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|
package packfile
import (
"bytes"
"hash/adler32"
"io/ioutil"
"gopkg.in/src-d/go-git.v4/plumbing"
)
// See https://github.com/jelmer/dulwich/blob/master/dulwich/pack.py and
// https://github.com/tarruda/node-git-core/blob/master/src/js/delta.js
// for more info
const (
// Standard chunk size used to generate fingerprints
s = 16
// https://github.com/git/git/blob/f7466e94375b3be27f229c78873f0acf8301c0a5/diff-delta.c#L428
// Max size of a copy operation (64KB)
maxCopySize = 64 * 1024
)
// GetDelta returns an EncodedObject of type OFSDeltaObject. Base and Target object,
// will be loaded into memory to be able to create the delta object.
// To generate target again, you will need the obtained object and "base" one.
// Error will be returned if base or target object cannot be read.
func GetDelta(base, target plumbing.EncodedObject) (plumbing.EncodedObject, error) {
br, err := base.Reader()
if err != nil {
return nil, err
}
tr, err := target.Reader()
if err != nil {
return nil, err
}
bb, err := ioutil.ReadAll(br)
if err != nil {
return nil, err
}
tb, err := ioutil.ReadAll(tr)
if err != nil {
return nil, err
}
db := DiffDelta(bb, tb)
delta := &plumbing.MemoryObject{}
_, err = delta.Write(db)
if err != nil {
return nil, err
}
delta.SetSize(int64(len(db)))
delta.SetType(plumbing.OFSDeltaObject)
return delta, nil
}
// DiffDelta returns the delta that transforms src into tgt.
func DiffDelta(src []byte, tgt []byte) []byte {
buf := bytes.NewBuffer(nil)
buf.Write(deltaEncodeSize(len(src)))
buf.Write(deltaEncodeSize(len(tgt)))
sindex := initMatch(src)
ibuf := bytes.NewBuffer(nil)
for i := 0; i < len(tgt); i++ {
offset, l := findMatch(src, tgt, sindex, i)
if l < s {
ibuf.WriteByte(tgt[i])
} else {
encodeInsertOperation(ibuf, buf)
rl := l
aOffset := offset
for {
if rl < maxCopySize {
buf.Write(encodeCopyOperation(aOffset, rl))
break
}
buf.Write(encodeCopyOperation(aOffset, maxCopySize))
rl -= maxCopySize
aOffset += maxCopySize
}
i += l - 1
}
}
encodeInsertOperation(ibuf, buf)
return buf.Bytes()
}
func encodeInsertOperation(ibuf, buf *bytes.Buffer) {
if ibuf.Len() == 0 {
return
}
b := ibuf.Bytes()
s := ibuf.Len()
o := 0
for {
if s <= 127 {
break
}
buf.WriteByte(byte(127))
buf.Write(b[o : o+127])
s -= 127
o += 127
}
buf.WriteByte(byte(s))
buf.Write(b[o : o+s])
ibuf.Reset()
}
func initMatch(src []byte) map[uint32]int {
i := 0
index := make(map[uint32]int)
for {
if i+s > len(src) {
break
}
ch := adler32.Checksum(src[i : i+s])
index[ch] = i
i += s
}
return index
}
func findMatch(src, tgt []byte, sindex map[uint32]int, tgtOffset int) (srcOffset, l int) {
if len(tgt) >= tgtOffset+s {
ch := adler32.Checksum(tgt[tgtOffset : tgtOffset+s])
var ok bool
srcOffset, ok = sindex[ch]
if !ok {
return
}
l = matchLength(src, tgt, tgtOffset, srcOffset)
}
return
}
func matchLength(src, tgt []byte, otgt, osrc int) int {
l := 0
for {
if (osrc >= len(src) || otgt >= len(tgt)) || src[osrc] != tgt[otgt] {
break
}
l++
osrc++
otgt++
}
return l
}
func deltaEncodeSize(size int) []byte {
var ret []byte
c := size & 0x7f
size >>= 7
for {
if size == 0 {
break
}
ret = append(ret, byte(c|0x80))
c = size & 0x7f
size >>= 7
}
ret = append(ret, byte(c))
return ret
}
func encodeCopyOperation(offset, length int) []byte {
code := 0x80
var opcodes []byte
var i uint
for i = 0; i < 4; i++ {
f := 0xff << (i * 8)
if offset&f != 0 {
opcodes = append(opcodes, byte(offset&f>>(i*8)))
code |= 0x01 << i
}
}
for i = 0; i < 3; i++ {
f := 0xff << (i * 8)
if length&f != 0 {
opcodes = append(opcodes, byte(length&f>>(i*8)))
code |= 0x10 << i
}
}
return append([]byte{byte(code)}, opcodes...)
}
|
