formats/packfile: type Hash instead of strings

7 files changed, 1104 insertions, 0 deletions

diff --git a/formats/packfile/delta.go b/formats/packfile/delta.go
new file mode 100644
index 0000000..86b556f
--- /dev/null
+++ b/formats/packfile/delta.go
@@ -0,0 +1,93 @@
+package packfile
+
+const delta_size_min = 4
+
+func deltaHeaderSize(b []byte) (uint, []byte) {
+	var size, j uint
+	var cmd byte
+	for {
+		cmd = b[j]
+		size |= (uint(cmd) & 0x7f) << (j * 7)
+		j++
+		if uint(cmd)&0xb80 == 0 || j == uint(len(b)) {
+			break
+		}
+	}
+	return size, b[j:]
+}
+
+func PatchDelta(src, delta []byte) []byte {
+	if len(delta) < delta_size_min {
+		return nil
+	}
+	size, delta := deltaHeaderSize(delta)
+	if size != uint(len(src)) {
+		return nil
+	}
+	size, delta = deltaHeaderSize(delta)
+	origSize := size
+
+	dest := make([]byte, 0)
+
+	// var offset uint
+	var cmd byte
+	for {
+		cmd = delta[0]
+		delta = delta[1:]
+		if (cmd & 0x80) != 0 {
+			var cp_off, cp_size uint
+			if (cmd & 0x01) != 0 {
+				cp_off = uint(delta[0])
+				delta = delta[1:]
+			}
+			if (cmd & 0x02) != 0 {
+				cp_off |= uint(delta[0]) << 8
+				delta = delta[1:]
+			}
+			if (cmd & 0x04) != 0 {
+				cp_off |= uint(delta[0]) << 16
+				delta = delta[1:]
+			}
+			if (cmd & 0x08) != 0 {
+				cp_off |= uint(delta[0]) << 24
+				delta = delta[1:]
+			}
+
+			if (cmd & 0x10) != 0 {
+				cp_size = uint(delta[0])
+				delta = delta[1:]
+			}
+			if (cmd & 0x20) != 0 {
+				cp_size |= uint(delta[0]) << 8
+				delta = delta[1:]
+			}
+			if (cmd & 0x40) != 0 {
+				cp_size |= uint(delta[0]) << 16
+				delta = delta[1:]
+			}
+			if cp_size == 0 {
+				cp_size = 0x10000
+			}
+			if cp_off+cp_size < cp_off ||
+				cp_off+cp_size > uint(len(src)) ||
+				cp_size > origSize {
+				break
+			}
+			dest = append(dest, src[cp_off:cp_off+cp_size]...)
+			size -= cp_size
+		} else if cmd != 0 {
+			if uint(cmd) > origSize {
+				break
+			}
+			dest = append(dest, delta[0:uint(cmd)]...)
+			size -= uint(cmd)
+			delta = delta[uint(cmd):]
+		} else {
+			return nil
+		}
+		if size <= 0 {
+			break
+		}
+	}
+	return dest
+}
diff --git a/formats/packfile/doc.go b/formats/packfile/doc.go
new file mode 100644
index 0000000..1fc28da
--- /dev/null
+++ b/formats/packfile/doc.go
@@ -0,0 +1,168 @@
+package packfile
+
+// Code from:
+// https://github.com/gitchain/gitchain/tree/master/git @ 4c2fabdf9
+//
+// GIT pack format
+// ===============
+//
+// == pack-*.pack files have the following format:
+//
+//    - A header appears at the beginning and consists of the following:
+//
+//      4-byte signature:
+//          The signature is: {'P', 'A', 'C', 'K'}
+//
+//      4-byte version number (network byte order):
+//          GIT currently accepts version number 2 or 3 but
+//          generates version 2 only.
+//
+//      4-byte number of objects contained in the pack (network byte order)
+//
+//      Observation: we cannot have more than 4G versions ;-) and
+//      more than 4G objects in a pack.
+//
+//    - The header is followed by number of object entries, each of
+//      which looks like this:
+//
+//      (undeltified representation)
+//      n-byte type and length (3-bit type, (n-1)*7+4-bit length)
+//      compressed data
+//
+//      (deltified representation)
+//      n-byte type and length (3-bit type, (n-1)*7+4-bit length)
+//      20-byte base object name
+//      compressed delta data
+//
+//      Observation: length of each object is encoded in a variable
+//      length format and is not constrained to 32-bit or anything.
+//
+//   - The trailer records 20-byte SHA1 checksum of all of the above.
+//
+// == Original (version 1) pack-*.idx files have the following format:
+//
+//   - The header consists of 256 4-byte network byte order
+//     integers.  N-th entry of this table records the number of
+//     objects in the corresponding pack, the first byte of whose
+//     object name is less than or equal to N.  This is called the
+//     'first-level fan-out' table.
