Substitute old pktline encoder/decoder with new pktline scanner (#84)

* replace old pktline package with new pktline scanner

* remove error checks on pktline.NewFromString

* fix deppend bug

* reduce memory garbage when pktline.NewFromStrings

* improve int to hex conversion to help gc

* make intToHex func private

* clean function names

13 files changed, 723 insertions, 417 deletions

diff --git a/clients/common/common.go b/clients/common/common.go
index 1518081..df1c233 100644
--- a/clients/common/common.go
+++ b/clients/common/common.go
@@ -11,7 +11,7 @@ import (
 	"strings"
 
 	"gopkg.in/src-d/go-git.v4/core"
-	"gopkg.in/src-d/go-git.v4/formats/pktline"
+	"gopkg.in/src-d/go-git.v4/formats/packp/pktline"
 	"gopkg.in/src-d/go-git.v4/storage/memory"
 )
 
@@ -204,8 +204,8 @@ func NewGitUploadPackInfo() *GitUploadPackInfo {
 	return &GitUploadPackInfo{Capabilities: NewCapabilities()}
 }
 
-func (r *GitUploadPackInfo) Decode(d *pktline.Decoder) error {
-	if err := r.read(d); err != nil {
+func (r *GitUploadPackInfo) Decode(s *pktline.Scanner) error {
+	if err := r.read(s); err != nil {
 		if err == ErrEmptyGitUploadPack {
 			return core.NewPermanentError(err)
 		}
@@ -216,16 +216,29 @@ func (r *GitUploadPackInfo) Decode(d *pktline.Decoder) error {
 	return nil
 }
 
-func (r *GitUploadPackInfo) read(d *pktline.Decoder) error {
-	lines, err := d.ReadAll()
-	if err != nil {
-		return err
-	}
-
+func (r *GitUploadPackInfo) read(s *pktline.Scanner) error {
 	isEmpty := true
 	r.Refs = make(memory.ReferenceStorage, 0)
-	for _, line := range lines {
-		if !r.isValidLine(line) {
+	smartCommentIgnore := false
+	for s.Scan() {
+		line := string(s.Bytes())
+
+		if smartCommentIgnore {
+			// some servers like Github add a flush-pkt after the smart http comment
+			// that we must ignore to prevent a premature termination of the read.
+			if len(line) == 0 {
+				continue
+			}
+			smartCommentIgnore = false
+		}
+
+		// exit on first flush-pkt
+		if len(line) == 0 {
+			break
+		}
+
+		if isSmartHttpComment(line) {
+			smartCommentIgnore = true
 			continue
 		}
 
@@ -240,11 +253,11 @@ func (r *GitUploadPackInfo) read(d *pktline.Decoder) error {
 		return ErrEmptyGitUploadPack
 	}
 
-	return nil
+	return s.Err()
 }
 
-func (r *GitUploadPackInfo) isValidLine(line string) bool {
-	return line[0] != '#'
+func isSmartHttpComment(line string) bool {
+	return line[0] == '#'
 }
 
 func (r *GitUploadPackInfo) readLine(line string) error {
@@ -280,21 +293,28 @@ func (r *GitUploadPackInfo) String() string {
 }
 
 func (r *GitUploadPackInfo) Bytes() []byte {
-	e := pktline.NewEncoder()
-	e.AddLine("# service=git-upload-pack")
-	e.AddFlush()
-	e.AddLine(fmt.Sprintf("%s HEAD\x00%s", r.Head().Hash(), r.Capabilities.String()))
+	payloads := []string{}
+	payloads = append(payloads, "# service=git-upload-pack\n")
+	// inserting a flush-pkt here violates the protocol spec, but some
+	// servers do it, like Github.com
+	payloads = append(payloads, "")
+
+	firstLine := fmt.Sprintf("%s HEAD\x00%s\n", r.Head().Hash(), r.Capabilities.String())
+	payloads = append(payloads, firstLine)
 
 	for _, ref := range r.Refs {
 		if ref.Type() != core.HashReference {
 			continue
 		}
 
-		e.AddLine(fmt.Sprintf("%s %s", ref.Hash(), ref.Name()))
+		ref := fmt.Sprintf("%s %s\n", ref.Hash(), ref.Name())
+		payloads = append(payloads, ref)
 	}
 
-	e.AddFlush()
-	b, _ := ioutil.ReadAll(e.Reader())
+	payloads = append(payloads, "")
+	pktlines, _ := pktline.NewFromStrings(payloads...)
+	b, _ := ioutil.ReadAll(pktlines)
+
 	return b
 }
 
@@ -318,21 +338,25 @@ func (r *GitUploadPackRequest) String() string {
 }
 
 func (r *GitUploadPackRequest) Reader() *strings.Reader {
-	e := pktline.NewEncoder()
+	payloads := []string{}
+
 	for _, want := range r.Wants {
-		e.AddLine(fmt.Sprintf("want %s", want))
+		payloads = append(payloads, fmt.Sprintf("want %s\n", want))
 	}
 
 	for _, have := range r.Haves {
-		e.AddLine(fmt.Sprintf("have %s", have))
+		payloads = append(payloads, fmt.Sprintf("have %s\n", have))
 	}
 
 	if r.Depth != 0 {
-		e.AddLine(fmt.Sprintf("deepen %d", r.Depth))
+		payloads = append(payloads, fmt.Sprintf("deepen %d\n", r.Depth))
 	}
 
-	e.AddFlush()
-	e.AddLine("done")
+	payloads = append(payloads, "")
+	payloads = append(payloads, "done\n")
+
+	pktlines, _ := pktline.NewFromStrings(payloads...)
+	b, _ := ioutil.ReadAll(pktlines)
 
