[0xff] {
+		if fi.parent != nil {
+			return fi.parent.GetHashByIndex(idx - fi.fanout[0xff])
+		}
+		return found, ErrMalformedCommitGraphFile
+	}
+
+	offset := fi.offsets[OIDLookupChunk] + int64(idx)*hash.Size
+	if _, err := fi.reader.ReadAt(found[:], offset); err != nil {
+		return found, err
+	}
+
+	return found, nil
+}
+
+func (fi *fileIndex) getHashesFromIndexes(indexes []uint32) ([]plumbing.Hash, error) {
+	hashes := make([]plumbing.Hash, len(indexes))
+
+	for i, idx := range indexes {
+		if idx >= fi.fanout[0xff] {
+			if fi.parent != nil {
+				hash, err := fi.parent.GetHashByIndex(idx - fi.fanout[0xff])
+				if err != nil {
+					return nil, err
+				}
+				hashes[i] = hash
+				continue
+			}
+
+			return nil, ErrMalformedCommitGraphFile
+		}
+
+		offset := fi.offsets[OIDLookupChunk] + int64(idx)*hash.Size
+		if _, err := fi.reader.ReadAt(hashes[i][:], offset); err != nil {
+			return nil, err
+		}
+	}
+
+	return hashes, nil
+}
+
+// Hashes returns all the hashes that are available in the index.
+func (fi *fileIndex) Hashes() []plumbing.Hash {
+	hashes := make([]plumbing.Hash, fi.fanout[0xff])
+	for i := uint32(0); i < fi.fanout[0xff]; i++ {
+		offset := fi.offsets[OIDLookupChunk] + int64(i)*hash.Size
+		if n, err := fi.reader.ReadAt(hashes[i][:], offset); err != nil || n < hash.Size {
+			return nil
+		}
+	}
+	if fi.parent != nil {
+		parentHashes := fi.parent.Hashes()
+		hashes = append(hashes, parentHashes...)
+	}
+	return hashes
+}