/*
	Copyright 2021 SANGFOR TECHNOLOGIES

	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 orcraft

import (
	"bufio"
	"bytes"
	"encoding/json"
	"fmt"
	"gitee.com/opengauss/ham4db/go/core/log"
	"hash"
	"hash/crc64"
	"io"
	"io/ioutil"
	"os"
	"path/filepath"
	"sort"
	"strings"
	"time"

	"github.com/hashicorp/raft"
)

const (
	testPath      = "permTest"
	snapPath      = "snapshots"
	metaFilePath  = "meta.json"
	stateFilePath = "state.bin"
	tmpSuffix     = ".tmp"
)

// FileSnapshotStore implements the SnapshotStore interface and allows
// snapshots to be made on the local disk.
type FileSnapshotStore struct {
	path   string
	retain int
}

type snapMetaSlice []*fileSnapshotMeta

// FileSnapshotSink implements SnapshotSink with a file.
type FileSnapshotSink struct {
	store *FileSnapshotStore
	dir   string
	meta  fileSnapshotMeta

	stateFile *os.File
	stateHash hash.Hash64
	buffered  *bufio.Writer

	closed bool
}

// fileSnapshotMeta is stored on disk. We also put a CRC
// on disk so that we can verify the snapshot.
type fileSnapshotMeta struct {
	raft.SnapshotMeta
	CRC []byte
}

// bufferedFile is returned when we open a snapshot. This way
// reads are buffered and the file still gets closed.
type bufferedFile struct {
	bh *bufio.Reader
	fh *os.File
}

func (b *bufferedFile) Read(p []byte) (n int, err error) {
	return b.bh.Read(p)
}

func (b *bufferedFile) Close() error {
	return b.fh.Close()
}

// NewFileSnapshotStoreWithLogger creates a new FileSnapshotStore based
// on a base directory. The `retain` parameter controls how many
// snapshots are retained. Must be at least 1.
func NewFileSnapshotStoreWithLogger(base string, retain int) (*FileSnapshotStore, error) {
	if retain < 1 {
		return nil, fmt.Errorf("must retain at least one snapshot")
	}

	// Ensure our path exists
	path := filepath.Join(base, snapPath)
	if err := os.MkdirAll(path, 0755); err != nil && !os.IsExist(err) {
		return nil, fmt.Errorf("snapshot path not accessible: %v", err)
	}

	// Setup the store
	store := &FileSnapshotStore{
		path:   path,
		retain: retain,
	}

	// Do a permissions test
	if err := store.testPermissions(); err != nil {
		return nil, fmt.Errorf("permissions test failed: %v", err)
	}
	return store, nil
}

// NewFileSnapshotStore creates a new FileSnapshotStore based
// on a base directory. The `retain` parameter controls how many
// snapshots are retained. Must be at least 1.
func NewFileSnapshotStore(base string, retain int, logOutput io.Writer) (*FileSnapshotStore, error) {
	if logOutput == nil {
		logOutput = os.Stderr
	}
	return NewFileSnapshotStoreWithLogger(base, retain)
}

// testPermissions tries to touch a file in our path to see if it works.
func (f *FileSnapshotStore) testPermissions() error {
	path := filepath.Join(f.path, testPath)
	fh, err := os.Create(path)
	if err != nil {
		return err
	}

	if err = fh.Close(); err != nil {
		return err
	}

	if err = os.Remove(path); err != nil {
		return err
	}
	return nil
}

// snapshotName generates a name for the snapshot.
func snapshotName(term, index uint64) string {
	now := time.Now()
	msec := now.UnixNano() / int64(time.Millisecond)
	return fmt.Sprintf("%d-%d-%d", term, index, msec)
}

// Create is used to start a new snapshot
func (f *FileSnapshotStore) Create(index, term uint64, peers []byte) (raft.SnapshotSink, error) {
	// Create a new path
	name := snapshotName(term, index)
	path := filepath.Join(f.path, name+tmpSuffix)
	log.Infof("snapshot: Creating new snapshot at %s", path)

	// Make the directory
	if err := os.MkdirAll(path, 0755); err != nil {
		_ = log.Errorf("snapshot: Failed to make snapshot directory: %v", err)
		return nil, err
	}

	// Create the sink
	sink := &FileSnapshotSink{
		store: f,
		dir:   path,
		meta: fileSnapshotMeta{
			SnapshotMeta: raft.SnapshotMeta{
				ID:    name,
				Index: index,
				Term:  term,
				Peers: peers,
			},
			CRC: nil,
		},
	}

	// Write out the meta data
	if err := sink.writeMeta(); err != nil {
		_ = log.Errorf("snapshot: Failed to write metadata: %v", err)
		return nil, err
	}

