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

1 files changed, 114 insertions, 0 deletions

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
+}