WIP

plumbing/format/commitgraph/commitgraph.go

@@ -0,0 +1,235 @@
+package commitgraph
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"math"
+	"time"
+
+	"gopkg.in/src-d/go-git.v4/plumbing"
+	"gopkg.in/src-d/go-git.v4/utils/binary"
+)
+
+// Node is a reduced representation of Commit as presented in the commit graph
+// file. It is merely useful as an optimization for walking the commit graphs.
+type Node struct {
+	// TreeHash is the hash of the root tree of the commit.
+	TreeHash plumbing.Hash
+	// ParentHashes are the hashes of the parent commits of the commit.
+	ParentIndexes []uint32
+	// Generation number is the pre-computed generation in the commit graph
+	// or zero if not available
+	Generation uint32
+	// When is the timestamp of the commit.
+	When time.Time
+}
+
+// Index represents a representation of commit graph that allows indexed
+// access to the nodes using commit object hash
+type Index interface {
+	// GetIndexByHash gets the index in the commit graph from commit hash, if available
+	GetIndexByHash(h plumbing.Hash) (uint32, error)
+	// GetHashByIndex gets the hash from node index, if available
+	GetHashByIndex(i uint32) (plumbing.Hash, error)
+	// GetNodeByIndex gets the commit node from the commit graph using index
+	// obtained from child node, if available
+	GetNodeByIndex(i uint32) (*Node, error)
+}
+
+var (
+	// ErrUnsupportedVersion is returned by OpenFileIndex when the commit graph
+	// file version is not supported.
+	ErrUnsupportedVersion = errors.New("Unsuported 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("Unsuported 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'}
+	oidFanoutSignature     = []byte{'O', 'I', 'D', 'F'}
+	oidLookupSignature     = []byte{'O', 'I', 'D', 'L'}
+	commitDataSignature    = []byte{'C', 'D', 'A', 'T'}
+	largeEdgeListSignature = []byte{'E', 'D', 'G', 'E'}
+
+	parentNone        = uint32(0x70000000)
+	parentOctopusUsed = uint32(0x80000000)
+	parentOctopusMask = uint32(0x7fffffff)
+	parentLast        = uint32(0x80000000)
+)
+
+type fileIndex struct {
+	reader              io.ReaderAt
+	fanout              [256]uint32
+	oidLookupOffset     int64
+	commitDataOffset    int64
+	largeEdgeListOffset int64
+}
+
+// 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 io.ReaderAt) (Index, error) {
+	// Verify file signature
+	var signature = make([]byte, 4)
+	if _, err := reader.ReadAt(signature, 0); err != nil {
+		return nil, err
+	}
+	if !bytes.Equal(signature, commitFileSignature) {
+		return nil, ErrMalformedCommitGraphFile
+	}
+
+	// Read and verify the file header
+	var header = make([]byte, 4)
+	if _, err := reader.ReadAt(header, 4); err != nil {
+		return nil, err
+	}
+	if header[0] != 1 {
+		return nil, ErrUnsupportedVersion
+	}
+	if header[1] != 1 {
+		return nil, ErrUnsupportedHash
+	}
+
+	// Read chunk headers
+	var chunkID = make([]byte, 4)
+	var oidFanoutOffset int64
+	var oidLookupOffset int64
+	var commitDataOffset int64
+	var largeEdgeListOffset int64
+	chunkCount := int(header[2])
+	for i := 0; i < chunkCount; i++ {
+		chunkHeader := io.NewSectionReader(reader, 8+(int64(i)*12), 12)
+		if _, err := io.ReadAtLeast(chunkHeader, chunkID, 4); err != nil {
+			return nil, err
+		}
+		chunkOffset, err := binary.ReadUint64(chunkHeader)
+		if err != nil {
+			return nil, err
+		}
+
+		if bytes.Equal(chunkID, oidFanoutSignature) {
+			oidFanoutOffset = int64(chunkOffset)
+		} else if bytes.Equal(chunkID, oidLookupSignature) {
+			oidLookupOffset = int64(chunkOffset)
+		} else if bytes.Equal(chunkID, commitDataSignature) {
+			commitDataOffset = int64(chunkOffset)
+		} else if bytes.Equal(chunkID, largeEdgeListSignature) {
+			largeEdgeListOffset = int64(chunkOffset)
+		}
+	}
+
+	if oidFanoutOffset <= 0 || oidLookupOffset <= 0 || commitDataOffset <= 0 {
+		return nil, ErrMalformedCommitGraphFile
+	}
+
+	// Read fanout table and calculate the file offsets into the lookup table
+	fanoutReader := io.NewSectionReader(reader, oidFanoutOffset, 256*4)
+	var fanout [256]uint32
+	for i := 0; i < 256; i++ {
+		var err error
+		if fanout[i], err = binary.ReadUint32(fanoutReader); err != nil {
+			return nil, err
+		}
+	}
+
+	return &fileIndex{reader, fanout, oidLookupOffset, commitDataOffset, largeEdgeListOffset}, nil
+}
+
+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.oidLookupOffset + int64(mid)*20
+		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
+		}
+	}
+
+	return 0, plumbing.ErrObjectNotFound
+}
+
+func (fi *fileIndex) GetNodeByIndex(idx uint32) (*Node, error) {
+	offset := fi.commitDataOffset + int64(idx)*36
+	commitDataReader := io.NewSectionReader(fi.reader, offset, 36)
+
+	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}
+		offset := fi.largeEdgeListOffset + 4*int64(parent2&parentOctopusMask)
+		parentReader := io.NewSectionReader(fi.reader, offset, math.MaxInt64)
+		for {
+			parent, err := binary.ReadUint32(parentReader)
+			if err != nil {
+				return nil, err
+			}
+			parentIndexes = append(parentIndexes, parent1&parentOctopusMask)
+			if parent&parentLast == parentLast {
+				break
+			}
+		}
+	} else if parent2 != parentNone {
+		parentIndexes = []uint32{parent1, parent2}
+	} else if parent1 != parentNone {
+		parentIndexes = []uint32{parent1}
+	}
+
+	return &Node{
+		TreeHash:      treeHash,
+		ParentIndexes: parentIndexes,
+		Generation:    uint32(genAndTime >> 34),
+		When:          time.Unix(int64(genAndTime&0x3FFFFFFFF), 0),
+	}, nil
+}
+
+func (fi *fileIndex) GetHashByIndex(i uint32) (plumbing.Hash, error) {
+	if i > fi.fanout[0xff] {
+		return plumbing.ZeroHash, plumbing.ErrObjectNotFound
+	}
+
+	var oid plumbing.Hash
+	offset := fi.oidLookupOffset + int64(i)*20
+	if _, err := fi.reader.ReadAt(oid[:], offset); err != nil {
+		return plumbing.ZeroHash, err
+	}
+
+	return oid, nil
+}

