package nnet

import (
	"bytes"
	"encoding/binary"
	"errors"
	"git.noahlan.cn/noahlan/nnet/packet"
)

type Packer struct {
	buf      *bytes.Buffer
	size     int  // 最近一次 length
	typ      byte // 最近一次 packet type
	flag     byte // 最近一次 flag
	routeMap *RouteMap
}

// packer constants.
const (
	headLength    = 5
	maxPacketSize = 64 * 1024

	msgRouteCompressMask = 0x01 // 0000 0001  last bit
	msgTypeMask          = 0x07 // 0000 0111  1-3 bit (需要>>)
	msgRouteLengthMask   = 0xFF // 1111 1111  last 8 bit
	msgHeadLength        = 0x02 // 0000 0010  2 bit
)

var (
	ErrPacketSizeExceed  = errors.New("packer: packet size exceed")
	ErrWrongMessageType  = errors.New("wrong message type")
	ErrRouteInfoNotFound = errors.New("route info not found in dictionary")
	ErrWrongMessage      = errors.New("wrong message")
)

func NewPacker(routeMap *RouteMap) *Packer {
	p := &Packer{
		buf:      bytes.NewBuffer(nil),
		routeMap: routeMap,
	}
	p.resetFlags()
	return p
}

func (d *Packer) resetFlags() {
	d.size = -1
	d.typ = byte(Unknown)
	d.flag = 0x00
}

func (d *Packer) routable(t MsgType) bool {
	return t == Request || t == Notify || t == Push
}

func (d *Packer) invalidType(t MsgType) bool {
	return t < Request || t > Push
}

func (d *Packer) Pack(header any, data []byte) ([]byte, error) {
	h, ok := header.(Header)
	if !ok {
		return nil, packet.ErrWrongPacketType
	}
	typ := h.PacketType

	if typ < Handshake || typ > Kick {
		return nil, packet.ErrWrongPacketType
	}

	if d.invalidType(h.MsgType) {
		return nil, ErrWrongMessageType
	}

	buf := make([]byte, 0)

	// packet type
	buf = append(buf, byte(h.PacketType))

	// length
	buf = append(buf, d.intToBytes(uint32(len(data)))...)

	// flag
	flag := byte(h.MsgType << 1) // 编译器提示,此处 byte 转换不能删
	code, compressed := d.routeMap.Routes[h.Route]
	if compressed {
		flag |= msgRouteCompressMask
	}
	buf = append(buf, flag)

	// msg id
	if h.MsgType == Request || h.MsgType == Response {
		n := h.ID
		// variant length encode
		for {
			b := byte(n % 128)
			n >>= 7
			if n != 0 {
				buf = append(buf, b+128)
			} else {
				buf = append(buf, b)
				break
			}
		}
	}

	// route
	if d.routable(h.MsgType) {
		if compressed {
			buf = append(buf, byte((code>>8)&0xFF))
			buf = append(buf, byte(code&0xFF))
		} else {
			buf = append(buf, byte(len(h.Route)))
			buf = append(buf, []byte(h.Route)...)
		}
	}

	// body
	buf = append(buf, data...)
	return buf, nil
}

// Encode packet data length to bytes(Big end)
func (d *Packer) intToBytes(n uint32) []byte {
	buf := make([]byte, 3)
	buf[0] = byte((n >> 16) & 0xFF)
	buf[1] = byte((n >> 8) & 0xFF)
	buf[2] = byte(n & 0xFF)
	return buf
}

func (d *Packer) Unpack(data []byte) ([]packet.IPacket, error) {
	d.buf.Write(data) // copy

	var (
		packets []packet.IPacket
		err     error
	)

	// 检查包长度
	if d.buf.Len() < headLength {
		return nil, err
	}

	// 第一次拆包
	if d.size < 0 {
		if err = d.readHeader(); err != nil {
			return nil, err
		}
	}

	for d.size <= d.buf.Len() {
		// 读取
		p := newPacket(Type(d.typ))
		p.MsgType = MsgType((d.flag >> 1) & msgTypeMask)

		if d.invalidType(p.MsgType) {
			return nil, ErrWrongMessageType
		}
		if p.MsgType == Request || p.MsgType == Response {
			id := uint64(0)
			// little end byte order
			// WARNING: must be stored in 64 bits integer
			// variant length encode
			c := 0
			for {
				b, err := d.buf.ReadByte()
				if err != nil {
					break
				}
				id += uint64(b&0x7F) << uint64(7*c)
				if b < 128 {
					break
				}
				c++
			}
			p.ID = id
		}
		if d.routable(p.MsgType) {
			if d.flag&msgRouteCompressMask == 1 {
				p.compressed = true
				code := binary.BigEndian.Uint16(d.buf.Next(2))
				route, ok := d.routeMap.Codes[code]
				if !ok {
					return nil, ErrRouteInfoNotFound
				}
				p.Route = route
			} else {
				p.compressed = false
				rl, _ := d.buf.ReadByte()
				if int(rl) > d.buf.Len() {
					return nil, ErrWrongMessage
				}
				p.Route = string(d.buf.Next(int(rl)))
			}
		}
		p.Length = uint32(d.size)
		p.Data = d.buf.Next(d.size)

		packets = append(packets, p)

		// 剩余数据不满足至少一个数据帧，重置数据帧长度
		// 数据缓存内存 保留至 下一次进入本方法以继续拆包
		if d.buf.Len() < headLength {
			d.resetFlags()
			break
		}
		// 读取下一个包 next
		if err = d.readHeader(); err != nil {
			return packets, err
		}
	}
	if packets == nil || len(packets) <= 0 {
		d.resetFlags()
		d.buf.Reset()
	}
	return packets, nil
}

func (d *Packer) readHeader() error {
	header := d.buf.Next(headLength)
	d.typ = header[0]
	if d.typ < Handshake || d.typ > Kick {
		return packet.ErrWrongPacketType
	}
	d.size = d.bytesToInt(header[1 : len(header)-1])
	d.flag = header[len(header)-1]

	// 最大包限定
	if d.size > maxPacketSize {
		return ErrPacketSizeExceed
	}
	return nil
}

// Decode packet data length byte to int(Big end)
func (d *Packer) bytesToInt(b []byte) int {
	result := 0
	for _, v := range b {
		result = result<<8 + int(v)
	}
	return result
}