1 files changed, 353 insertions, 0 deletions
diff --git a/formats/packfile/parser.go b/formats/packfile/parser.go
new file mode 100644
index 0000000..d3463bd
--- /dev/null
+++ b/formats/packfile/parser.go
@@ -0,0 +1,353 @@
+package packfile
+
+import (
+	"bytes"
+	"compress/zlib"
+	"encoding/binary"
+	"fmt"
+	"io"
+
+	"gopkg.in/src-d/go-git.v3/core"
+	"gopkg.in/src-d/go-git.v3/storage/memory"
+)
+
+var (
+	// ErrEmptyPackfile is returned by ReadHeader when no data is found in the packfile
+	ErrEmptyPackfile = NewError("empty packfile")
+	// ErrBadSignature is returned by ReadHeader when the signature in the packfile is incorrect.
+	ErrBadSignature = NewError("malformed pack file signature")
+	// ErrUnsupportedVersion is returned by ReadHeader when the packfile version is
+	// different than VersionSupported.
+	ErrUnsupportedVersion = NewError("unsupported packfile version")
+)
+
+const (
+	// VersionSupported is the packfile version supported by this parser.
+	VersionSupported = 2
+)
+
+// A Parser is a collection of functions to read and process data form a packfile.
+// Values from this type are not zero-value safe. See the NewParser function bellow.
+type Parser struct {
+	ReadRecaller
+}
+
+// NewParser returns a new Parser that reads from the packfile represented by r.
+func NewParser(r ReadRecaller) *Parser {
+	return &Parser{ReadRecaller: r}
+}
+
+// ReadInt32 reads 4 bytes and returns them as a Big Endian int32.
+func (p Parser) readInt32() (uint32, error) {
+	var v uint32
+	if err := binary.Read(p, binary.BigEndian, &v); err != nil {
+		return 0, err
+	}
+
+	return v, nil
+}
+
+// ReadSignature reads an returns the signature field in the packfile.
+func (p *Parser) ReadSignature() ([]byte, error) {
+	var sig = make([]byte, 4)
+	if _, err := io.ReadFull(p, sig); err != nil {
+		return []byte{}, err
+	}
+
+	return sig, nil
+}
+
+// IsValidSignature returns if sig is a valid packfile signature.
+func (p Parser) IsValidSignature(sig []byte) bool {
+	return bytes.Equal(sig, []byte{'P', 'A', 'C', 'K'})
+}
+
+// ReadVersion reads and returns the version field of a packfile.
+func (p *Parser) ReadVersion() (uint32, error) {
+	return p.readInt32()
+}
+
+// IsSupportedVersion returns whether version v is supported by the parser.
+// The current supported version is VersionSupported, defined above.
+func (p *Parser) IsSupportedVersion(v uint32) bool {
+	return v == VersionSupported
+}
+
+// ReadCount reads and returns the count of objects field of a packfile.
+func (p *Parser) ReadCount() (uint32, error) {
+	return p.readInt32()
+}
+
+// ReadHeader reads the whole packfile header (signature, version and
+// object count). It returns the object count and performs checks on the
+// validity of the signature and the version fields.
+func (p Parser) ReadHeader() (uint32, error) {
+	sig, err := p.ReadSignature()
+	if err != nil {
+		if err == io.EOF {
+			return 0, ErrEmptyPackfile
+		}
+		return 0, err
+	}
+
+	if !p.IsValidSignature(sig) {
+		return 0, ErrBadSignature
+	}
+
+	ver, err := p.ReadVersion()
+	if err != nil {
+		return 0, err
+	}
+
+	if !p.IsSupportedVersion(ver) {
+		return 0, ErrUnsupportedVersion.AddDetails("%d", ver)
+	}
+
+	count, err := p.ReadCount()
+	if err != nil {
+		return 0, err
+	}
+
+	return count, nil
+}
+
+// ReadObjectTypeAndLength reads and returns the object type and the
+// length field from an object entry in a packfile.