-	return e.Reader()
+	return strings.NewReader(string(b))
 }
diff --git a/clients/common/common_test.go b/clients/common/common_test.go
index cc6c6d1..66a49e1 100644
--- a/clients/common/common_test.go
+++ b/clients/common/common_test.go
@@ -5,9 +5,10 @@ import (
 	"encoding/base64"
 	"testing"
 
-	. "gopkg.in/check.v1"
 	"gopkg.in/src-d/go-git.v4/core"
-	"gopkg.in/src-d/go-git.v4/formats/pktline"
+	"gopkg.in/src-d/go-git.v4/formats/packp/pktline"
+
+	. "gopkg.in/check.v1"
 )
 
 func Test(t *testing.T) { TestingT(t) }
@@ -48,7 +49,7 @@ func (s *SuiteCommon) TestGitUploadPackInfo(c *C) {
 	b, _ := base64.StdEncoding.DecodeString(GitUploadPackInfoFixture)
 
 	i := NewGitUploadPackInfo()
-	err := i.Decode(pktline.NewDecoder(bytes.NewBuffer(b)))
+	err := i.Decode(pktline.NewScanner(bytes.NewBuffer(b)))
 	c.Assert(err, IsNil)
 
 	name := i.Capabilities.SymbolicReference("HEAD")
@@ -70,7 +71,7 @@ func (s *SuiteCommon) TestGitUploadPackInfoNoHEAD(c *C) {
 	b, _ := base64.StdEncoding.DecodeString(GitUploadPackInfoNoHEADFixture)
 
 	i := NewGitUploadPackInfo()
-	err := i.Decode(pktline.NewDecoder(bytes.NewBuffer(b)))
+	err := i.Decode(pktline.NewScanner(bytes.NewBuffer(b)))
 	c.Assert(err, IsNil)
 
 	name := i.Capabilities.SymbolicReference("HEAD")
@@ -86,7 +87,7 @@ func (s *SuiteCommon) TestGitUploadPackInfoEmpty(c *C) {
 	b := bytes.NewBuffer(nil)
 
 	i := NewGitUploadPackInfo()
-	err := i.Decode(pktline.NewDecoder(b))
+	err := i.Decode(pktline.NewScanner(b))
 	c.Assert(err, ErrorMatches, "permanent.*empty.*")
 }
 
diff --git a/clients/http/git_upload_pack.go b/clients/http/git_upload_pack.go
index 888d279..eb8db0b 100644
--- a/clients/http/git_upload_pack.go
+++ b/clients/http/git_upload_pack.go
@@ -2,6 +2,7 @@ package http
 
 import (
 	"bufio"
+	"bytes"
 	"fmt"
 	"io"
 	"net/http"
@@ -9,7 +10,7 @@ import (
 
 	"gopkg.in/src-d/go-git.v4/clients/common"
 	"gopkg.in/src-d/go-git.v4/core"
-	"gopkg.in/src-d/go-git.v4/formats/pktline"
+	"gopkg.in/src-d/go-git.v4/formats/packp/pktline"
 )
 
 // GitUploadPackService git-upoad-pack service over HTTP
@@ -77,7 +78,7 @@ func (s *GitUploadPackService) Info() (*common.GitUploadPackInfo, error) {
 	defer res.Body.Close()
 
 	i := common.NewGitUploadPackInfo()
-	return i, i.Decode(pktline.NewDecoder(res.Body))
+	return i, i.Decode(pktline.NewScanner(res.Body))
 }
 
 // Fetch request and returns a reader to a packfile
@@ -101,27 +102,22 @@ func (s *GitUploadPackService) Fetch(r *common.GitUploadPackRequest) (io.ReadClo
 		return nil, err
 	}
 
-	if err := s.discardResponseInfo(reader); err != nil {
+	if err := discardResponseInfo(reader); err != nil {
 		return nil, err
 	}
 
 	return reader, nil
 }
 
-func (s *GitUploadPackService) discardResponseInfo(r io.Reader) error {
-	decoder := pktline.NewDecoder(r)
-	for {
-		line, err := decoder.ReadLine()
-		if err != nil {
-			break
-		}
-
-		if line == "NAK\n" {
+func discardResponseInfo(r io.Reader) error {
+	s := pktline.NewScanner(r)
+	for s.Scan() {
+		if bytes.Equal(s.Bytes(), []byte{'N', 'A', 'K', '\n'}) {
 			break
 		}
 	}
 
-	return nil
+	return s.Err()
 }
 
 func (s *GitUploadPackService) doRequest(method, url string, content *strings.Reader) (*http.Response, error) {
diff --git a/clients/ssh/git_upload_pack.go b/clients/ssh/git_upload_pack.go
index d83aadb..513e528 100644
--- a/clients/ssh/git_upload_pack.go
+++ b/clients/ssh/git_upload_pack.go
@@ -11,7 +11,7 @@ import (
 	"strings"
 
 	"gopkg.in/src-d/go-git.v4/clients/common"
-	"gopkg.in/src-d/go-git.v4/formats/pktline"
+	"gopkg.in/src-d/go-git.v4/formats/packp/pktline"
 
 	"golang.org/x/crypto/ssh"
 )
@@ -123,7 +123,7 @@ func (s *GitUploadPackService) Info() (i *common.GitUploadPackInfo, err error) {
 	}
 
 	i = common.NewGitUploadPackInfo()
-	return i, i.Decode(pktline.NewDecoder(bytes.NewReader(out)))
+	return i, i.Decode(pktline.NewScanner(bytes.NewReader(out)))
 }
 
