package zmlink

import (
	"bytes"
	"encoding/binary"
	"errors"
	"io"
	"math"
	"sync/atomic"
)

type Payload interface {
	Size() uint32
	Bytes() []byte
	Type() int8
	SetPkgIdx(i uint64)
	GetPkgIdx() uint64
	Ack(data []byte) Payload
}

/*
| 头 | 长度位 | 类型位 | 报文编号 | 载荷 |
|:----|:----|:----|:----|:----|
| ZM | uint32 | int8 | uint64 | string |
*/
const (
	payloadHeader    = "ZM"
	payloadHeaderLen = len(payloadHeader)
)

var payloadIdx uint64

// BytesPayload bytes payload
type BytesPayload struct {
	pkgType int8
	pkgIdx  uint64
	data    []byte // 荷载 (V)
}

func (b *BytesPayload) Ack(data []byte) Payload {
	return &BytesPayload{
		pkgType: -b.pkgType,
		pkgIdx:  b.pkgIdx,
		data:    data,
	}
}

func (b *BytesPayload) SetPkgIdx(i uint64) {
	b.pkgIdx = i
}

func (b *BytesPayload) GetPkgIdx() uint64 {
	if b.pkgIdx == 0 {
		b.pkgIdx = atomic.AddUint64(&payloadIdx, 1)
	}
	return b.pkgIdx
}

func (b *BytesPayload) Size() uint32 {
	return uint32(payloadHeaderLen + 4 + 1 + 8 + len(b.Bytes()))
}

func (b *BytesPayload) Bytes() []byte {
	return b.data
}

func (b *BytesPayload) Type() int8 {
	return b.pkgType
}

// NewBytesPayload 构建一个 []byte 荷载
func NewBytesPayload(t int8, data []byte) (Payload, error) {
	if len(data) > math.MaxUint32-(payloadHeaderLen+4+1+8) {
		return nil, errors.New("data length must be less than uint32")
	}
	return &BytesPayload{
		pkgType: t,
		data:    data,
	}, nil
}

func DecodePayLoad(data []byte) (Payload, error) {
	buffer := bytes.NewReader(data)
	header := make([]byte, payloadHeaderLen)
	_, _ = buffer.Read(header)
	size := make([]byte, 4)
	_, _ = buffer.Read(size)
	pkgType, err := buffer.ReadByte()
	if err != nil {
		return nil, err
	}
	pkgIdx := make([]byte, 8)
	_, _ = buffer.Read(pkgIdx)
	pData, _ := io.ReadAll(buffer)
	payload, err := NewBytesPayload(int8(pkgType), pData)
	if err != nil {
		return nil, err
	}
	payload.SetPkgIdx(binary.BigEndian.Uint64(pkgIdx))
	return payload, nil
}

func EncodePayLoad(payload Payload) ([]byte, error) {
	// header
	buffer := bytes.NewBuffer([]byte(payloadHeader))
	// pkgSize
	b := make([]byte, 4)
	binary.BigEndian.PutUint32(b, payload.Size())
	buffer.Write(b)
	// type
	buffer.WriteByte(byte(payload.Type()))
	// pkgIndex
	b = make([]byte, 8)
	binary.BigEndian.PutUint64(b, payload.GetPkgIdx())
	buffer.Write(b)
	// data
	buffer.Write(payload.Bytes())
	buffer.Bytes()
	return io.ReadAll(buffer)
}