+//
+//   - The header is followed by sorted 24-byte entries, one entry
+//     per object in the pack.  Each entry is:
+//
+//     4-byte network byte order integer, recording where the
+//     object is stored in the packfile as the offset from the
+//     beginning.
+//
+//     20-byte object name.
+//
+//   - The file is concluded with a trailer:
+//
+//     A copy of the 20-byte SHA1 checksum at the end of
+//     corresponding packfile.
+//
+//     20-byte SHA1-checksum of all of the above.
+//
+// Pack Idx file:
+//
+//     --  +--------------------------------+
+// fanout  | fanout[0] = 2 (for example)    |-.
+// table   +--------------------------------+ |
+//         | fanout[1]                      | |
+//         +--------------------------------+ |
+//         | fanout[2]                      | |
+//         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
+//         | fanout[255] = total objects    |---.
+//     --  +--------------------------------+ | |
+// main        | offset                         | | |
+// index       | object name 00XXXXXXXXXXXXXXXX | | |
+// table       +--------------------------------+ | |
+//         | offset                         | | |
+//         | object name 00XXXXXXXXXXXXXXXX | | |
+//         +--------------------------------+<+ |
+//       .-| offset                         |   |
+//       | | object name 01XXXXXXXXXXXXXXXX |   |
+//       | +--------------------------------+   |
+//       | | offset                         |   |
+//       | | object name 01XXXXXXXXXXXXXXXX |   |
+//       | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~   |
+//       | | offset                         |   |
+//       | | object name FFXXXXXXXXXXXXXXXX |   |
+//     --| +--------------------------------+<--+
+// trailer   | | packfile checksum              |
+//       | +--------------------------------+
+//       | | idxfile checksum               |
+//       | +--------------------------------+
+//           .-------.
+//                   |
+// Pack file entry: <+
+//
+//      packed object header:
+//     1-byte size extension bit (MSB)
+//            type (next 3 bit)
+//            size0 (lower 4-bit)
+//         n-byte sizeN (as long as MSB is set, each 7-bit)
+//         size0..sizeN form 4+7+7+..+7 bit integer, size0
+//         is the least significant part, and sizeN is the
+//         most significant part.
+//      packed object data:
+//         If it is not DELTA, then deflated bytes (the size above
+//         is the size before compression).
+//     If it is REF_DELTA, then
+//       20-byte base object name SHA1 (the size above is the
+//         size of the delta data that follows).
+//           delta data, deflated.
+//     If it is OFS_DELTA, then
+//       n-byte offset (see below) interpreted as a negative
+//         offset from the type-byte of the header of the
+//         ofs-delta entry (the size above is the size of
+//         the delta data that follows).
+//       delta data, deflated.
+//
+//      offset encoding:
+//       n bytes with MSB set in all but the last one.
+//       The offset is then the number constructed by
+//       concatenating the lower 7 bit of each byte, and
+//       for n >= 2 adding 2^7 + 2^14 + ... + 2^(7*(n-1))
+//       to the result.
+//
+//
+//
+// == Version 2 pack-*.idx files support packs larger than 4 GiB, and
+//    have some other reorganizations.  They have the format:
+//
+//   - A 4-byte magic number '\377tOc' which is an unreasonable
+//     fanout[0] value.
+//
+//   - A 4-byte version number (= 2)
+//
+//   - A 256-entry fan-out table just like v1.
+//
+//   - A table of sorted 20-byte SHA1 object names.  These are
+//     packed together without offset values to reduce the cache
+//     footprint of the binary search for a specific object name.
+//
+//   - A table of 4-byte CRC32 values of the packed object data.
+//     This is new in v2 so compressed data can be copied directly
+//     from pack to pack during repacking without undetected
+//     data corruption.
+//
+//   - A table of 4-byte offset values (in network byte order).
+//     These are usually 31-bit pack file offsets, but large
+//     offsets are encoded as an index into the next table with
+//     the msbit set.
+//
+//   - A table of 8-byte offset entries (empty for pack files less
+//     than 2 GiB).  Pack files are organized with heavily used
+//     objects toward the front, so most object references should
+//     not need to refer to this table.
+//
+//   - The same trailer as a v1 pack file:
+//
+//     A copy of the 20-byte SHA1 checksum at the end of
+//     corresponding packfile.
+//
+//     20-byte SHA1-checksum of all of the above.