 // Disconnect the SSH client.
diff --git a/formats/packp/pktline/pktline.go b/formats/packp/pktline/pktline.go
new file mode 100644
index 0000000..58c36fe
--- /dev/null
+++ b/formats/packp/pktline/pktline.go
@@ -0,0 +1,114 @@
+// Package pktline implements reading and creating pkt-lines as per
+// https://github.com/git/git/blob/master/Documentation/technical/protocol-common.txt.
+package pktline
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"strings"
+)
+
+const (
+	// MaxPayloadSize is the maximum payload size of a pkt-line in bytes.
+	MaxPayloadSize = 65516
+)
+
+var (
+	flush = []byte{'0', '0', '0', '0'}
+)
+
+// PktLine values represent a succession of pkt-lines.
+// Values from this type are not zero-value safe, see the functions New
+// and NewFromString below.
+type PktLine struct {
+	io.Reader
+}
+
+// ErrPayloadTooLong is returned by New and NewFromString when any of
+// the provided payloads is bigger than MaxPayloadSize.
+var ErrPayloadTooLong = errors.New("payload is too long")
+
+// New returns the concatenation of several pkt-lines, each of them with
+// the payload specified by the contents of each input byte slice.  An
+// empty payload byte slice will produce a flush-pkt.
+func New(payloads ...[]byte) (PktLine, error) {
+	ret := []io.Reader{}
+	for _, p := range payloads {
+		if err := add(&ret, p); err != nil {
+			return PktLine{}, err
+		}
+	}
+
+	return PktLine{io.MultiReader(ret...)}, nil
+}
+
+func add(dst *[]io.Reader, e []byte) error {
+	if len(e) > MaxPayloadSize {
+		return ErrPayloadTooLong
+	}
+
+	if len(e) == 0 {
+		*dst = append(*dst, bytes.NewReader(flush))
+		return nil
+	}
+
+	n := len(e) + 4
+	*dst = append(*dst, bytes.NewReader(int16ToHex(n)))
+	*dst = append(*dst, bytes.NewReader(e))
+
+	return nil
+}
+
+// susbtitutes fmt.Sprintf("%04x", n) to avoid memory garbage
+// generation.
+func int16ToHex(n int) []byte {
+	var ret [4]byte
+	ret[0] = byteToAsciiHex(byte(n & 0xf000 >> 12))
+	ret[1] = byteToAsciiHex(byte(n & 0x0f00 >> 8))
+	ret[2] = byteToAsciiHex(byte(n & 0x00f0 >> 4))
+	ret[3] = byteToAsciiHex(byte(n & 0x000f))
+
+	return ret[:]
+}
+
+// turns a byte into its hexadecimal ascii representation.  Example:
+// from 11 (0xb) into 'b'.
+func byteToAsciiHex(n byte) byte {
+	if n < 10 {
+		return byte('0' + n)
+	}
+
+	return byte('a' - 10 + n)
+}
+
+// NewFromStrings returns the concatenation of several pkt-lines, each
+// of them with the payload specified by the contents of each input
+// string.  An empty payload string will produce a flush-pkt.
+func NewFromStrings(payloads ...string) (PktLine, error) {
+	ret := []io.Reader{}
+	for _, p := range payloads {
+		if err := addString(&ret, p); err != nil {
+			return PktLine{}, err
+		}
+	}
+
+	return PktLine{io.MultiReader(ret...)}, nil
+}
+
+func addString(dst *[]io.Reader, s string) error {
+	if len(s) > MaxPayloadSize {
+		return ErrPayloadTooLong
+	}
+
+	if len(s) == 0 {
+		*dst = append(*dst, bytes.NewReader(flush))
+		return nil
+	}
+
+	n := len(s) + 4
+	*dst = append(*dst, bytes.NewReader(int16ToHex(n)))
+	*dst = append(*dst, strings.NewReader(s))
+
+	return nil
+}
diff --git a/formats/packp/pktline/pktline_test.go b/formats/packp/pktline/pktline_test.go
new file mode 100644
index 0000000..3c18f53
--- /dev/null
+++ b/formats/packp/pktline/pktline_test.go
@@ -0,0 +1,199 @@
+package pktline_test
+
+import (
+	"io"
+	"io/ioutil"
+	"os"
+	"strings"
+	"testing"
+
+	"gopkg.in/src-d/go-git.v4/formats/packp/pktline"
+
+	. "gopkg.in/check.v1"
+)
+
+func Test(t *testing.T) { TestingT(t) }
+
+type SuitePktLine struct {
+}
+
+var _ = Suite(&SuitePktLine{})
+
+func (s *SuitePktLine) TestNew(c *C) {
+	for i, test := range [...]struct {
+		input    [][]byte
+		expected []byte
+	}{
+		{
+			input:    [][]byte{},
+			expected: []byte{},
+		}, {
+			input: [][]byte{
+				[]byte(nil),
+			},
+			expected: []byte("0000"),
+		}, {
+			input: [][]byte{
+				[]byte{},
+			},
+			expected: []byte("0000"),
+		}, {
+			input: [][]byte{
+				[]byte(""),
+			},
+			expected: []byte("0000"),
+		}, {
+			input: [][]byte{
+				[]byte("hello\n"),
+			},
+			expected: []byte("000ahello\n"),
+		}, {
+			input: [][]byte{
+				[]byte("hello\n"),
+				[]byte("world!\n"),
+				[]byte(""),
+				[]byte("foo"),
+				[]byte(""),
+			},
+			expected: []byte("000ahello\n000bworld!\n00000007foo0000"),
+		}, {
+			input: [][]byte{
