package wal

import (
	"bufio"
	"encoding/binary"
	"hash"
	"io"

	"github.com/coreos/etcd/crc"
	"github.com/coreos/etcd/raft/raftpb"
	"github.com/coreos/etcd/wal/walpb"
)

type decoder struct {
	br  *bufio.Reader
	c   io.Closer
	crc hash.Hash32
}

func newDecoder(rc io.ReadCloser) *decoder {
	return &decoder{
		br:  bufio.NewReader(rc),
		c:   rc,
		crc: crc.New(0, crcTable),
	}
}

func (d *decoder) decode(rec *walpb.Record) error {
	rec.Reset()
	l, err := readInt64(d.br)
	if err != nil {
		return err
	}
	data := make([]byte, l)
	if _, err = io.ReadFull(d.br, data); err != nil {
		return err
	}
	if err := rec.Unmarshal(data); err != nil {
		return err
	}
	// skip crc checking if the record type is crcType
	if rec.Type == crcType {
		return nil
	}
	d.crc.Write(rec.Data)
	return rec.Validate(d.crc.Sum32())
}

func (d *decoder) updateCRC(prevCrc uint32) {
	d.crc = crc.New(prevCrc, crcTable)
}

func (d *decoder) lastCRC() uint32 {
	return d.crc.Sum32()
}

func (d *decoder) close() error {
	return d.c.Close()
}

func mustUnmarshalInfo(d []byte) raftpb.Info {
	var i raftpb.Info
	if err := i.Unmarshal(d); err != nil {
		// crc matched, but we cannot unmarshal the struct?!
		// we must be the next winner of the $1B lottery.
		panic(err)
	}
	return i
}

func mustUnmarshalEntry(d []byte) raftpb.Entry {
	var e raftpb.Entry
	if err := e.Unmarshal(d); err != nil {
		panic(err)
	}
	return e
}

func mustUnmarshalState(d []byte) raftpb.State {
	var s raftpb.State
	if err := s.Unmarshal(d); err != nil {
		panic(err)
	}
	return s
}

func readInt64(r io.Reader) (int64, error) {
	var n int64
	err := binary.Read(r, binary.LittleEndian, &n)
	return n, err
}