+//
+// From:
+// https://www.kernel.org/pub/software/scm/git/docs/v1.7.5/technical/pack-protocol.txt
diff --git a/formats/packfile/objects.go b/formats/packfile/objects.go
new file mode 100644
index 0000000..4c7ee75
--- /dev/null
+++ b/formats/packfile/objects.go
@@ -0,0 +1,237 @@
+package packfile
+
+import (
+	"bytes"
+	"crypto/sha1"
+	"encoding/hex"
+	"fmt"
+	"strconv"
+	"time"
+)
+
+type ObjectType string
+
+const (
+	CommitObject ObjectType = "commit"
+	TreeObject   ObjectType = "tree"
+	BlobObject   ObjectType = "blob"
+)
+
+// Object generic object interface
+type Object interface {
+	Type() ObjectType
+	Hash() Hash
+}
+
+// Hash SHA1 hased content
+type Hash [20]byte
+
+// ComputeHash compute the hash for a given objType and content
+func ComputeHash(t ObjectType, content []byte) Hash {
+	h := []byte(t)
+	h = append(h, ' ')
+	h = strconv.AppendInt(h, int64(len(content)), 10)
+	h = append(h, 0)
+	h = append(h, content...)
+
+	return Hash(sha1.Sum(h))
+}
+
+func (h Hash) String() string {
+	return hex.EncodeToString(h[:])
+}
+
+// Commit points to a single tree, marking it as what the project looked like
+// at a certain point in time. It contains meta-information about that point
+// in time, such as a timestamp, the author of the changes since the last
+// commit, a pointer to the previous commit(s), etc.
+// http://schacon.github.io/gitbook/1_the_git_object_model.html
+type Commit struct {
+	Tree      Hash
+	Parents   []Hash
+	Author    Signature
+	Committer Signature
+	Message   string
+	hash      Hash
+}
+
+// ParseCommit transform a byte slice into a Commit struct
+func ParseCommit(b []byte) (*Commit, error) {
+	//	b64 := base64.StdEncoding.EncodeToString(b)
+	//fmt.Printf("%q\n", b64)
+
+	o := &Commit{hash: ComputeHash(CommitObject, b)}
+
+	lines := bytes.Split(b, []byte{'\n'})
+	for i := range lines {
+		if len(lines[i]) > 0 {
+			var err error
+
+			split := bytes.SplitN(lines[i], []byte{' '}, 2)
+			switch string(split[0]) {
+			case "tree":
+				_, err = hex.Decode(o.Tree[:], split[1])
+			case "parent":
+				var h Hash
+				_, err = hex.Decode(h[:], split[1])
+				if err == nil {
+					o.Parents = append(o.Parents, h)
+				}
+			case "author":
+				o.Author = ParseSignature(split[1])
+			case "committer":
+				o.Committer = ParseSignature(split[1])
+			}
+
+			if err != nil {
+				return nil, err
+			}
+		} else {
+			o.Message = string(bytes.Join(append(lines[i+1:]), []byte{'\n'}))
+			break
+		}
+
+	}
+
+	return o, nil
+}
+
+// Type returns the object type
+func (o *Commit) Type() ObjectType {
+	return CommitObject
+}
+
+// Hash returns the computed hash of the commit
+func (o *Commit) Hash() Hash {
+	return o.hash
+}
+
+type Tree struct {
+	Entries []TreeEntry
+	hash    Hash
+}
+
+type TreeEntry struct {
+	Name string
+	Hash Hash
+}
+
+func NewTree(body []byte) (*Tree, error) {
+	o := &Tree{hash: ComputeHash(TreeObject, body)}
+
+	if len(body) == 0 {
+		return o, nil
+	}
+
+	for {
+		split := bytes.SplitN(body, []byte{0}, 2)
+		split1 := bytes.SplitN(split[0], []byte{' '}, 2)
+
+		entry := TreeEntry{}
+		entry.Name = string(split1[1])
+		copy(entry.Hash[:], split[1][0:20])
+
+		o.Entries = append(o.Entries, entry)
+
+		body = split[1][20:]
+		if len(split[1]) == 20 {
+			break
+		}
+	}
+
+	return o, nil
+}
+
+func (o *Tree) Type() ObjectType {
+	return TreeObject
+}
+
+func (o *Tree) Hash() Hash {
+	return o.hash
+}
+
+type Blob struct {
+	Len  int
+	hash Hash
+}
+
+func NewBlob(b []byte) (*Blob, error) {
+	return &Blob{
+		Len:  len(b),
+		hash: ComputeHash(BlobObject, b),
+	}, nil
+}
+
+func (o *Blob) Type() ObjectType {
+	return BlobObject
+}
+
+func (o *Blob) Hash() Hash {
+	return o.hash
+}
+
+type ContentCallback func(hash Hash, content []byte)
+
+// Signature represents an action signed by a person
+type Signature struct {
+	Name  string
+	Email string
+	When  time.Time
+}
+
+// ParseSignature parse a byte slice returning a new action signature.