+func (p Parser) ReadObjectTypeAndLength() (core.ObjectType, int64, error) {
+	t, c, err := p.readType()
+	if err != nil {
+		return t, 0, err
+	}
+
+	l, err := p.readLength(c)
+
+	return t, l, err
+}
+
+func (p Parser) readType() (core.ObjectType, byte, error) {
+	var c byte
+	var err error
+	if c, err = p.ReadByte(); err != nil {
+		return core.ObjectType(0), 0, err
+	}
+	typ := parseType(c)
+
+	return typ, c, nil
+}
+
+var (
+	maskContinue    = uint8(128) // 1000 0000
+	maskType        = uint8(112) // 0111 0000
+	maskFirstLength = uint8(15)  // 0000 1111
+	firstLengthBits = uint8(4)   // the first byte has 4 bits to store the length
+	maskLength      = uint8(127) // 0111 1111
+	lengthBits      = uint8(7)   // subsequent bytes has 7 bits to store the length
+)
+
+func parseType(b byte) core.ObjectType {
+	return core.ObjectType((b & maskType) >> firstLengthBits)
+}
+
+// the length is codified in the last 4 bits of the first byte and in
+// the last 7 bits of subsequent bytes.  Last byte has a 0 MSB.
+func (p Parser) readLength(first byte) (int64, error) {
+	length := int64(first & maskFirstLength)
+
+	c := first
+	shift := firstLengthBits
+	var err error
+	for moreBytesInLength(c) {
+		if c, err = p.ReadByte(); err != nil {
+			return 0, err
+		}
+
+		length += int64(c&maskLength) << shift
+		shift += lengthBits
+	}
+
+	return length, nil
+}
+
+func moreBytesInLength(c byte) bool {
+	return c&maskContinue > 0
+}
+
+// ReadObject reads and returns a git object from an object entry in the packfile.
+// Non-deltified and deltified objects are supported.
+func (p Parser) ReadObject() (core.Object, error) {
+	start, err := p.Offset()
+	if err != nil {
+		return nil, err
+	}
+
+	var typ core.ObjectType
+	typ, _, err = p.ReadObjectTypeAndLength()
+	if err != nil {
+		return nil, err
+	}
+
+	var cont []byte
+	switch typ {
+	case core.CommitObject, core.TreeObject, core.BlobObject, core.TagObject:
+		cont, err = p.ReadNonDeltaObjectContent()
+	case core.REFDeltaObject:
+		cont, typ, err = p.ReadREFDeltaObjectContent()
+	case core.OFSDeltaObject:
+		cont, typ, err = p.ReadOFSDeltaObjectContent(start)
+	default:
+		err = ErrInvalidObject.AddDetails("tag %q", typ)
+	}
+	if err != nil {
+		return nil, err
+	}
+
+	return memory.NewObject(typ, int64(len(cont)), cont), nil
+}
+
+// ReadNonDeltaObjectContent reads and returns a non-deltified object
+// from it zlib stream in an object entry in the packfile.
+func (p Parser) ReadNonDeltaObjectContent() ([]byte, error) {
+	return p.readZip()
+}
+
+func (p Parser) readZip() ([]byte, error) {
+	buf := bytes.NewBuffer(nil)
+	err := p.inflate(buf)
+
+	return buf.Bytes(), err
+}
+
+func (p Parser) inflate(w io.Writer) (err error) {
+	zr, err := zlib.NewReader(p)
+	if err != nil {
+		if err != zlib.ErrHeader {
+			return fmt.Errorf("zlib reading error: %s", err)
+		}
+	}
+
+	defer func() {
+		closeErr := zr.Close()
+		if err == nil {
+			err = closeErr
+		}
+	}()
+
+	_, err = io.Copy(w, zr)
+
+	return err
+}
+
+// ReadREFDeltaObjectContent reads and returns an object specified by a
+// REF-Delta entry in the packfile, form the hash onwards.
