// Copyright 2015 CoreOS, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package snap

import (
	"errors"
	"fmt"
	"hash/crc32"
	"io/ioutil"
	"log"
	"os"
	"path"
	"sort"
	"strings"

	"github.com/coreos/etcd/pkg/pbutil"
	"github.com/coreos/etcd/raft"
	"github.com/coreos/etcd/raft/raftpb"
	"github.com/coreos/etcd/snap/snappb"
)

const (
	snapSuffix = ".snap"
)

var (
	ErrNoSnapshot    = errors.New("snap: no available snapshot")
	ErrEmptySnapshot = errors.New("snap: empty snapshot")
	ErrCRCMismatch   = errors.New("snap: crc mismatch")
	crcTable         = crc32.MakeTable(crc32.Castagnoli)
)

type Snapshotter struct {
	dir string
}

func New(dir string) *Snapshotter {
	return &Snapshotter{
		dir: dir,
	}
}

func (s *Snapshotter) SaveSnap(snapshot raftpb.Snapshot) error {
	if raft.IsEmptySnap(snapshot) {
		return nil
	}
	return s.save(&snapshot)
}

func (s *Snapshotter) save(snapshot *raftpb.Snapshot) error {
	fname := fmt.Sprintf("%016x-%016x%s", snapshot.Metadata.Term, snapshot.Metadata.Index, snapSuffix)
	b := pbutil.MustMarshal(snapshot)
	crc := crc32.Update(0, crcTable, b)
	snap := snappb.Snapshot{Crc: crc, Data: b}
	d, err := snap.Marshal()
	if err != nil {
		return err
	}
	return ioutil.WriteFile(path.Join(s.dir, fname), d, 0666)
}

func (s *Snapshotter) Load() (*raftpb.Snapshot, error) {
	names, err := s.snapNames()
	if err != nil {
		return nil, err
	}
	var snap *raftpb.Snapshot
	for _, name := range names {
		if snap, err = loadSnap(s.dir, name); err == nil {
			break
		}
	}
	return snap, err
}

func loadSnap(dir, name string) (*raftpb.Snapshot, error) {
	fpath := path.Join(dir, name)
	snap, err := Read(fpath)
	if err != nil {
		renameBroken(fpath)
	}
	return snap, err
}

// Read reads the snapshot named by snapname and returns the snapshot.
func Read(snapname string) (*raftpb.Snapshot, error) {
	b, err := ioutil.ReadFile(snapname)
	if err != nil {
		log.Printf("snap: snapshotter cannot read file %v: %v", snapname, err)
		return nil, err
	}

	var serializedSnap snappb.Snapshot
	if err = serializedSnap.Unmarshal(b); err != nil {
		log.Printf("snap: corrupted snapshot file %v: %v", snapname, err)
		return nil, err
	}

	if len(serializedSnap.Data) == 0 || serializedSnap.Crc == 0 {
		log.Printf("snap: unexpected empty snapshot")
		return nil, ErrEmptySnapshot
	}

	crc := crc32.Update(0, crcTable, serializedSnap.Data)
	if crc != serializedSnap.Crc {
		log.Printf("snap: corrupted snapshot file %v: crc mismatch", snapname)
		return nil, ErrCRCMismatch
	}

	var snap raftpb.Snapshot
	if err = snap.Unmarshal(serializedSnap.Data); err != nil {
		log.Printf("snap: corrupted snapshot file %v: %v", snapname, err)
		return nil, err
	}
	return &snap, nil
}

// snapNames returns the filename of the snapshots in logical time order (from newest to oldest).
// If there is no available snapshots, an ErrNoSnapshot will be returned.
func (s *Snapshotter) snapNames() ([]string, error) {
	dir, err := os.Open(s.dir)
	if err != nil {
		return nil, err
	}
	defer dir.Close()
	names, err := dir.Readdirnames(-1)
	if err != nil {
		return nil, err
	}
	snaps := checkSuffix(names)
	if len(snaps) == 0 {
		return nil, ErrNoSnapshot
	}
	sort.Sort(sort.Reverse(sort.StringSlice(snaps)))
	return snaps, nil
}

func checkSuffix(names []string) []string {
	snaps := []string{}
	for i := range names {
		if strings.HasSuffix(names[i], snapSuffix) {
			snaps = append(snaps, names[i])
		} else {
			log.Printf("snap: unexpected non-snap file %v", names[i])
		}
	}
	return snaps
}

func renameBroken(path string) {
	brokenPath := path + ".broken"
	if err := os.Rename(path, brokenPath); err != nil {
		log.Printf("snap: cannot rename broken snapshot file %v to %v: %v", path, brokenPath, err)
	}
}