package packfile

import (
	"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 (
	maxCopyLen = 0xffff
)

// 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 baseBuf into targetBuf.
func DiffDelta(baseBuf []byte, targetBuf []byte) []byte {
	var outBuff []byte

	outBuff = append(outBuff, deltaEncodeSize(len(baseBuf))...)
	outBuff = append(outBuff, deltaEncodeSize(len(targetBuf))...)

	sm := newMatcher(baseBuf, targetBuf)
	for _, op := range sm.GetOpCodes() {
		switch {
		case op.Tag == tagEqual:
			copyStart := op.I1
			copyLen := op.I2 - op.I1
			for {
				if copyLen <= 0 {
					break
				}
				var toCopy int
				if copyLen < maxCopyLen {
					toCopy = copyLen
				} else {
					toCopy = maxCopyLen
				}

				outBuff = append(outBuff, encodeCopyOperation(copyStart, toCopy)...)
				copyStart += toCopy
				copyLen -= toCopy
			}
		case op.Tag == tagReplace || op.Tag == tagInsert:
			s := op.J2 - op.J1
			o := op.J1
			for {
				if s <= 127 {
					break
				}
				outBuff = append(outBuff, byte(127))
				outBuff = append(outBuff, targetBuf[o:o+127]...)
				s -= 127
				o += 127
			}
			outBuff = append(outBuff, byte(s))
			outBuff = append(outBuff, targetBuf[o:o+s]...)
		}
	}

	return outBuff
}

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...)
}