+				[]byte(strings.Repeat("a", pktline.MaxPayloadSize)),
+			},
+			expected: []byte("fff0" + strings.Repeat("a", pktline.MaxPayloadSize)),
+		},
+	} {
+		r, err := pktline.New(test.input...)
+		c.Assert(err, IsNil, Commentf("input %d = %v", i, test.input))
+
+		obtained, err := ioutil.ReadAll(r)
+		c.Assert(err, IsNil, Commentf("input %d = %v", i, test.input))
+
+		c.Assert(obtained, DeepEquals, test.expected,
+			Commentf("input %d = %v", i, test.input))
+	}
+}
+
+func (s *SuitePktLine) TestNewErrPayloadTooLong(c *C) {
+	for _, input := range [...][][]byte{
+		[][]byte{
+			[]byte(strings.Repeat("a", pktline.MaxPayloadSize+1)),
+		},
+		[][]byte{
+			[]byte("hello world!"),
+			[]byte(""),
+			[]byte(strings.Repeat("a", pktline.MaxPayloadSize+1)),
+		},
+		[][]byte{
+			[]byte("hello world!"),
+			[]byte(strings.Repeat("a", pktline.MaxPayloadSize+1)),
+			[]byte("foo"),
+		},
+	} {
+		_, err := pktline.New(input...)
+
+		c.Assert(err, Equals, pktline.ErrPayloadTooLong,
+			Commentf("%v\n", input))
+	}
+}
+
+func (s *SuitePktLine) TestNewFromStrings(c *C) {
+	for _, test := range [...]struct {
+		input    []string
+		expected []byte
+	}{
+		{
+			input:    []string(nil),
+			expected: []byte{},
+		}, {
+			input:    []string{},
+			expected: []byte{},
+		}, {
+			input:    []string{""},
+			expected: []byte("0000"),
+		}, {
+			input:    []string{"hello\n"},
+			expected: []byte("000ahello\n"),
+		}, {
+			input:    []string{"hello\n", "world!\n", "", "foo", ""},
+			expected: []byte("000ahello\n000bworld!\n00000007foo0000"),
+		}, {
+			input: []string{
+				strings.Repeat("a", pktline.MaxPayloadSize),
+			},
+			expected: []byte("fff0" + strings.Repeat("a", pktline.MaxPayloadSize)),
+		},
+	} {
+		r, err := pktline.NewFromStrings(test.input...)
+		c.Assert(err, IsNil)
+
+		obtained, err := ioutil.ReadAll(r)
+		c.Assert(err, IsNil)
+
+		c.Assert(obtained, DeepEquals, test.expected,
+			Commentf("input = %v\n", test.input))
+	}
+}
+
+func (s *SuitePktLine) TestNewFromStringsErrPayloadTooLong(c *C) {
+	for _, input := range [...][]string{
+		[]string{
+			strings.Repeat("a", pktline.MaxPayloadSize+1),
+		},
+		[]string{
+			"hello world!",
+			"",
+			strings.Repeat("a", pktline.MaxPayloadSize+1),
+		},
+		[]string{
+			"hello world!",
+			strings.Repeat("a", pktline.MaxPayloadSize+1),
+			"foo",
+		},
+	} {
+		_, err := pktline.NewFromStrings(input...)
+
+		c.Assert(err, Equals, pktline.ErrPayloadTooLong,
+			Commentf("%v\n", input))
+	}
+}
+
+func ExampleNew() {
+	// These are the payloads we want to turn into pkt-lines,
+	// the empty slice at the end will generate a flush-pkt.
+	payloads := [][]byte{
+		[]byte{'h', 'e', 'l', 'l', 'o', '\n'},
+		[]byte{'w', 'o', 'r', 'l', 'd', '!', '\n'},
+		[]byte{},
+	}
+
+	// Create the pkt-lines, ignoring errors...
+	pktlines, _ := pktline.New(payloads...)
+
+	// Send the raw data to stdout, ignoring errors...
+	_, _ = io.Copy(os.Stdout, pktlines)
+
+	// Output: 000ahello
+	// 000bworld!
+	// 0000
+}
+
+func ExampleNewFromStrings() {
+	// These are the payloads we want to turn into pkt-lines,
+	// the empty string at the end will generate a flush-pkt.
+	payloads := []string{
+		"hello\n",
+		"world!\n",
+		"",
+	}
+
+	// Create the pkt-lines, ignoring errors...
+	pktlines, _ := pktline.NewFromStrings(payloads...)
+
+	// Send the raw data to stdout, ignoring errors...
+	_, _ = io.Copy(os.Stdout, pktlines)
+
+	// Output: 000ahello
+	// 000bworld!
+	// 0000
+}
diff --git a/formats/packp/pktline/scanner.go b/formats/packp/pktline/scanner.go
new file mode 100644
index 0000000..3ce2adf
--- /dev/null
+++ b/formats/packp/pktline/scanner.go
@@ -0,0 +1,133 @@
+package pktline
+
+import (
+	"errors"
+	"io"
+)
+
+const (
+	lenSize = 4
+)
+
+// ErrInvalidPktLen is returned by Err() when an invalid pkt-len is found.
+var ErrInvalidPktLen = errors.New("invalid pkt-len found")
+
+// Scanner provides a convenient interface for reading the payloads of a
+// series of pkt-lines.  It takes an io.Reader providing the source,
+// which then can be tokenized through repeated calls to the Scan
+// method.
+//
+// After each Scan call, the Bytes method will return the payload of the
+// corresponding pkt-line on a shared buffer, which will be 65516 bytes
+// or smaller.  Flush pkt-lines are represented by empty byte slices.
+//
+// Scanning stops at EOF or the first I/O error.
+type Scanner struct {