plumbing/format/commitgraph/commitgraph_test.go

@@ -0,0 +1,35 @@
+package commitgraph_test
+
+import (
+	"testing"
+
+	"golang.org/x/exp/mmap"
+
+	. "gopkg.in/check.v1"
+	"gopkg.in/src-d/go-git-fixtures.v3"
+	"gopkg.in/src-d/go-git.v4/plumbing"
+	"gopkg.in/src-d/go-git.v4/plumbing/format/commitgraph"
+)
+
+func Test(t *testing.T) { TestingT(t) }
+
+type CommitgraphSuite struct {
+	fixtures.Suite
+}
+
+var _ = Suite(&CommitgraphSuite{})
+
+func (s *CommitgraphSuite) TestDecode(c *C) {
+	reader, err := mmap.Open("C:\\Projects\\testgit\\.git\\objects\\info\\commit-graph")
+	c.Assert(err, IsNil)
+	index, err := commitgraph.OpenFileIndex(reader)
+	c.Assert(err, IsNil)
+
+	nodeIndex, err := index.GetIndexByHash(plumbing.NewHash("5aa811d3c2f6d5d6e928a4acacd15248928c26d0"))
+	c.Assert(err, IsNil)
+	node, err := index.GetNodeByIndex(nodeIndex)
+	c.Assert(err, IsNil)
+	c.Assert(len(node.ParentIndexes), Equals, 0)
+
+	reader.Close()
+}