+func ParseSignature(signature []byte) Signature {
+	ret := Signature{}
+	if len(signature) == 0 {
+		return ret
+	}
+
+	from := 0
+	state := 'n' // n: name, e: email, t: timestamp, z: timezone
+	for i := 0; ; i++ {
+		var c byte
+		var end bool
+		if i < len(signature) {
+			c = signature[i]
+		} else {
+			end = true
+		}
+
+		switch state {
+		case 'n':
+			if c == '<' || end {
+				if i == 0 {
+					break
+				}
+				ret.Name = string(signature[from : i-1])
+				state = 'e'
+				from = i + 1
+			}
+		case 'e':
+			if c == '>' || end {
+				ret.Email = string(signature[from:i])
+				i++
+				state = 't'
+				from = i + 1
+			}
+		case 't':
+			if c == ' ' || end {
+				t, err := strconv.ParseInt(string(signature[from:i]), 10, 64)
+				if err == nil {
+					ret.When = time.Unix(t, 0)
+				}
+				end = true
+			}
+		}
+
+		if end {
+			break
+		}
+	}
+
+	return ret
+}
+
+func (s *Signature) String() string {
+	return fmt.Sprintf("%q <%s> @ %s", s.Name, s.Email, s.When)
+}
diff --git a/formats/packfile/objects_test.go b/formats/packfile/objects_test.go
new file mode 100644
index 0000000..70f4ae6
--- /dev/null
+++ b/formats/packfile/objects_test.go
@@ -0,0 +1,88 @@
+package packfile
+
+import (
+	"encoding/base64"
+	"time"
+
+	. "gopkg.in/check.v1"
+)
+
+type ObjectsSuite struct{}
+
+var _ = Suite(&ObjectsSuite{})
+
+func (s *ObjectsSuite) TestComputeHash(c *C) {
+	hash := ComputeHash("blob", []byte(""))
+	c.Assert(hash.String(), Equals, "e69de29bb2d1d6434b8b29ae775ad8c2e48c5391")
+
+	hash = ComputeHash("blob", []byte("Hello, World!\n"))
+	c.Assert(hash.String(), Equals, "8ab686eafeb1f44702738c8b0f24f2567c36da6d")
+}
+
+var CommitFixture = "dHJlZSBjMmQzMGZhOGVmMjg4NjE4ZjY1ZjZlZWQ2ZTE2OGUwZDUxNDg4NmY0CnBhcmVudCBiMDI5NTE3ZjYzMDBjMmRhMGY0YjY1MWI4NjQyNTA2Y2Q2YWFmNDVkCnBhcmVudCBiOGU0NzFmNThiY2JjYTYzYjA3YmRhMjBlNDI4MTkwNDA5YzJkYjQ3CmF1dGhvciBNw6F4aW1vIEN1YWRyb3MgPG1jdWFkcm9zQGdtYWlsLmNvbT4gMTQyNzgwMjQzNCArMDIwMApjb21taXR0ZXIgTcOheGltbyBDdWFkcm9zIDxtY3VhZHJvc0BnbWFpbC5jb20+IDE0Mjc4MDI0MzQgKzAyMDAKCk1lcmdlIHB1bGwgcmVxdWVzdCAjMSBmcm9tIGRyaXBvbGxlcy9mZWF0dXJlCgpDcmVhdGluZyBjaGFuZ2Vsb2c="
+
+func (s *ObjectsSuite) TestParseCommit(c *C) {
+	data, _ := base64.StdEncoding.DecodeString(CommitFixture)
+	commit, err := ParseCommit(data)
+	c.Assert(err, IsNil)
+
+	c.Assert(commit.Tree.String(), Equals, "c2d30fa8ef288618f65f6eed6e168e0d514886f4")
+	c.Assert(commit.Parents, HasLen, 2)
+	c.Assert(commit.Parents[0].String(), Equals, "b029517f6300c2da0f4b651b8642506cd6aaf45d")
+	c.Assert(commit.Parents[1].String(), Equals, "b8e471f58bcbca63b07bda20e428190409c2db47")
+	c.Assert(commit.Author.Email, Equals, "mcuadros@gmail.com")
+	c.Assert(commit.Author.Name, Equals, "Máximo Cuadros")
+	c.Assert(commit.Author.When.Unix(), Equals, int64(1427802434))
+	c.Assert(commit.Committer.Email, Equals, "mcuadros@gmail.com")
+	c.Assert(commit.Message, Equals, "Merge pull request #1 from dripolles/feature\n\nCreating changelog")
+}
+
+func (s *ObjectsSuite) TestCommitHash(c *C) {
+	data, _ := base64.StdEncoding.DecodeString(CommitFixture)
+	commit, err := ParseCommit(data)
+
+	c.Assert(err, IsNil)
+	c.Assert(commit.Hash().String(), Equals, "a5b8b09e2f8fcb0bb99d3ccb0958157b40890d69")
+}
+
+func (s *ObjectsSuite) TestParseSignature(c *C) {
+	cases := map[string]Signature{