+	r       io.Reader     // The reader provided by the client
+	err     error         // Sticky error
+	payload []byte        // Last pkt-payload
+	len     [lenSize]byte // Last pkt-len
+}
+
+// NewScanner returns a new Scanner to read from r.
+func NewScanner(r io.Reader) *Scanner {
+	return &Scanner{
+		r: r,
+	}
+}
+
+// Err returns the first error encountered by the Scanner.
+func (s *Scanner) Err() error {
+	return s.err
+}
+
+// Scan advances the Scanner to the next pkt-line, whose payload will
+// then be available through the Bytes method.  Scanning stops at EOF
+// or the first I/O error.  After Scan returns false, the Err method
+// will return any error that occurred during scanning, except that if
+// it was io.EOF, Err will return nil.
+func (s *Scanner) Scan() bool {
+	var l int
+	l, s.err = s.readPayloadLen()
+	if s.err == io.EOF {
+		s.err = nil
+		return false
+	}
+	if s.err != nil {
+		return false
+	}
+
+	if cap(s.payload) < l {
+		s.payload = make([]byte, 0, l)
+	}
+
+	if _, s.err = io.ReadFull(s.r, s.payload[:l]); s.err != nil {
+		return false
+	}
+	s.payload = s.payload[:l]
+
+	return true
+}
+
+// Bytes returns the most recent payload generated by a call to Scan.
+// The underlying array may point to data that will be overwritten by a
+// subsequent call to Scan. It does no allocation.
+func (s *Scanner) Bytes() []byte {
+	return s.payload
+}
+
+// Method readPayloadLen returns the payload length by reading the
+// pkt-len and substracting the pkt-len size.
+func (s *Scanner) readPayloadLen() (int, error) {
+	if _, err := io.ReadFull(s.r, s.len[:]); err != nil {
+		if err == io.EOF {
+			return 0, err
+		}
+		return 0, ErrInvalidPktLen
+	}
+
+	n, err := hexDecode(s.len)
+	if err != nil {
+		return 0, err
+	}
+
+	switch {
+	case n == 0:
+		return 0, nil
+	case n <= lenSize:
+		return 0, ErrInvalidPktLen
+	case n > MaxPayloadSize+lenSize:
+		return 0, ErrInvalidPktLen
+	default:
+		return n - lenSize, nil
+	}
+}
+
+// Turns the hexadecimal representation of a number in a byte slice into
+// a number. This function substitute strconv.ParseUint(string(buf), 16,
+// 16) and/or hex.Decode, to avoid generating new strings, thus helping the
+// GC.
+func hexDecode(buf [lenSize]byte) (int, error) {
+	var ret int
+	for i := 0; i < lenSize; i++ {
+		n, err := asciiHexToByte(buf[i])
+		if err != nil {
+			return 0, ErrInvalidPktLen
+		}
+		ret = 16*ret + int(n)
+	}
+	return ret, nil
+}
+
+// turns the hexadecimal ascii representation of a byte into its
+// numerical value.  Example: from 'b' to 11 (0xb).
+func asciiHexToByte(b byte) (byte, error) {
+	switch {
+	case b >= '0' && b <= '9':
+		return b - '0', nil
+	case b >= 'a' && b <= 'f':
+		return b - 'a' + 10, nil
+	default:
+		return 0, ErrInvalidPktLen
+	}
+}
diff --git a/formats/packp/pktline/scanner_test.go b/formats/packp/pktline/scanner_test.go
new file mode 100644
index 0000000..08ca51f
--- /dev/null
+++ b/formats/packp/pktline/scanner_test.go
@@ -0,0 +1,208 @@
+package pktline_test
+
+import (
+	"fmt"
+	"io"
+	"strings"
+
+	"gopkg.in/src-d/go-git.v4/formats/packp/pktline"
+
+	. "gopkg.in/check.v1"
+)
+
+type SuiteScanner struct{}
+
+var _ = Suite(&SuiteScanner{})
+
+func (s *SuiteScanner) TestInvalid(c *C) {
+	for _, test := range [...]string{
+		"0001", "0002", "0003", "0004",
+		"0001asdfsadf", "0004foo",
+		"fff1", "fff2",
+		"gorka",
+		"0", "003",
+		"   5a", "5   a", "5   \n",
+		"-001", "-000",
+	} {
+		r := strings.NewReader(test)
+		sc := pktline.NewScanner(r)
+		_ = sc.Scan()
+		c.Assert(sc.Err(), ErrorMatches, pktline.ErrInvalidPktLen.Error(),
+			Commentf("data = %q", test))
+	}
+}
+
+func (s *SuiteScanner) TestEmptyReader(c *C) {
+	r := strings.NewReader("")
+	sc := pktline.NewScanner(r)
+	hasPayload := sc.Scan()
+	c.Assert(hasPayload, Equals, false)
+	c.Assert(sc.Err(), Equals, nil)
+}
+
+func (s *SuiteScanner) TestFlush(c *C) {
+	r, err := pktline.NewFromStrings("")
+	c.Assert(err, IsNil)
+	sc := pktline.NewScanner(r)
+	c.Assert(sc.Scan(), Equals, true)
+	payload := sc.Bytes()
+	c.Assert(len(payload), Equals, 0)
+}
+
+func (s *SuiteScanner) TestPktLineTooShort(c *C) {
+	r := strings.NewReader("010cfoobar")
+
+	sc := pktline.NewScanner(r)
+
+	c.Assert(sc.Scan(), Equals, false)
+	c.Assert(sc.Err(), ErrorMatches, "unexpected EOF")
+}
+
+func (s *SuiteScanner) TestScanAndPayload(c *C) {
+	for _, test := range [...]string{
