// Copyright 2017 The Prometheus Authors
// 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 chunks

import (
	"bufio"
	"encoding/binary"
	"fmt"
	"hash"
	"hash/crc32"
	"io"
	"io/ioutil"
	"os"
	"path/filepath"
	"strconv"

	"github.com/pkg/errors"
	"github.com/prometheus/tsdb/chunkenc"
	"github.com/prometheus/tsdb/fileutil"
)

const (
	// MagicChunks is 4 bytes at the head of a series file.
	MagicChunks = 0x85BD40DD
)

// Meta holds information about a chunk of data.
type Meta struct {
	// Ref and Chunk hold either a reference that can be used to retrieve
	// chunk data or the data itself.
	// Generally, only one of them is set.
	Ref   uint64
	Chunk chunkenc.Chunk

	MinTime, MaxTime int64 // time range the data covers
}

// writeHash writes the chunk encoding and raw data into the provided hash.
func (cm *Meta) writeHash(h hash.Hash) error {
	if _, err := h.Write([]byte{byte(cm.Chunk.Encoding())}); err != nil {
		return err
	}
	if _, err := h.Write(cm.Chunk.Bytes()); err != nil {
		return err
	}
	return nil
}

var (
	errInvalidSize     = fmt.Errorf("invalid size")
	errInvalidFlag     = fmt.Errorf("invalid flag")
	errInvalidChecksum = fmt.Errorf("invalid checksum")
)

var castagnoliTable *crc32.Table

func init() {
	castagnoliTable = crc32.MakeTable(crc32.Castagnoli)
}

// newCRC32 initializes a CRC32 hash with a preconfigured polynomial, so the
// polynomial may be easily changed in one location at a later time, if necessary.
func newCRC32() hash.Hash32 {
	return crc32.New(castagnoliTable)
}

// Writer implements the ChunkWriter interface for the standard
// serialization format.
type Writer struct {
	dirFile *os.File
	files   []*os.File
	wbuf    *bufio.Writer
	n       int64
	crc32   hash.Hash

	segmentSize int64
}

const (
	defaultChunkSegmentSize = 512 * 1024 * 1024

	chunksFormatV1 = 1
)

// NewWriter returns a new writer against the given directory.
func NewWriter(dir string) (*Writer, error) {
	if err := os.MkdirAll(dir, 0777); err != nil {
		return nil, err
	}
	dirFile, err := fileutil.OpenDir(dir)
	if err != nil {
		return nil, err
	}
	cw := &Writer{
		dirFile:     dirFile,
		n:           0,
		crc32:       newCRC32(),
		segmentSize: defaultChunkSegmentSize,
	}
	return cw, nil
}

func (w *Writer) tail() *os.File {
	if len(w.files) == 0 {
		return nil
	}
	return w.files[len(w.files)-1]
}

// finalizeTail writes all pending data to the current tail file,
// truncates its size, and closes it.
func (w *Writer) finalizeTail() error {
	tf := w.tail()
	if tf == nil {
		return nil
	}

	if err := w.wbuf.Flush(); err != nil {
		return err
	}
	if err := fileutil.Fsync(tf); err != nil {
		return err
	}
	// As the file was pre-allocated, we truncate any superfluous zero bytes.
	off, err := tf.Seek(0, io.SeekCurrent)
	if err != nil {
		return err
	}
	if err := tf.Truncate(off); err != nil {
		return err
	}

	return tf.Close()
}

func (w *Writer) cut() error {
	// Sync current tail to disk and close.
	if err := w.finalizeTail(); err != nil {
		return err
	}

	p, _, err := nextSequenceFile(w.dirFile.Name())
	if err != nil {
		return err
	}
	f, err := os.OpenFile(p, os.O_WRONLY|os.O_CREATE, 0666)
	if err != nil {
		return err
	}
	if err = fileutil.Preallocate(f, w.segmentSize, true); err != nil {
		return err
	}
	if err = w.dirFile.Sync(); err != nil {
		return err
	}

	// Write header metadata for new file.

	metab := make([]byte, 8)
	binary.BigEndian.PutUint32(metab[:4], MagicChunks)
	metab[4] = chunksFormatV1

	if _, err := f.Write(metab); err != nil {
		return err
	}

	w.files = append(w.files, f)
	if w.wbuf != nil {
		w.wbuf.Reset(f)
	} else {
		w.wbuf = bufio.NewWriterSize(f, 8*1024*1024)
	}
	w.n = 8

	return nil
}

func (w *Writer) write(b []byte) error {
	n, err := w.wbuf.Write(b)
	w.n += int64(n)
	return err
}

func (w *Writer) WriteChunks(chks ...Meta) error {
	// Calculate maximum space we need and cut a new segment in case
	// we don't fit into the current one.
	maxLen := int64(binary.MaxVarintLen32) // The number of chunks.
	for _, c := range chks {
		maxLen += binary.MaxVarintLen32 + 1 // The number of bytes in the chunk and its encoding.
		maxLen += int64(len(c.Chunk.Bytes()))
	}
	newsz := w.n + maxLen

	if w.wbuf == nil || w.n > w.segmentSize || newsz > w.segmentSize && maxLen <= w.segmentSize {
		if err := w.cut(); err != nil {
			return err
		}
	}