+		`Foo Bar <foo@bar.com> 1257894000 +0100`: {
+			Name:  "Foo Bar",
+			Email: "foo@bar.com",
+			When:  time.Unix(1257894000, 0),
+		},
+		`Foo Bar <> 1257894000 +0100`: {
+			Name:  "Foo Bar",
+			Email: "",
+			When:  time.Unix(1257894000, 0),
+		},
+		` <> 1257894000`: {
+			Name:  "",
+			Email: "",
+			When:  time.Unix(1257894000, 0),
+		},
+		`Foo Bar <foo@bar.com>`: {
+			Name:  "Foo Bar",
+			Email: "foo@bar.com",
+			When:  time.Time{},
+		},
+		``: {
+			Name:  "",
+			Email: "",
+			When:  time.Time{},
+		},
+		`<`: {
+			Name:  "",
+			Email: "",
+			When:  time.Time{},
+		},
+	}
+
+	for raw, exp := range cases {
+		got := ParseSignature([]byte(raw))
+		c.Assert(got.Name, Equals, exp.Name)
+		c.Assert(got.Email, Equals, exp.Email)
+		c.Assert(got.When.Unix(), Equals, exp.When.Unix())
+	}
+}
diff --git a/formats/packfile/packfile.go b/formats/packfile/packfile.go
new file mode 100644
index 0000000..d70f396
--- /dev/null
+++ b/formats/packfile/packfile.go
@@ -0,0 +1,82 @@
+package packfile
+
+import "fmt"
+
+type Packfile struct {
+	Version     uint32
+	Size        int64
+	ObjectCount int
+	Checksum    []byte
+	Commits     map[Hash]*Commit
+	Trees       map[Hash]*Tree
+	Blobs       map[Hash]*Blob
+}
+
+func NewPackfile() *Packfile {
+	return &Packfile{
+		Commits: make(map[Hash]*Commit, 0),
+		Trees:   make(map[Hash]*Tree, 0),
+		Blobs:   make(map[Hash]*Blob, 0),
+	}
+}
+
+type BlobEntry struct {
+	path string
+	*Blob
+}
+
+type SubtreeEntry struct {
+	path string
+	*Tree
+	TreeCh
+}
+
+type treeEntry interface {
+	isTreeEntry()
+	Path() string
+}
+
+func (b BlobEntry) isTreeEntry()    {}
+func (b BlobEntry) Path() string    { return b.path }
+func (b SubtreeEntry) isTreeEntry() {}
+func (b SubtreeEntry) Path() string { return b.path }
+
+type TreeCh <-chan treeEntry
+
+func (p *Packfile) WalkCommit(commitHash Hash) (TreeCh, error) {
+	commit, ok := p.Commits[commitHash]
+	if !ok {
+		return nil, fmt.Errorf("Unable to find %q commit", commitHash)
+	}
+
+	return p.WalkTree(p.Trees[commit.Tree]), nil
+}
+
+func (p *Packfile) WalkTree(tree *Tree) TreeCh {
+	return p.walkTree(tree, "")
+}
+
+func (p *Packfile) walkTree(tree *Tree, pathPrefix string) TreeCh {
+	ch := make(chan treeEntry)
+
+	if tree == nil {
+		close(ch)
+		return ch
+	}
+
+	go func() {
+		defer func() {
+			close(ch)
+		}()
+		for _, e := range tree.Entries {
+			path := pathPrefix + e.Name
+			if blob, ok := p.Blobs[e.Hash]; ok {
+				ch <- BlobEntry{path, blob}
+			} else if subtree, ok := p.Trees[e.Hash]; ok {
+				ch <- SubtreeEntry{path, subtree, p.walkTree(subtree, path+"/")}
+			}
+		}
+	}()
+
+	return ch
+}
diff --git a/formats/packfile/reader.go b/formats/packfile/reader.go
new file mode 100644
index 0000000..ccf4822
--- /dev/null
+++ b/formats/packfile/reader.go
@@ -0,0 +1,401 @@
+package packfile
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"io"
+
+	"github.com/klauspost/compress/zlib"
+)
+
+const MaxObjectsLimit = 1000000
+
+var ErrMaxSize = fmt.Errorf("Max size exceeded for in-memory client")
+
+type TrackingByteReader struct {
+	r    io.Reader
+	n, l int
+}
+
+func (t *TrackingByteReader) Pos() int { return t.n }
+
+func (t *TrackingByteReader) Read(p []byte) (n int, err error) {
+	n, err = t.r.Read(p)
+	if err != nil {
+		return 0, err
+	}
+	t.n += n