+		"a",
+		"a\n",
+		strings.Repeat("a", 100),
+		strings.Repeat("a", 100) + "\n",
+		strings.Repeat("\x00", 100),
+		strings.Repeat("\x00", 100) + "\n",
+		strings.Repeat("a", pktline.MaxPayloadSize),
+		strings.Repeat("a", pktline.MaxPayloadSize-1) + "\n",
+	} {
+		r, err := pktline.NewFromStrings(test)
+		c.Assert(err, IsNil, Commentf("input len=%x, contents=%.10q\n", len(test), test))
+		sc := pktline.NewScanner(r)
+
+		c.Assert(sc.Scan(), Equals, true,
+			Commentf("test = %.20q...", test))
+		obtained := sc.Bytes()
+		c.Assert(obtained, DeepEquals, []byte(test),
+			Commentf("in = %.20q out = %.20q", test, string(obtained)))
+	}
+}
+
+func (s *SuiteScanner) TestSkip(c *C) {
+	for _, test := range [...]struct {
+		input    []string
+		n        int
+		expected []byte
+	}{
+		{
+			input: []string{
+				"first",
+				"second",
+				"third",
+				""},
+			n:        1,
+			expected: []byte("second"),
+		},
+		{
+			input: []string{
+				"first",
+				"second",
+				"third",
+				""},
+			n:        2,
+			expected: []byte("third"),
+		},
+	} {
+		r, err := pktline.NewFromStrings(test.input...)
+		c.Assert(err, IsNil)
+		sc := pktline.NewScanner(r)
+		for i := 0; i < test.n; i++ {
+			c.Assert(sc.Scan(), Equals, true,
+				Commentf("scan error = %s", sc.Err()))
+		}
+		c.Assert(sc.Scan(), Equals, true,
+			Commentf("scan error = %s", sc.Err()))
+		obtained := sc.Bytes()
+		c.Assert(obtained, DeepEquals, test.expected,
+			Commentf("\nin = %.20q\nout = %.20q\nexp = %.20q",
+				test.input, obtained, test.expected))
+	}
+}
+
+func (s *SuiteScanner) TestEOF(c *C) {
+	r, err := pktline.NewFromStrings("first", "second")
+	c.Assert(err, IsNil)
+	sc := pktline.NewScanner(r)
+	for sc.Scan() {
+	}
+	c.Assert(sc.Err(), IsNil)
+}
+
+// A section are several non flush-pkt lines followed by a flush-pkt, which
+// how the git protocol sends long messages.
+func (s *SuiteScanner) TestReadSomeSections(c *C) {
+	nSections := 2
+	nLines := 4
+	data := sectionsExample(c, nSections, nLines)
+	sc := pktline.NewScanner(data)
+
+	sectionCounter := 0
+	lineCounter := 0
+	for sc.Scan() {
+		if len(sc.Bytes()) == 0 {
+			sectionCounter++
+		}
+		lineCounter++
+	}
+	c.Assert(sc.Err(), IsNil)
+	c.Assert(sectionCounter, Equals, nSections)
+	c.Assert(lineCounter, Equals, (1+nLines)*nSections)
+}
+
+// returns nSection sections, each of them with nLines pkt-lines (not
+// counting the flush-pkt:
+//
+// 0009 0.0\n
+// 0009 0.1\n
+// ...
+// 0000
+// and so on
+func sectionsExample(c *C, nSections, nLines int) io.Reader {
+	ss := []string{}
+	for section := 0; section < nSections; section++ {
+		for line := 0; line < nLines; line++ {
+			line := fmt.Sprintf(" %d.%d\n", section, line)
+			ss = append(ss, line)
+		}
+		ss = append(ss, "")
+	}
+
+	ret, err := pktline.NewFromStrings(ss...)
+	c.Assert(err, IsNil)
+
+	return ret
+}
+
+func ExampleScanner() {
+	// A reader is needed as input.
+	input := strings.NewReader("000ahello\n" +
+		"000bworld!\n" +
+		"0000",
+	)
+
+	// Create the scanner...
+	s := pktline.NewScanner(input)
+
+	// and scan every pkt-line found in the input.
+	for s.Scan() {
+		payload := s.Bytes()
+		if len(payload) == 0 { // zero sized payloads correspond to flush-pkts.
+			fmt.Println("FLUSH-PKT DETECTED\n")
+		} else { // otherwise, you will be able to access the full payload.
+			fmt.Printf("PAYLOAD = %q\n", string(payload))
+		}
+	}
+
+	// this will catch any error when reading from the input, if any.
+	if s.Err() != nil {
+		fmt.Println(s.Err())
+	}
+
+	// Output:
+	// PAYLOAD = "hello\n"
+	// PAYLOAD = "world!\n"
+	// FLUSH-PKT DETECTED
+}
diff --git a/formats/pktline/decoder.go b/formats/pktline/decoder.go
deleted file mode 100644
index 789ba7d..0000000
--- a/formats/pktline/decoder.go
+++ /dev/null
@@ -1,111 +0,0 @@
-package pktline
-
-import (
-	"errors"
-	"io"
-	"strconv"
-)
-
-var (
-	// ErrUnderflow is triggered when a line is shorter than the described length
-	ErrUnderflow = errors.New("unexpected string length (underflow)")
-	// ErrInvalidHeader invalid pktline header
-	ErrInvalidHeader = errors.New("invalid header")
-	// ErrInvalidLen ivanlid line length found, < 0
-	ErrInvalidLen = errors.New("invalid length")
-)
-
-// Decoder implements a pkt-line format decoder
-type Decoder struct {
-	r io.Reader
-}
-
-// NewDecoder returns a new Decoder
-func NewDecoder(r io.Reader) *Decoder {