+func (p Parser) ReadREFDeltaObjectContent() ([]byte, core.ObjectType, error) {
+	refHash, err := p.ReadHash()
+	if err != nil {
+		return nil, core.ObjectType(0), err
+	}
+
+	refObj, err := p.RecallByHash(refHash)
+	if err != nil {
+		return nil, core.ObjectType(0), err
+	}
+
+	content, err := p.ReadSolveDelta(refObj.Content())
+	if err != nil {
+		return nil, refObj.Type(), err
+	}
+
+	return content, refObj.Type(), nil
+}
+
+// ReadHash reads a hash.
+func (p Parser) ReadHash() (core.Hash, error) {
+	var h core.Hash
+	if _, err := io.ReadFull(p, h[:]); err != nil {
+		return core.ZeroHash, err
+	}
+
+	return h, nil
+}
+
+// ReadSolveDelta reads and returns the base patched with the contents
+// of a zlib compressed diff data in the delta portion of an object
+// entry in the packfile.
+func (p Parser) ReadSolveDelta(base []byte) ([]byte, error) {
+	diff, err := p.readZip()
+	if err != nil {
+		return nil, err
+	}
+
+	return PatchDelta(base, diff), nil
+}
+
+// ReadOFSDeltaObjectContent reads an returns an object specified by an
+// OFS-delta entry in the packfile from it negative offset onwards.  The
+// start parameter is the offset of this particular object entry (the
+// current offset minus the already processed type and length).
+func (p Parser) ReadOFSDeltaObjectContent(start int64) (
+	[]byte, core.ObjectType, error) {
+
+	jump, err := p.ReadNegativeOffset()
+	if err != nil {
+		return nil, core.ObjectType(0), err
+	}
+
+	ref, err := p.RecallByOffset(start + jump)
+	if err != nil {
+		return nil, core.ObjectType(0), err
+	}
+
+	content, err := p.ReadSolveDelta(ref.Content())
+	if err != nil {
+		return nil, ref.Type(), err
+	}
+
+	return content, ref.Type(), nil
+}
+
+// ReadNegativeOffset reads and returns an offset from a OFS DELTA
+// object entry in a packfile. OFS DELTA offsets are specified in Git
+// VLQ special format:
+//
+// Ordinary VLQ has some redundancies, example:  the number 358 can be
+// encoded as the 2-octet VLQ 0x8166 or the 3-octet VLQ 0x808166 or the
+// 4-octet VLQ 0x80808166 and so forth.
+//
+// To avoid these redundancies, the VLQ format used in Git removes this
+// prepending redundancy and extends the representable range of shorter
+// VLQs by adding an offset to VLQs of 2 or more octets in such a way
+// that the lowest possible value for such an (N+1)-octet VLQ becomes
+// exactly one more than the maximum possible value for an N-octet VLQ.
+// In particular, since a 1-octet VLQ can store a maximum value of 127,
+// the minimum 2-octet VLQ (0x8000) is assigned the value 128 instead of
+// 0. Conversely, the maximum value of such a 2-octet VLQ (0xff7f) is
+// 16511 instead of just 16383. Similarly, the minimum 3-octet VLQ
+// (0x808000) has a value of 16512 instead of zero, which means
+// that the maximum 3-octet VLQ (0xffff7f) is 2113663 instead of
+// just 2097151.  And so forth.
+//
+// This is how the offset is saved in C:
+//
+//     dheader[pos] = ofs & 127;
+//     while (ofs >>= 7)
+//         dheader[--pos] = 128 | (--ofs & 127);
+//
+func (p Parser) ReadNegativeOffset() (int64, error) {
+	var c byte
+	var err error
+
+	if c, err = p.ReadByte(); err != nil {
+		return 0, err
+	}
+
+	var offset = int64(c & maskLength)
+	for moreBytesInLength(c) {
+		offset++
+		if c, err = p.ReadByte(); err != nil {
+			return 0, err
+		}
+		offset = (offset << lengthBits) + int64(c&maskLength)
+	}
+
+	return -offset, nil
+}