+	if t.n >= t.l {
+		return n, ErrMaxSize
+	}
+	return n, err
+}
+
+func (t *TrackingByteReader) ReadByte() (c byte, err error) {
+	var p [1]byte
+	n, err := t.r.Read(p[:])
+	if err != nil {
+		return 0, err
+	}
+	if n > 1 {
+		return 0, fmt.Errorf("read %d bytes, should have read just 1", n)
+	}
+	t.n += n // n is 1
+	return p[0], nil
+}
+
+type PackfileReader struct {
+	r *TrackingByteReader
+
+	objects map[Hash]packfileObject
+	offsets map[int]Hash
+	deltas  []packfileDelta
+
+	contentCallback ContentCallback
+}
+
+type packfileObject struct {
+	bytes []byte
+	typ   int8
+}
+
+type packfileDelta struct {
+	hash  Hash
+	delta []byte
+}
+
+func NewPackfileReader(r io.Reader, l int, fn ContentCallback) (*PackfileReader, error) {
+	return &PackfileReader{
+		r:               &TrackingByteReader{r: r, n: 0, l: l},
+		objects:         make(map[Hash]packfileObject, 0),
+		offsets:         make(map[int]Hash, 0),
+		contentCallback: fn,
+	}, nil
+}
+
+func (pr *PackfileReader) Pos() int { return pr.r.Pos() }
+
+func (pr *PackfileReader) Read() (*Packfile, error) {
+	packfile := NewPackfile()
+
+	if err := pr.validateHeader(); err != nil {
+		if err == io.EOF {
+			// This is an empty repo. It's OK.
+			return packfile, nil
+		}
+		return nil, err
+	}
+
+	ver, err := pr.readInt32()
+	if err != nil {
+		return nil, err
+	}
+
+	count, err := pr.readInt32()
+	if err != nil {
+		return nil, err
+	}
+
+	packfile.Version = uint32(ver)
+	packfile.ObjectCount = int(count)
+
+	if packfile.ObjectCount > MaxObjectsLimit {
+		return nil, NewError("too many objects (%d)", packfile.ObjectCount)
+	}
+
+	if err := pr.readObjects(packfile); err != nil {
+		return nil, err
+	}
+
+	packfile.Size = int64(pr.r.Pos())
+
+	return packfile, nil
+}
+
+func (pr *PackfileReader) validateHeader() error {
+	var header = make([]byte, 4)
+	if _, err := pr.r.Read(header); err != nil {
+		return err
+	}
+
+	if !bytes.Equal(header, []byte{'P', 'A', 'C', 'K'}) {
+		return NewError("Pack file does not start with 'PACK'")
+	}
+
+	return nil
+}
+
+func (pr *PackfileReader) readInt32() (uint32, error) {
+	var value uint32
+	if err := binary.Read(pr.r, binary.BigEndian, &value); err != nil {
+		return 0, err
+	}
+
+	return value, nil
+}
+
+func (pr *PackfileReader) readObjects(packfile *Packfile) error {
+	// This code has 50-80 µs of overhead per object not counting zlib inflation.
+	// Together with zlib inflation, it's 400-410 µs for small objects.
+	// That's 1 sec for ~2450 objects, ~4.20 MB, or ~250 ms per MB,
+	// of which 12-20 % is _not_ zlib inflation (ie. is our code).
+
+	for i := 0; i < packfile.ObjectCount; i++ {
+		var pos = pr.Pos()
+		obj, err := pr.readObject(packfile)
+		if err != nil && err != io.EOF {
+			return err
+		}
+
+		pr.offsets[pos] = obj.hash
+
+		if err == io.EOF {
+			break
+		}
+	}
+
+	return nil
+}
+
+func (pr *PackfileReader) readObject(packfile *Packfile) (*objectReader, error) {
+	o, err := newObjectReader(pr, packfile)
+	if err != nil {
+		return nil, err
+	}
+
+	switch o.typ {
+	case OBJ_REF_DELTA:
+		err = o.readREFDelta()
+	case OBJ_OFS_DELTA:
+		err = o.readOFSDelta()
+	case OBJ_COMMIT, OBJ_TREE, OBJ_BLOB, OBJ_TAG:
+		err = o.readObject()
+	default:
+		err = NewError("Invalid git object tag %q", o.typ)
+	}
+
+	if err != nil {
+		return nil, err
+	}
+
