// Copyright 2014, 2015 by Sascha L. Teichmann
// Use of this source code is governed by the MIT license
// that can be found in the LICENSE file.

package common

import (
	"bytes"
	"compress/zlib"
	"encoding/binary"
	"errors"
	"io"
	"io/ioutil"
	"log"
	"sync"
)

// Error returned if a Producer has run to its end.
var (
	ErrNoMoreBlocks           = errors.New("No more blocks.")
	ErrMapContentSizeMismatch = errors.New("Content size does not match.")
	ErrBlockTruncated         = errors.New("Block is truncated.")
)

const (
	mapBlockSize = 16
	nodeCount    = mapBlockSize * mapBlockSize * mapBlockSize
)

type (
	// Block data from Minetest database.
	Block struct {
		Coord Coord
		Data  []byte
	}
	// BlockProducer is used to over a existing Minetest database
	// and return its content block by block.
	BlockProducer interface {
		// error is ErrNoMoreBlocks if it run out of blocks.
		Next(*Block) error
		// Closes the open database connections.
		Close() error
	}

	// BlockConsumer is used to store blocks in a new Minetest database.
	BlockConsumer interface {
		Consume(*Block) error
		// Closes the open database connections.
		Close() error
	}

	DecodedBlock struct {
		Version     byte
		Transparent bool
		MapContent  []byte
		AirID       int32
		IgnoreID    int32
		IndexMap    map[int32]int32
	}
)

// zlibEmpty is a minimal zlib stream with zero length.
// zlib.NewReader needs a valid zlib stream to start with
// even if Reset is called directly afterwards.
var zlibEmpty = []byte{
	0x78, 0x9c, 0x00, 0x00,
	0x00, 0xff, 0xff, 0x01,
	0x00, 0x00, 0xff, 0xff,
	0x00, 0x00, 0x00, 0x01}

// zlibReaderPool is a pool of zlib Readers to be reused
// for decoding the compressed parts of database blocks.
// Decoding blocks relies heavly on zlib decompression.
// Reusing the internal structures of already allocated
// zlib readers speeds up the decoding significantly.
var zlibReaderPool = sync.Pool{
	New: func() interface{} {
		reader, _ := zlib.NewReader(bytes.NewBuffer(zlibEmpty))
		return reader
	},
}

// The content of the map and the meta data are compressed with zlib.
// Unfortunately the byte length of this two structures are not stored
// explicitly in the block data. To access the informations behind
// them (e.g. the node id mappings) we have to count the bytes consumed
// by the zlib reader and continue our extraction process behind this
// offset. posBuf implements such a counting reader source.
type posBuf struct {
	Data []byte
	Pos  int
}

func NewDecodedBlock(data []byte, colors *Colors) (db *DecodedBlock, err error) {

	dataLen := len(data)
	if dataLen < 4 {
		return nil, ErrBlockTruncated
	}

	version := data[0]

	contentWidth := int(data[2])
	paramsWidth := int(data[3])

	uncompressedLen := nodeCount * (contentWidth + paramsWidth)

	var offset int
	switch {
	case version >= 27:
		offset = 6
	case version >= 22:
		offset = 4
	default:
		offset = 2
	}

	zr := zlibReaderPool.Get().(interface {
		io.ReadCloser
		zlib.Resetter
	})
	defer func() {
		zr.Close() // This sould not be necessary.
		zlibReaderPool.Put(zr)
	}()

	buf := posBuf{Data: data[offset:]}
	if err = zr.Reset(&buf, nil); err != nil {
		return
	}

	mapContent := make([]byte, uncompressedLen)

	var k int
	k, err = io.ReadFull(zr, mapContent)
	if err != nil {
		return
	}

	if k != uncompressedLen {
		err = ErrMapContentSizeMismatch
		return
	}

	// There is a bug before Go 1.7 that enforces
	// to add 4 as an offset after the compressed
	// geometry data. This is resolved via build tags
	// and definitions in pre17offset.go and
	// post17offset.go.
	offset += buf.Pos + afterCompressOfs
	buf.Pos = 0
	if offset >= dataLen {
		return nil, ErrBlockTruncated
	}
	buf.Data = data[offset:]

	if err = zr.(zlib.Resetter).Reset(&buf, nil); err != nil {
		return
	}

	// Discard the meta data.
	if _, err = io.Copy(ioutil.Discard, zr); err != nil {
		return
	}

