package adapter

import (
	"encoding/binary"
	"errors"
	"io"
)

// 协议常量.
const (
	// MessageTypeData 表示数据消息.
	MessageTypeData byte = 0x01
	// MessageTypeAck 表示 ACK 确认.
	MessageTypeAck byte = 0x02
	// MessageTypeNack 表示 NACK 否定确认.
	MessageTypeNack byte = 0x03

	// 协议限制.
	maxTopicLength    = 65535
	maxUUIDLength     = 255
	maxPayloadSize    = 1 << 30
	topicLengthSize   = 2
	uuidLengthSize    = 1
	payloadLengthSize = 4
)

// 预定义错误.
var (
	ErrNilMessage      = errors.New("message is nil")
	ErrTopicTooLong    = errors.New("topic too long")
	ErrUUIDTooLong     = errors.New("uuid too long")
	ErrPayloadTooLarge = errors.New("payload too large")
)

// TCPMessage 表示 TCP 传输的消息.
type TCPMessage struct {
	Type    byte   // 消息类型
	Topic   string // 主题
	UUID    string // 消息 UUID
	Payload []byte // 消息内容
}

// EncodeTCPMessage 将消息编码为字节数组.
// 格式: [消息类型 1字节][Topic长度 2字节][Topic][UUID长度 1字节][UUID][Payload长度 4字节][Payload].
func EncodeTCPMessage(msg *TCPMessage) ([]byte, error) {
	if msg == nil {
		return nil, ErrNilMessage
	}

	topicLen := len(msg.Topic)
	if topicLen > maxTopicLength {
		return nil, ErrTopicTooLong
	}

	uuidLen := len(msg.UUID)
	if uuidLen > maxUUIDLength {
		return nil, ErrUUIDTooLong
	}

	payloadLen := len(msg.Payload)
	if payloadLen > maxPayloadSize {
		return nil, ErrPayloadTooLarge
	}

	// 计算总长度
	totalLen := 1 + topicLengthSize + topicLen + uuidLengthSize + uuidLen + payloadLengthSize + payloadLen
	buf := make([]byte, totalLen)

	offset := 0

	// 写入消息类型
	buf[offset] = msg.Type
	offset++

	// 写入 Topic 长度和内容
	binary.BigEndian.PutUint16(buf[offset:], uint16(topicLen))
	offset += topicLengthSize
	copy(buf[offset:], []byte(msg.Topic))
	offset += topicLen

	// 写入 UUID 长度和内容
	buf[offset] = byte(uuidLen)
	offset++
	copy(buf[offset:], []byte(msg.UUID))
	offset += uuidLen

	// 写入 Payload 长度和内容
	binary.BigEndian.PutUint32(buf[offset:], uint32(payloadLen))
	offset += payloadLengthSize
	copy(buf[offset:], msg.Payload)

	return buf, nil
}

// DecodeTCPMessage 从字节数组解码消息.
func DecodeTCPMessage(reader io.Reader) (*TCPMessage, error) {
	msg := &TCPMessage{}

	// 读取消息类型
	msgTypeBuf := make([]byte, 1)
	if _, err := io.ReadFull(reader, msgTypeBuf); err != nil {
		return nil, err
	}
	msg.Type = msgTypeBuf[0]

	// 读取 Topic 长度
	topicLenBuf := make([]byte, topicLengthSize)
	if _, err := io.ReadFull(reader, topicLenBuf); err != nil {
		return nil, err
	}
	topicLen := binary.BigEndian.Uint16(topicLenBuf)

	// 读取 Topic
	topicBuf := make([]byte, topicLen)
	if _, err := io.ReadFull(reader, topicBuf); err != nil {
		return nil, err
	}
	msg.Topic = string(topicBuf)

	// 读取 UUID 长度
	uuidLenBuf := make([]byte, 1)
	if _, err := io.ReadFull(reader, uuidLenBuf); err != nil {
		return nil, err
	}
	uuidLen := uuidLenBuf[0]

	// 读取 UUID
	uuidBuf := make([]byte, uuidLen)
	if _, err := io.ReadFull(reader, uuidBuf); err != nil {
		return nil, err
	}
	msg.UUID = string(uuidBuf)

	// 读取 Payload 长度
	payloadLenBuf := make([]byte, payloadLengthSize)
	if _, err := io.ReadFull(reader, payloadLenBuf); err != nil {
		return nil, err
	}
	payloadLen := binary.BigEndian.Uint32(payloadLenBuf)

	// 读取 Payload
	payloadBuf := make([]byte, payloadLen)
	if _, err := io.ReadFull(reader, payloadBuf); err != nil {
		return nil, err
	}
	msg.Payload = payloadBuf

	return msg, nil
}