Source: diff_delta.go in package github.com/go-git/go-git/v5/plumbing/format/packfile

package packfile

import (
	"bytes"

	"github.com/go-git/go-git/v5/plumbing"
	"github.com/go-git/go-git/v5/utils/ioutil"
)

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

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) {
	return getDelta(new(deltaIndex), base, target)
}

func getDelta(index *deltaIndex, base, target plumbing.EncodedObject) (o plumbing.EncodedObject, err error) {
	br, err := base.Reader()
	if err != nil {
		return nil, err
	}

	defer ioutil.CheckClose(br, &err)

	tr, err := target.Reader()
	if err != nil {
		return nil, err
	}

	defer ioutil.CheckClose(tr, &err)

	bb := bufPool.Get().(*bytes.Buffer)
	defer bufPool.Put(bb)
	bb.Reset()

	_, err = bb.ReadFrom(br)
	if err != nil {
		return nil, err
	}

	tb := bufPool.Get().(*bytes.Buffer)
	defer bufPool.Put(tb)
	tb.Reset()

	_, err = tb.ReadFrom(tr)
	if err != nil {
		return nil, err
	}

	db := diffDelta(index, bb.Bytes(), tb.Bytes())
	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, tgt []byte) []byte {
	return diffDelta(new(deltaIndex), src, tgt)
}

func diffDelta(index *deltaIndex, src []byte, tgt []byte) []byte {
	buf := bufPool.Get().(*bytes.Buffer)
	defer bufPool.Put(buf)
	buf.Reset()
	buf.Write(deltaEncodeSize(len(src)))
	buf.Write(deltaEncodeSize(len(tgt)))

	if len(index.entries) == 0 {
		index.init(src)
	}

	ibuf := bufPool.Get().(*bytes.Buffer)
	defer bufPool.Put(ibuf)
	ibuf.Reset()
	for i := 0; i < len(tgt); i++ {
		offset, l := index.findMatch(src, tgt, i)

couldn't find a match, just write the current byte and continue

			ibuf.WriteByte(tgt[i])

src is less than blksz, copy the rest of the target to avoid calls to findMatch

			for ; i < len(tgt); i++ {
				ibuf.WriteByte(tgt[i])
			}

remaining target is less than blksz, copy what's left of it and avoid calls to findMatch

			for j := i; j < i+l; j++ {
				ibuf.WriteByte(tgt[j])
			}
			i += l - 1
		} else {
			encodeInsertOperation(ibuf, buf)

			rl := l
			aOffset := offset
			for rl > 0 {
				if rl < maxCopySize {
					buf.Write(encodeCopyOperation(aOffset, rl))
					break
				}

				buf.Write(encodeCopyOperation(aOffset, maxCopySize))
				rl -= maxCopySize
				aOffset += maxCopySize
			}

			i += l - 1
		}
	}

	encodeInsertOperation(ibuf, buf)

buf.Bytes() is only valid until the next modifying operation on the buffer. Copy it.

	return append([]byte{}, 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 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...)