	offset += buf.Pos

	switch {
	case version <= 21:
		offset += 2
	case version == 23:
		offset++
	case version == 24:
		if offset >= dataLen {
			return nil, ErrBlockTruncated
		}
		ver := data[offset]
		offset++
		if ver == 1 {
			if offset+1 >= dataLen {
				return nil, ErrBlockTruncated
			}
			num := int(binary.BigEndian.Uint16(data[offset:]))
			offset += 2 + 10*num
		}
	}

	offset++
	if offset+1 >= dataLen {
		return nil, ErrBlockTruncated
	}
	numStaticObjects := int(binary.BigEndian.Uint16(data[offset:]))
	offset += 2
	for i := 0; i < numStaticObjects; i++ {
		offset += 13
		if offset+1 >= dataLen {
			return nil, ErrBlockTruncated
		}
		dataSize := int(binary.BigEndian.Uint16(data[offset:]))
		offset += dataSize + 2
	}
	offset += 4

	airID, ignoreID := int32(-1), int32(-1)
	indexMap := make(map[int32]int32)
	var transparent bool
	if version >= 22 {
		offset++
		if offset+1 >= dataLen {
			return nil, ErrBlockTruncated
		}
		numMappings := int(binary.BigEndian.Uint16(data[offset:]))
		offset += 2

		// Be a bit more tolerant with truncated node name table.
		// We should probably issue an error here, too!?
		const outOfBounds = "Offset in node id table out of bounds. Ignored."

		for i := 0; i < numMappings; i++ {
			if offset+1 >= dataLen {
				log.Println(outOfBounds)
				break
			}
			nodeID := int32(binary.BigEndian.Uint16(data[offset:]))
			offset += 2
			if offset+1 >= dataLen {
				log.Println(outOfBounds)
				break
			}
			nameLen := int(binary.BigEndian.Uint16(data[offset:]))
			offset += 2
			if offset+nameLen-1 >= dataLen {
				log.Println(outOfBounds)
				break
			}
			name := string(data[offset : offset+nameLen])
			offset += nameLen
			switch name {
			case "air":
				airID = nodeID
			case "ignore":
				ignoreID = nodeID
			default:
				if index, found := colors.NameIndex[name]; found {
					indexMap[nodeID] = index
					if !transparent && colors.IsTransparent(index) {
						transparent = true
					}
				} else {
					logMissing(name)
				}
			}
		}
	}

	db = &DecodedBlock{
		Version:     version,
		Transparent: transparent,
		MapContent:  mapContent,
		AirID:       airID,
		IgnoreID:    ignoreID,
		IndexMap:    indexMap}

	return
}

var missingColors = struct {
	sync.Mutex
	cols map[string]struct{}
}{cols: map[string]struct{}{}}

func logMissing(name string) {
	missingColors.Lock()
	defer missingColors.Unlock()
	if _, found := missingColors.cols[name]; !found {
		missingColors.cols[name] = struct{}{}
		log.Printf("Missing color entry for %s.\n", name)
	}
}

func (db *DecodedBlock) AirOnly() bool {
	return db.AirID != -1 && len(db.IndexMap) == 0
}

func (db *DecodedBlock) Content(x, y, z int) (content int32, found bool) {
	pos := z<<8 + y<<4 + x

	switch {
	case db.Version >= 24:
		pos <<= 1
		content = int32(db.MapContent[pos])<<8 | int32(db.MapContent[pos+1])
	case db.Version >= 20:
		if db.MapContent[pos] <= 0x80 {
			content = int32(db.MapContent[pos])
		} else {
			content = int32(db.MapContent[pos])<<4 | int32(db.MapContent[pos+0x2000])>>4
		}
	default:
		return
	}
	if content != db.AirID && content != db.IgnoreID {
		content, found = db.IndexMap[content]
	}
	return
}

func (pb *posBuf) Read(p []byte) (int, error) {
	pl := len(p)
	ml := len(pb.Data)
	if pb.Pos >= ml {
		return 0, io.EOF
	}
	rest := ml - pb.Pos
	if pl > rest {
		copy(p, pb.Data[pb.Pos:])
		pb.Pos = ml
		return rest, io.EOF
	}
	copy(p, pb.Data[pb.Pos:pb.Pos+pl])
	pb.Pos += pl
	return pl, nil
}

func (pb *posBuf) ReadByte() (byte, error) {
	if pb.Pos >= len(pb.Data) {
		return 0, io.EOF
	}
	c := pb.Data[pb.Pos]
	pb.Pos++
	return c, nil
}