	// Open the state file
	statePath := filepath.Join(path, stateFilePath)
	fh, err := os.Create(statePath)
	if err != nil {
		_ = log.Errorf("snapshot: Failed to create state file: %v", err)
		return nil, err
	}
	sink.stateFile = fh

	// Create a CRC64 hash
	sink.stateHash = crc64.New(crc64.MakeTable(crc64.ECMA))

	// Wrap both the hash and file in a MultiWriter with buffering
	multi := io.MultiWriter(sink.stateFile, sink.stateHash)
	sink.buffered = bufio.NewWriter(multi)

	// Done
	return sink, nil
}

// List returns available snapshots in the store.
func (f *FileSnapshotStore) List() ([]*raft.SnapshotMeta, error) {
	// Get the eligible snapshots
	snapshots, err := f.getSnapshots()
	if err != nil {
		_ = log.Errorf("snapshot: Failed to get snapshots: %v", err)
		return nil, err
	}

	var snapMeta []*raft.SnapshotMeta
	for _, meta := range snapshots {
		snapMeta = append(snapMeta, &meta.SnapshotMeta)
		if len(snapMeta) == f.retain {
			break
		}
	}
	return snapMeta, nil
}

// getSnapshots returns all the known snapshots.
func (f *FileSnapshotStore) getSnapshots() ([]*fileSnapshotMeta, error) {
	// Get the eligible snapshots
	snapshots, err := ioutil.ReadDir(f.path)
	if err != nil {
		_ = log.Errorf("snapshot: Failed to scan snapshot dir: %v", err)
		return nil, err
	}

	// Populate the metadata
	var snapMeta []*fileSnapshotMeta
	for _, snap := range snapshots {
		// Ignore any files
		if !snap.IsDir() {
			continue
		}

		// Ignore any temporary snapshots
		dirName := snap.Name()
		if strings.HasSuffix(dirName, tmpSuffix) {
			_ = log.Warningf("snapshot: Found temporary snapshot: %v", dirName)
			continue
		}

		// Try to read the meta data
		meta, err := f.readMeta(dirName)
		if err != nil {
			_ = log.Warningf("snapshot: Failed to read metadata for %v: %v", dirName, err)
			continue
		}

		// Append, but only return up to the retain count
		snapMeta = append(snapMeta, meta)
	}

	// Sort the snapshot, reverse so we get new -> old
	sort.Sort(sort.Reverse(snapMetaSlice(snapMeta)))

	return snapMeta, nil
}

// readMeta is used to read the meta data for a given named backup
func (f *FileSnapshotStore) readMeta(name string) (*fileSnapshotMeta, error) {
	// Open the meta file
	metaPath := filepath.Join(f.path, name, metaFilePath)
	fh, err := os.Open(metaPath)
	if err != nil {
		return nil, err
	}
	defer fh.Close()

	// Buffer the file IO
	buffered := bufio.NewReader(fh)

	// Read in the JSON
	meta := &fileSnapshotMeta{}
	dec := json.NewDecoder(buffered)
	if err := dec.Decode(meta); err != nil {
		return nil, err
	}
	return meta, nil
}

// Open takes a snapshot ID and returns a ReadCloser for that snapshot.
func (f *FileSnapshotStore) Open(id string) (*raft.SnapshotMeta, io.ReadCloser, error) {
	// Get the metadata
	meta, err := f.readMeta(id)
	if err != nil {
		_ = log.Errorf("snapshot: Failed to get meta data to open snapshot: %v", err)
		return nil, nil, err
	}

	// Open the state file
	statePath := filepath.Join(f.path, id, stateFilePath)
	fh, err := os.Open(statePath)
	if err != nil {
		_ = log.Errorf("snapshot: Failed to open state file: %v", err)
		return nil, nil, err
	}

	// Create a CRC64 hash
	stateHash := crc64.New(crc64.MakeTable(crc64.ECMA))

	// Compute the hash
	_, err = io.Copy(stateHash, fh)
	if err != nil {
		_ = log.Errorf("snapshot: Failed to read state file: %v", err)
		fh.Close()
		return nil, nil, err
	}

	// Verify the hash
	computed := stateHash.Sum(nil)
	if bytes.Compare(meta.CRC, computed) != 0 {
		_ = log.Errorf("snapshot: CRC checksum failed (stored: %v computed: %v)",
			meta.CRC, computed)
		fh.Close()
		return nil, nil, fmt.Errorf("CRC mismatch")
	}

	// Seek to the start
	if _, err := fh.Seek(0, 0); err != nil {
		_ = log.Errorf("snapshot: State file seek failed: %v", err)
		fh.Close()
		return nil, nil, err
	}

