package dehub

import (
	"crypto/sha256"
	"encoding/binary"
	"fmt"
	"hash"
	"sort"

	"gopkg.in/src-d/go-git.v4/plumbing/object"
)

var (
	defaultHashHelperAlgo = sha256.New
)

type hashHelper struct {
	hash.Hash
	varintBuf []byte
}

// if h is nil it then defaultHashHelperAlgo will be used
func newHashHelper(h hash.Hash) *hashHelper {
	if h == nil {
		h = defaultHashHelperAlgo()
	}
	s := &hashHelper{
		Hash:      h,
		varintBuf: make([]byte, binary.MaxVarintLen64),
	}
	return s
}

func (s *hashHelper) sum(prefix []byte) []byte {
	out := make([]byte, len(prefix), len(prefix)+s.Hash.Size())
	copy(out, prefix)
	return s.Hash.Sum(out)
}

func (s *hashHelper) writeUint(i uint64) {
	n := binary.PutUvarint(s.varintBuf, i)
	if _, err := s.Write(s.varintBuf[:n]); err != nil {
		panic(fmt.Sprintf("error writing %x to sha256 sum: %v", s.varintBuf[:n], err))
	}
}

func (s *hashHelper) writeStr(str string) {
	s.writeUint(uint64(len(str)))
	s.Write([]byte(str))
}

func (s *hashHelper) writeTreeDiff(from, to *object.Tree) {
	filesChanged, err := calcDiff(from, to)
	if err != nil {
		panic(err.Error())
	}

	sort.Slice(filesChanged, func(i, j int) bool {
		return filesChanged[i].path < filesChanged[j].path
	})

	s.writeUint(uint64(len(filesChanged)))
	for _, fileChanged := range filesChanged {
		s.writeStr(fileChanged.path)
		s.Write(fileChanged.fromMode.Bytes())
		s.Write(fileChanged.fromHash[:])
		s.Write(fileChanged.toMode.Bytes())
		s.Write(fileChanged.toHash[:])
	}

}

var (
	changeHashVersion  = []byte{0}
	commentHashVersion = []byte{0}
)

// if h is nil it then defaultHashHelperAlgo will be used
func genChangeHash(h hash.Hash, msg string, from, to *object.Tree) []byte {
	s := newHashHelper(h)
	s.writeStr(msg)
	s.writeTreeDiff(from, to)
	return s.sum(changeHashVersion)
}

// if h is nil it then defaultHashHelperAlgo will be used
func genCommentHash(h hash.Hash, comment string) []byte {
	s := newHashHelper(h)
	s.writeStr(comment)
	return s.sum(commentHashVersion)
}