	var (
		b   = [binary.MaxVarintLen32]byte{}
		seq = uint64(w.seq()) << 32
	)
	for i := range chks {
		chk := &chks[i]

		chk.Ref = seq | uint64(w.n)

		n := binary.PutUvarint(b[:], uint64(len(chk.Chunk.Bytes())))

		if err := w.write(b[:n]); err != nil {
			return err
		}
		b[0] = byte(chk.Chunk.Encoding())
		if err := w.write(b[:1]); err != nil {
			return err
		}
		if err := w.write(chk.Chunk.Bytes()); err != nil {
			return err
		}

		w.crc32.Reset()
		if err := chk.writeHash(w.crc32); err != nil {
			return err
		}
		if err := w.write(w.crc32.Sum(b[:0])); err != nil {
			return err
		}
	}

	return nil
}

func (w *Writer) seq() int {
	return len(w.files) - 1
}

func (w *Writer) Close() error {
	if err := w.finalizeTail(); err != nil {
		return err
	}

	// close dir file (if not windows platform will fail on rename)
	return w.dirFile.Close()
}

// ByteSlice abstracts a byte slice.
type ByteSlice interface {
	Len() int
	Range(start, end int) []byte
}

type realByteSlice []byte

func (b realByteSlice) Len() int {
	return len(b)
}

func (b realByteSlice) Range(start, end int) []byte {
	return b[start:end]
}

func (b realByteSlice) Sub(start, end int) ByteSlice {
	return b[start:end]
}

// Reader implements a SeriesReader for a serialized byte stream
// of series data.
type Reader struct {
	// The underlying bytes holding the encoded series data.
	bs []ByteSlice

	// Closers for resources behind the byte slices.
	cs []io.Closer

	pool chunkenc.Pool
}

func newReader(bs []ByteSlice, cs []io.Closer, pool chunkenc.Pool) (*Reader, error) {
	cr := Reader{pool: pool, bs: bs, cs: cs}

	for i, b := range cr.bs {
		if b.Len() < 4 {
			return nil, errors.Wrapf(errInvalidSize, "validate magic in segment %d", i)
		}
		// Verify magic number.
		if m := binary.BigEndian.Uint32(b.Range(0, 4)); m != MagicChunks {
			return nil, fmt.Errorf("invalid magic number %x", m)
		}
	}
	return &cr, nil
}

// NewReader returns a new chunk reader against the given byte slices.
func NewReader(bs []ByteSlice, pool chunkenc.Pool) (*Reader, error) {
	if pool == nil {
		pool = chunkenc.NewPool()
	}
	return newReader(bs, nil, pool)
}

// NewDirReader returns a new Reader against sequentially numbered files in the
// given directory.
func NewDirReader(dir string, pool chunkenc.Pool) (*Reader, error) {
	files, err := sequenceFiles(dir)
	if err != nil {
		return nil, err
	}
	if pool == nil {
		pool = chunkenc.NewPool()
	}

	var bs []ByteSlice
	var cs []io.Closer

	for _, fn := range files {
		f, err := fileutil.OpenMmapFile(fn)
		if err != nil {
			return nil, errors.Wrapf(err, "mmap files")
		}
		cs = append(cs, f)
		bs = append(bs, realByteSlice(f.Bytes()))
	}
	return newReader(bs, cs, pool)
}

func (s *Reader) Close() error {
	return closeAll(s.cs...)
}

func (s *Reader) Chunk(ref uint64) (chunkenc.Chunk, error) {
	var (
		seq = int(ref >> 32)
		off = int((ref << 32) >> 32)
	)
	if seq >= len(s.bs) {
		return nil, errors.Errorf("reference sequence %d out of range", seq)
	}
	b := s.bs[seq]

	if off >= b.Len() {
		return nil, errors.Errorf("offset %d beyond data size %d", off, b.Len())
	}
	// With the minimum chunk length this should never cause us reading
	// over the end of the slice.
	r := b.Range(off, off+binary.MaxVarintLen32)

	l, n := binary.Uvarint(r)
	if n < 0 {
		return nil, fmt.Errorf("reading chunk length failed")
	}
	r = b.Range(off+n, off+n+int(l))

	return s.pool.Get(chunkenc.Encoding(r[0]), r[1:1+l])
}

func nextSequenceFile(dir string) (string, int, error) {
	names, err := fileutil.ReadDir(dir)
	if err != nil {
		return "", 0, err
	}

	i := uint64(0)
	for _, n := range names {
		j, err := strconv.ParseUint(n, 10, 64)
		if err != nil {
			continue
		}
		i = j
	}
	return filepath.Join(dir, fmt.Sprintf("%0.6d", i+1)), int(i + 1), nil
}

func sequenceFiles(dir string) ([]string, error) {
	files, err := ioutil.ReadDir(dir)
	if err != nil {
		return nil, err
	}
	var res []string

	for _, fi := range files {
		if _, err := strconv.ParseUint(fi.Name(), 10, 64); err != nil {
			continue
		}
		res = append(res, filepath.Join(dir, fi.Name()))
	}
	return res, nil
}

func closeAll(cs ...io.Closer) (err error) {
	for _, c := range cs {
		if e := c.Close(); e != nil {
			err = e
		}
	}
	return err
}