-	return &Decoder{r}
-}
-
-// ReadLine reads and return one pkt-line line from the reader
-func (d *Decoder) ReadLine() (string, error) {
-	return d.readLine()
-}
-
-func (d *Decoder) readLine() (string, error) {
-	raw := make([]byte, HeaderLength)
-	if _, err := io.ReadFull(d.r, raw); err != nil {
-		return "", err
-	}
-
-	header, err := strconv.ParseInt(string(raw), 16, 16)
-	if err != nil {
-		return "", ErrInvalidHeader
-	}
-
-	if header == 0 {
-		return "", nil
-	}
-
-	exp := int(header - HeaderLength)
-	if exp < 0 {
-		return "", ErrInvalidLen
-	}
-
-	line := make([]byte, exp)
-	if read, err := io.ReadFull(d.r, line); err != nil {
-		if err == io.ErrUnexpectedEOF && read < exp {
-			return "", ErrUnderflow
-		}
-
-		return "", err
-	}
-
-	return string(line), nil
-}
-
-// ReadBlock reads and return multiple pkt-line lines, it stops at the end
-// of the reader or if a flush-pkt is reached
-func (d *Decoder) ReadBlock() ([]string, error) {
-	var o []string
-
-	for {
-		line, err := d.readLine()
-		if err == io.EOF {
-			return o, nil
-		}
-
-		if err != nil {
-			return o, err
-		}
-
-		if err == nil && line == "" {
-			return o, nil
-		}
-
-		o = append(o, line)
-	}
-}
-
-// ReadAll read and returns all the lines
-func (d *Decoder) ReadAll() ([]string, error) {
-	result, err := d.ReadBlock()
-	if err != nil {
-		return result, err
-	}
-
-	for {
-		lines, err := d.ReadBlock()
-		if err == io.EOF {
-			return result, nil
-		}
-
-		if err != nil {
-			return result, err
-		}
-
-		if err == nil && len(lines) == 0 {
-			return result, nil
-		}
-
-		result = append(result, lines...)
-	}
-}
diff --git a/formats/pktline/decoder_test.go b/formats/pktline/decoder_test.go
deleted file mode 100644
index e3cd2f4..0000000
--- a/formats/pktline/decoder_test.go
+++ /dev/null
@@ -1,85 +0,0 @@
-package pktline
-
-import (
-	"strings"
-	"testing"
-
-	. "gopkg.in/check.v1"
-)
-
-func Test(t *testing.T) { TestingT(t) }
-
-type DecoderSuite struct{}
-
-var _ = Suite(&DecoderSuite{})
-
-func (s *DecoderSuite) TestReadLine(c *C) {
-	j := NewDecoder(strings.NewReader("0006a\n"))
-
-	line, err := j.ReadLine()
-	c.Assert(err, IsNil)
-	c.Assert(line, Equals, "a\n")
-}
-
-func (s *DecoderSuite) TestReadLineInvalidHeader(c *C) {
-	j := NewDecoder(strings.NewReader("foo\n"))
-
-	_, err := j.ReadLine()
-	c.Assert(err, Equals, ErrInvalidHeader)
-}
-
-func (s *DecoderSuite) TestReadLineBufferUnderflow(c *C) {
-	j := NewDecoder(strings.NewReader("00e7a\n"))
-
-	line, err := j.ReadLine()
-	c.Assert(err, Equals, ErrUnderflow)
-	c.Assert(line, Equals, "")
-}
-
-func (s *DecoderSuite) TestReadLineInvalidLen(c *C) {
-	j := NewDecoder(strings.NewReader("0001foo\n"))
-
-	line, err := j.ReadLine()
-	c.Assert(err, Equals, ErrInvalidLen)
-	c.Assert(line, Equals, "")
-}
-
-func (s *DecoderSuite) TestReadBlock(c *C) {
-	j := NewDecoder(strings.NewReader("0006a\n"))
-
-	lines, err := j.ReadBlock()
-	c.Assert(err, IsNil)
-	c.Assert(lines, HasLen, 1)
-	c.Assert(lines[0], Equals, "a\n")
-}
-
-func (s *DecoderSuite) TestReadBlockWithFlush(c *C) {
-	j := NewDecoder(strings.NewReader("0006a\n0006b\n00000006c\n"))
-
-	lines, err := j.ReadBlock()
-	c.Assert(err, IsNil)
-	c.Assert(lines, HasLen, 2)
-	c.Assert(lines[0], Equals, "a\n")
-	c.Assert(lines[1], Equals, "b\n")
-}
-
-func (s *DecoderSuite) TestReadBlockInvalidLen(c *C) {
-	j := NewDecoder(strings.NewReader("0001foo\n"))
-
-	lines, err := j.ReadBlock()
-	c.Assert(err, Equals, ErrInvalidLen)
-	c.Assert(lines, HasLen, 0)
-}
-
-func (s *DecoderSuite) TestReadAll(c *C) {
-	j := NewDecoder(strings.NewReader("0006a\n0006b\n00000006c\n0006d\n0006e\n"))
-
-	lines, err := j.ReadAll()
-	c.Assert(err, IsNil)
-	c.Assert(lines, HasLen, 5)
-	c.Assert(lines[0], Equals, "a\n")
-	c.Assert(lines[1], Equals, "b\n")
-	c.Assert(lines[2], Equals, "c\n")
-	c.Assert(lines[3], Equals, "d\n")
-	c.Assert(lines[4], Equals, "e\n")
-}
diff --git a/formats/pktline/doc.go b/formats/pktline/doc.go
deleted file mode 100644
index 1e14ec0..0000000
--- a/formats/pktline/doc.go
+++ /dev/null
@@ -1,59 +0,0 @@
-// Package pktline implements a encoder/decoder of pkt-line format
-package pktline
-
-// pkt-line Format
-// ---------------
-//
-// Much (but not all) of the payload is described around pkt-lines.
-//
-// A pkt-line is a variable length binary string.  The first four bytes
-// of the line, the pkt-len, indicates the total length of the line,