+	return o, err
+}
+
+const (
+	OBJ_COMMIT    = 1
+	OBJ_TREE      = 2
+	OBJ_BLOB      = 3
+	OBJ_TAG       = 4
+	OBJ_OFS_DELTA = 6
+	OBJ_REF_DELTA = 7
+)
+
+const SIZE_LIMIT uint64 = 1 << 32 // 4GB
+
+type objectReader struct {
+	pr    *PackfileReader
+	pf    *Packfile
+	hash  Hash
+	steps int
+
+	typ  int8
+	size uint64
+}
+
+func newObjectReader(pr *PackfileReader, pf *Packfile) (*objectReader, error) {
+	o := &objectReader{pr: pr, pf: pf}
+
+	var buf [1]byte
+	if _, err := o.Read(buf[:]); err != nil {
+		return nil, err
+	}
+
+	o.typ = int8((buf[0] >> 4) & 7)
+	o.size = uint64(buf[0] & 15)
+	o.steps++ // byte we just read to get `o.typ` and `o.size`
+
+	var shift uint = 4
+	for buf[0]&0x80 == 0x80 {
+		if _, err := o.Read(buf[:]); err != nil {
+			return nil, err
+		}
+
+		o.size += uint64(buf[0]&0x7f) << shift
+		o.steps++ // byte we just read to update `o.size`
+		shift += 7
+	}
+
+	return o, nil
+}
+
+func (o *objectReader) readREFDelta() error {
+	var ref Hash
+	if _, err := o.Read(ref[:]); err != nil {
+		return err
+	}
+
+	buf, err := o.inflate()
+	if err != nil {
+		return err
+	}
+
+	referenced, ok := o.pr.objects[ref]
+	if !ok {
+		o.pr.deltas = append(o.pr.deltas, packfileDelta{hash: ref, delta: buf[:]})
+	} else {
+		patched := PatchDelta(referenced.bytes, buf[:])
+		if patched == nil {
+			return NewError("error while patching %x", ref)
+		}
+		o.typ = referenced.typ
+		err = o.addObject(patched)
+		if err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+func decodeOffset(src io.ByteReader, steps int) (int, error) {
+	b, err := src.ReadByte()
+	if err != nil {
+		return 0, err
+	}
+	var offset = int(b & 0x7f)
+	for (b & 0x80) != 0 {
+		offset += 1 // WHY?
+		b, err = src.ReadByte()
+		if err != nil {
+			return 0, err
+		}
+		offset = (offset << 7) + int(b&0x7f)
+	}
+	// offset needs to be aware of the bytes we read for `o.typ` and `o.size`
+	offset += steps
+	return -offset, nil
+}
+
+func (o *objectReader) readOFSDelta() error {
+	var pos = o.pr.Pos()
+
+	// read negative offset
+	offset, err := decodeOffset(o.pr.r, o.steps)
+	if err != nil {
+		return err
+	}
+
+	buf, err := o.inflate()
+	if err != nil {
+		return err
+	}
+
+	ref := o.pr.offsets[pos+offset]
+	referenced, ok := o.pr.objects[ref]
+	if !ok {
+		return NewError("can't find a pack entry at %d", pos+offset)
+	} else {
+		patched := PatchDelta(referenced.bytes, buf)
+		if patched == nil {
+			return NewError("error while patching %q", ref)
+		}
+		o.typ = referenced.typ
+		err = o.addObject(patched)
+		if err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+func (o *objectReader) readObject() error {
+	buf, err := o.inflate()
+	if err != nil {
+		return err
+	}
+
+	return o.addObject(buf)
+}
+
+func (o *objectReader) addObject(bytes []byte) error {
+	var hash Hash
+
+	switch o.typ {
+	case OBJ_COMMIT:
+		c, err := ParseCommit(bytes)
+		if err != nil {
+			return err
+		}
+		o.pf.Commits[c.Hash()] = c
+		hash = c.Hash()
+	case OBJ_TREE:
+		c, err := NewTree(bytes)
+		if err != nil {
+			return err
+		}
+		o.pf.Trees[c.Hash()] = c
+		hash = c.Hash()
+	case OBJ_BLOB:
+		c, err := NewBlob(bytes)
+		if err != nil {
+			return err
+		}
+		o.pf.Blobs[c.Hash()] = c
+		hash = c.Hash()
+
+		if o.pr.contentCallback != nil {
+			o.pr.contentCallback(hash, bytes)
+		}
