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package git
import (
"bytes"
"compress/zlib"
"context"
"crypto/sha1"
"encoding/binary"
"fmt"
"io"
)
// GitStorage interface for storage backends that can provide Git objects
type GitStorage interface {
GetObject(ctx context.Context, repo, objectID string) ([]byte, error)
GetReachableObjects(ctx context.Context, repo string, wants []string, haves []string) ([]string, error)
}
type PackGenerator struct {
storage GitStorage
}
func NewPackGenerator(storage GitStorage) *PackGenerator {
return &PackGenerator{
storage: storage,
}
}
func (pg *PackGenerator) GeneratePack(ctx context.Context, repo string, wants []string, haves []string) ([]byte, error) {
// Get all objects to include in pack
objectSHAs, err := pg.storage.GetReachableObjects(ctx, repo, wants, haves)
if err != nil {
return nil, fmt.Errorf("failed to get reachable objects: %w", err)
}
if len(objectSHAs) == 0 {
// Return empty pack
return pg.generateEmptyPack(), nil
}
// Read all objects
var objects []*GitObject
for _, sha := range objectSHAs {
objData, err := pg.storage.GetObject(ctx, repo, sha)
if err != nil {
return nil, fmt.Errorf("failed to read required object %s: %w", sha, err)
}
obj, err := ParseGitObject(objData, sha)
if err != nil {
return nil, fmt.Errorf("failed to parse required object %s: %w", sha, err)
}
objects = append(objects, obj)
}
return pg.generatePackFromObjects(objects)
}
func (pg *PackGenerator) generateEmptyPack() []byte {
var buf bytes.Buffer
// Pack header: "PACK"
buf.Write([]byte("PACK"))
// Version (4 bytes, big-endian): version 2
binary.Write(&buf, binary.BigEndian, uint32(2))
// Number of objects (4 bytes, big-endian): 0
binary.Write(&buf, binary.BigEndian, uint32(0))
// Calculate SHA1 checksum of the pack content
hash := sha1.New()
hash.Write(buf.Bytes())
checksum := hash.Sum(nil)
// Append checksum
buf.Write(checksum)
return buf.Bytes()
}
func (pg *PackGenerator) generatePackFromObjects(objects []*GitObject) ([]byte, error) {
var buf bytes.Buffer
// Pack header: "PACK"
buf.Write([]byte("PACK"))
// Version (4 bytes, big-endian): version 2
binary.Write(&buf, binary.BigEndian, uint32(2))
// Number of objects (4 bytes, big-endian)
binary.Write(&buf, binary.BigEndian, uint32(len(objects)))
// Write each object
for _, obj := range objects {
err := pg.writePackObject(&buf, obj)
if err != nil {
return nil, fmt.Errorf("failed to write object %s: %w", obj.SHA, err)
}
}
// Calculate SHA1 checksum of the pack content
hash := sha1.New()
hash.Write(buf.Bytes())
checksum := hash.Sum(nil)
// Append checksum
buf.Write(checksum)
return buf.Bytes(), nil
}
func (pg *PackGenerator) writePackObject(buf *bytes.Buffer, obj *GitObject) error {
// Reconstruct original object data (type + size + null + content)
var objData bytes.Buffer
objData.WriteString(string(obj.Type))
objData.WriteString(" ")
objData.WriteString(fmt.Sprintf("%d", obj.Size))
objData.WriteByte(0)
objData.Write(obj.Data)
// Compress the object data
var compressed bytes.Buffer
zw := zlib.NewWriter(&compressed)
_, err := zw.Write(objData.Bytes())
if err != nil {
return err
}
err = zw.Close()
if err != nil {
return err
}
compressedData := compressed.Bytes()
// Write object header
err = pg.writePackObjectHeader(buf, obj.Type, int64(len(objData.Bytes())))
if err != nil {
return err
}
// Write compressed data
buf.Write(compressedData)
return nil
}
func (pg *PackGenerator) writePackObjectHeader(buf *bytes.Buffer, objType ObjectType, size int64) error {
// Pack object header encoding:
// First byte: MTTT SSSS
// M = more bytes follow (MSB)
// TTT = object type (3 bits)
// SSSS = size (4 bits, LSB)
// Additional bytes (if M=1): MMMM MMMM (7 bits of size, MSB indicates more bytes)
var typeValue uint8
switch objType {
case ObjectTypeCommit:
typeValue = 1
case ObjectTypeTree:
typeValue = 2
case ObjectTypeBlob:
typeValue = 3
case ObjectTypeTag:
typeValue = 4
default:
return fmt.Errorf("unsupported object type: %s", objType)
}
// First byte
firstByte := (typeValue << 4) | uint8(size&0x0F)
size >>= 4
if size > 0 {
firstByte |= 0x80 // Set more-bytes flag
}
buf.WriteByte(firstByte)
// Additional bytes for size
for size > 0 {
nextByte := uint8(size & 0x7F)
size >>= 7
if size > 0 {
nextByte |= 0x80 // Set more-bytes flag
}
buf.WriteByte(nextByte)
}
return nil
}
// PackWriter handles writing pack data with proper Git protocol framing
type PackWriter struct {
writer io.Writer
}
func NewPackWriter(w io.Writer) *PackWriter {
return &PackWriter{writer: w}
}
func (pw *PackWriter) WritePacketLine(data string) error {
length := len(data) + 4
_, err := fmt.Fprintf(pw.writer, "%04x%s", length, data)
return err
}
func (pw *PackWriter) WriteFlush() error {
_, err := pw.writer.Write([]byte("0000"))
return err
}
func (pw *PackWriter) WritePack(packData []byte) error {
// For side-band protocol, we need to frame the pack data
// Band 1: pack data
const maxPacketSize = 65515 // Max packet size minus 4-byte length and 1-byte band
for len(packData) > 0 {
chunkSize := len(packData)
if chunkSize > maxPacketSize {
chunkSize = maxPacketSize
}
chunk := packData[:chunkSize]
packData = packData[chunkSize:]
// Write packet with band indicator
length := len(chunk) + 5 // 4-byte length + 1-byte band + data
_, err := fmt.Fprintf(pw.writer, "%04x\x01", length)
if err != nil {
return err
}
_, err = pw.writer.Write(chunk)
if err != nil {
return err
}
}
return nil
}
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