-// in hexadecimal.  The pkt-len includes the 4 bytes used to contain
-// the length's hexadecimal representation.
-//
-// A pkt-line MAY contain binary data, so implementors MUST ensure
-// pkt-line parsing/formatting routines are 8-bit clean.
-//
-// A non-binary line SHOULD BE terminated by an LF, which if present
-// MUST be included in the total length.
-//
-// The maximum length of a pkt-line's data component is 65520 bytes.
-// Implementations MUST NOT send pkt-line whose length exceeds 65524
-// (65520 bytes of payload + 4 bytes of length data).
-//
-// Implementations SHOULD NOT send an empty pkt-line ("0004").
-//
-// A pkt-line with a length field of 0 ("0000"), called a flush-pkt,
-// is a special case and MUST be handled differently than an empty
-// pkt-line ("0004").
-//
-// ----
-//   pkt-line     =  data-pkt / flush-pkt
-//
-//   data-pkt     =  pkt-len pkt-payload
-//   pkt-len      =  4*(HEXDIG)
-//   pkt-payload  =  (pkt-len - 4)*(OCTET)
-//
-//   flush-pkt    = "0000"
-// ----
-//
-// Examples (as C-style strings):
-//
-// ----
-//   pkt-line          actual value
-//   ---------------------------------
-//   "0006a\n"         "a\n"
-//   "0005a"           "a"
-//   "000bfoobar\n"    "foobar\n"
-//   "0004"            ""
-// ----
-//
-// Extracted from:
-// https://github.com/git/git/blob/master/Documentation/technical/protocol-common.txt
-
-const (
-	// HeaderLength length of the pktline header
-	HeaderLength = 4
-	// MaxLength max line length
-	MaxLength = 65524
-)
diff --git a/formats/pktline/encoder.go b/formats/pktline/encoder.go
deleted file mode 100644
index 18efa25..0000000
--- a/formats/pktline/encoder.go
+++ /dev/null
@@ -1,64 +0,0 @@
-package pktline
-
-import (
-	"errors"
-	"fmt"
-	"strings"
-)
-
-var (
-	//ErrOverflow is triggered when the line length exceed the MaxLength
-	ErrOverflow = errors.New("unexpected string length (overflow)")
-)
-
-// Encoder implements a pkt-line format encoder
-type Encoder struct {
-	lines []string
-}
-
-// NewEncoder returns a new Encoder
-func NewEncoder() *Encoder {
-	return &Encoder{make([]string, 0)}
-}
-
-// AddLine encode and adds a line to the encoder
-func (e *Encoder) AddLine(line string) error {
-	le, err := EncodeFromString(line + "\n")
-	if err != nil {
-		return err
-	}
-
-	e.lines = append(e.lines, le)
-	return nil
-}
-
-// AddFlush adds a flush-pkt to the encoder
-func (e *Encoder) AddFlush() {
-	e.lines = append(e.lines, "0000")
-}
-
-// Reader returns a string.Reader over the encoder
-func (e *Encoder) Reader() *strings.Reader {
-	data := strings.Join(e.lines, "")
-
-	return strings.NewReader(data)
-}
-
-// EncodeFromString encodes a string to pkt-line format
-func EncodeFromString(line string) (string, error) {
-	return Encode([]byte(line))
-}
-
-// Encode encodes a byte slice to pkt-line format
-func Encode(line []byte) (string, error) {
-	if line == nil {
-		return "0000", nil
-	}
-
-	l := len(line) + HeaderLength
-	if l > MaxLength {
-		return "", ErrOverflow
-	}
-
-	return fmt.Sprintf("%04x%s", l, line), nil
-}
diff --git a/formats/pktline/encoder_test.go b/formats/pktline/encoder_test.go
deleted file mode 100644
index f718c33..0000000
--- a/formats/pktline/encoder_test.go
+++ /dev/null
@@ -1,50 +0,0 @@
-package pktline
-
-import (
-	"bytes"
-	"io/ioutil"
-	"strings"
-
-	. "gopkg.in/check.v1"
-)
-
-type EncoderSuite struct{}
-
-var _ = Suite(&EncoderSuite{})
-
-func (s *EncoderSuite) TestEncode(c *C) {
-	line, err := Encode([]byte("a\n"))
-	c.Assert(err, IsNil)
-	c.Assert(string(line), Equals, "0006a\n")
-}
-
-func (s *EncoderSuite) TestEncodeNil(c *C) {
-	line, err := Encode(nil)
-	c.Assert(err, IsNil)
-	c.Assert(string(line), Equals, "0000")
-}
-
-func (s *EncoderSuite) TestEncodeOverflow(c *C) {
-	_, err := Encode(bytes.Repeat([]byte{'0'}, MaxLength+1))
-	c.Assert(err, Equals, ErrOverflow)
-}
-
-func (s *EncoderSuite) TestEncodeFromString(c *C) {
-	line, err := EncodeFromString("a\n")
-	c.Assert(err, IsNil)
-	c.Assert(string(line), Equals, "0006a\n")
-}
-
-func (s *EncoderSuite) TestEncoder(c *C) {
-	e := NewEncoder()
-	c.Assert(e.AddLine("a"), IsNil)
-	e.AddFlush()
-	c.Assert(e.AddLine("b"), IsNil)
-
-	over := strings.Repeat("0", MaxLength+1)
-	c.Assert(e.AddLine(over), Equals, ErrOverflow)
-
-	r := e.Reader()
-	a, _ := ioutil.ReadAll(r)
-	c.Assert(string(a), Equals, "0006a\n00000006b\n")
-}