+	}
+
+	o.pr.objects[hash] = packfileObject{bytes: bytes, typ: o.typ}
+	o.hash = hash
+
+	return nil
+}
+
+func (o *objectReader) inflate() ([]byte, error) {
+	zr, err := zlib.NewReader(o.pr.r)
+	if err != nil {
+		if err == zlib.ErrHeader {
+			return nil, zlib.ErrHeader
+		} else {
+			return nil, NewError("error opening packfile's object zlib: %v", err)
+		}
+	}
+	defer zr.Close()
+
+	if o.size > SIZE_LIMIT {
+		return nil, NewError("the object size exceeed the allowed limit: %d", o.size)
+	}
+
+	var buf bytes.Buffer
+	io.Copy(&buf, zr) // also: io.CopyN(&buf, zr, int64(o.size))
+
+	var bufLen = buf.Len()
+	if bufLen != int(o.size) {
+		return nil, NewError("inflated size mismatch, expected %d, got %d", o.size, bufLen)
+	}
+
+	return buf.Bytes(), nil
+}
+
+func (o *objectReader) Read(p []byte) (int, error) {
+	return o.pr.r.Read(p)
+}
+
+func (o *objectReader) ReadByte() (byte, error) {
+	return o.pr.r.ReadByte()
+}
+
+type ReaderError struct {
+	Msg string // description of error
+}
+
+func NewError(format string, args ...interface{}) error {
+	return &ReaderError{Msg: fmt.Sprintf(format, args...)}
+}
+
+func (e *ReaderError) Error() string { return e.Msg }
diff --git a/formats/packfile/reader_test.go b/formats/packfile/reader_test.go
new file mode 100644
index 0000000..04f2948
--- /dev/null
+++ b/formats/packfile/reader_test.go
@@ -0,0 +1,35 @@
+package packfile
+
+import (
+	"bytes"
+	"encoding/base64"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+var packFileWithEmptyObjects = "UEFDSwAAAAIAAAALnw54nKXMQWoDMQxA0b1PoX2hSLIm44FSAlmXnEG2NYlhXAfHgdLb5Cy9WAM5Qpb/Lf7oZqArUpakyYtQjCoxZ5lmWXwwyuzJbHqAuYt2+x6QoyCyhYCKIa67lGameSLWvPh5JU0hsCg7vY1z6/D1d/8ptcHhprm3Kxz7KL/wUdOz96eqZXtPrX4CCeOOPU8Eb0iI7qG1jGGvXdxaNoPs/gHeNkp8lA94nKXMQUpDMRCA4X1OMXtBZpI3L3kiRXAtPcMkmWjgxZSYQultPEsv1oJHcPl/i38OVRC0IXF0lshrJorZEcpKmTEJYbA+B3aFzEmGfk9gpqJEsmnZNutXF71i1IURU/G0bsWWwJ6NnOdXH/Bx+73U1uH9LHn0HziOWa/w2tJfv302qftz6u0AtFh0wQdmeEJCNA9tdU7938WUuivEF5CczR11ZEsNnw54nKWMUQoCIRRF/13F+w/ijY6jQkTQd7SGpz5LyAxzINpNa2ljTbSEPu/hnNsbM4TJTzqyt561GdUUmJKT6K2MeiCVgnZWoY/iRo2vHVS0URrUS+e+dkqIEp11HMhh9IaUkRM6QXM/1waH9+uRS4X9TLHVOxxbz0/YlPDbu1OhfFmHWrYwjBKVNVaNsMIBUSy05N75vxeR8oXBiw8GoErCnwt4nKXMzQkCMRBA4XuqmLsgM2M2ZkAWwbNYQ341sCEQsyB2Yy02pmAJHt93eKOnBFpMNJqtl5CFxVIMomViomQSEWP2JrN3yq3j1jqc369HqQ1Oq4u93eHSR3nCoYZfH6/VlWUbWp2BNOPO7i1OsEFCVF+tZYz030XlsiRw6gPZ0jxaqwV4nDM0MDAzMVFIZHg299HsTRevOXt3a64rj7px6ElP8ERDiGQSQ2uoXe8RrcodS5on+J4/u8HjD4NDKFQyRS8tPx+rbgDt3yiEMHicAwAAAAABPnicS0wEAa4kMOACACTjBKdkZXici7aaYAUAA3gBYKoDeJwzNDAwMzFRSGR4NvfR7E0Xrzl7d2uuK4+6cehJT/BEQ4hkEsOELYFJvS2eX47UJdVttFQrenrmzQwA13MaiDd4nEtMBAEuAApMAlGtAXicMzQwMDMxUUhkeDb30exNF685e3drriuPunHoSU/wRACvkA258N/i8hVXx9CiAZzvFXNIhCuSFmE="
+
+func TestReadPackfile(t *testing.T) {
+	data, _ := base64.StdEncoding.DecodeString(packFileWithEmptyObjects)
+	d := bytes.NewReader(data)
+
+	r, err := NewPackfileReader(d, 8<<20, nil)
+	assert.Nil(t, err)
+
+	p, err := r.Read()
+	assert.Nil(t, err)
+
+	assert.Equal(t, 11, p.ObjectCount)
+	assert.Equal(t, 4, len(p.Commits))
+	assert.Equal(t, 4, len(p.Trees))
+}
+
+func TestReadPackfileInvalid(t *testing.T) {
+	r, err := NewPackfileReader(bytes.NewReader([]byte("dasdsadasas")), 8<<20, nil)
+	assert.Nil(t, err)
+
+	_, err = r.Read()
+	_, ok := err.(*ReaderError)
+	assert.True(t, ok)
+}