	// Return a buffered file
	buffered := &bufferedFile{
		bh: bufio.NewReader(fh),
		fh: fh,
	}

	return &meta.SnapshotMeta, buffered, nil
}

// ReapSnapshots reaps any snapshots beyond the retain count.
func (f *FileSnapshotStore) ReapSnapshots(currentSnapshotMeta *fileSnapshotMeta) error {

	reapSnapshot := func(snapshot *fileSnapshotMeta) error {
		path := filepath.Join(f.path, snapshot.ID)
		log.Infof("snapshot: reaping snapshot %v", path)
		if err := os.RemoveAll(path); err != nil {
			_ = log.Errorf("snapshot: Failed to reap snapshot %v: %v", path, err)
			return err
		}
		return nil
	}
	snapshots, err := f.getSnapshots()
	if err != nil {
		_ = log.Errorf("snapshot: Failed to get snapshots: %v", err)
		return err
	}

	deprecatedSnapshotsReaped := false
	for _, snapshot := range snapshots {
		if snapshot.Term > currentSnapshotMeta.Term ||
			snapshot.Term == currentSnapshotMeta.Term && snapshot.Index > currentSnapshotMeta.Index {
			reapSnapshot(snapshot)
			deprecatedSnapshotsReaped = true
		}
	}

	if deprecatedSnapshotsReaped {
		// re-read list, since we've removed files
		snapshots, err = f.getSnapshots()
		if err != nil {
			_ = log.Errorf("snapshot: Failed to get snapshots: %v", err)
			return err
		}
	}
	for i := f.retain; i < len(snapshots); i++ {
		reapSnapshot(snapshots[i])
	}
	return nil
}

// ID returns the ID of the snapshot, can be used with Open()
// after the snapshot is finalized.
func (s *FileSnapshotSink) ID() string {
	return s.meta.ID
}

// Write is used to append to the state file. We write to the
// buffered IO object to reduce the amount of context switches.
func (s *FileSnapshotSink) Write(b []byte) (int, error) {
	return s.buffered.Write(b)
}

// Close is used to indicate a successful end.
func (s *FileSnapshotSink) Close() error {
	// Make sure close is idempotent
	if s.closed {
		return nil
	}
	s.closed = true

	// Close the open handles
	if err := s.finalize(); err != nil {
		_ = log.Errorf("snapshot: Failed to finalize snapshot: %v", err)
		return err
	}

	// Write out the meta data
	if err := s.writeMeta(); err != nil {
		_ = log.Errorf("snapshot: Failed to write metadata: %v", err)
		return err
	}

	// Move the directory into place
	newPath := strings.TrimSuffix(s.dir, tmpSuffix)
	if err := os.Rename(s.dir, newPath); err != nil {
		_ = log.Errorf("snapshot: Failed to move snapshot into place: %v", err)
		return err
	}

	// Reap any old snapshots
	if err := s.store.ReapSnapshots(&s.meta); err != nil {
		return err
	}

	return nil
}

// Cancel is used to indicate an unsuccessful end.
func (s *FileSnapshotSink) Cancel() error {
	// Make sure close is idempotent
	if s.closed {
		return nil
	}
	s.closed = true

	// Close the open handles
	if err := s.finalize(); err != nil {
		_ = log.Errorf("snapshot: Failed to finalize snapshot: %v", err)
		return err
	}

	// Attempt to remove all artifacts
	return os.RemoveAll(s.dir)
}

// finalize is used to close all of our resources.
func (s *FileSnapshotSink) finalize() error {
	// Flush any remaining data
	if err := s.buffered.Flush(); err != nil {
		return err
	}

	// Get the file size
	stat, statErr := s.stateFile.Stat()

	// Close the file
	if err := s.stateFile.Close(); err != nil {
		return err
	}

	// Set the file size, check after we close
	if statErr != nil {
		return statErr
	}
	s.meta.Size = stat.Size()

	// Set the CRC
	s.meta.CRC = s.stateHash.Sum(nil)
	return nil
}

// writeMeta is used to write out the metadata we have.
func (s *FileSnapshotSink) writeMeta() error {
	// Open the meta file
	metaPath := filepath.Join(s.dir, metaFilePath)
	fh, err := os.Create(metaPath)
	if err != nil {
		return err
	}
	defer fh.Close()

	// Buffer the file IO
	buffered := bufio.NewWriter(fh)
	defer buffered.Flush()

	// Write out as JSON
	enc := json.NewEncoder(buffered)
	if err := enc.Encode(&s.meta); err != nil {
		return err
	}
	return nil
}

// Implement the sort interface for []*fileSnapshotMeta.
func (s snapMetaSlice) Len() int {
	return len(s)
}

func (s snapMetaSlice) Less(i, j int) bool {
	if s[i].Term != s[j].Term {
		return s[i].Term < s[j].Term
	}
	if s[i].Index != s[j].Index {
		return s[i].Index < s[j].Index
	}
	return s[i].ID < s[j].ID
}

func (s snapMetaSlice) Swap(i, j int) {
	s[i], s[j] = s[j], s[i]
}