package loadsim

import (
	"encoding/binary"
	"fmt"
	"time"
)

type FrameFactory struct {
	protocol Protocol
	base     []byte
}

func NewFrameFactory(protocol Protocol, template string) (*FrameFactory, error) {
	base, err := FrameTemplate(protocol, template)
	if err != nil {
		return nil, err
	}
	return &FrameFactory{protocol: protocol, base: base}, nil
}

func (f *FrameFactory) Frame(connectionIndex int, frameIndex int64) ([]byte, error) {
	switch f.protocol {
	case ProtocolJT808:
		return mutateJT808Frame(f.base, connectionIndex, frameIndex)
	case ProtocolGB32960:
		return mutateGB32960Frame(f.base, connectionIndex, frameIndex)
	default:
		out := append([]byte(nil), f.base...)
		return out, nil
	}
}

func mutateJT808Frame(base []byte, connectionIndex int, frameIndex int64) ([]byte, error) {
	if len(base) < 2 || base[0] != 0x7e || base[len(base)-1] != 0x7e {
		return nil, fmt.Errorf("jt808 template must include 0x7e delimiters")
	}
	payload, err := jt808Unescape(base[1 : len(base)-1])
	if err != nil {
		return nil, err
	}
	if len(payload) < 13 {
		return nil, fmt.Errorf("jt808 template too short: %d", len(payload))
	}
	phone := fmt.Sprintf("%012d", 139000000000+connectionIndex%100000000)
	copy(payload[4:10], encodeBCD(phone, 6))
	binary.BigEndian.PutUint16(payload[10:12], uint16((int(frameIndex)+connectionIndex)%65536))
	bodySize := int(binary.BigEndian.Uint16(payload[2:4]) & 0x03ff)
	bodyStart := 12
	if len(payload) >= bodyStart+bodySize+1 && bodySize >= 28 {
		writeJT808Time(payload[bodyStart+22:bodyStart+28], time.Now().Add(time.Duration(frameIndex)*time.Second))
	}
	payload[len(payload)-1] = jt808Checksum(payload[:len(payload)-1])
	escaped := jt808Escape(payload)
	out := make([]byte, 0, len(escaped)+2)
	out = append(out, 0x7e)
	out = append(out, escaped...)
	out = append(out, 0x7e)
	return out, nil
}

func mutateGB32960Frame(base []byte, connectionIndex int, frameIndex int64) ([]byte, error) {
	if len(base) < 25 || base[0] != '#' || base[1] != '#' {
		return nil, fmt.Errorf("gb32960 template too short or missing start symbols")
	}
	frame := append([]byte(nil), base...)
	vin := fmt.Sprintf("LNSIM%012d", connectionIndex%1000000000000)
	copy(frame[4:21], []byte(vin[:17]))
	bodyLen := int(binary.BigEndian.Uint16(frame[22:24]))
	bodyStart := 24
	if len(frame) >= bodyStart+bodyLen+1 && bodyLen >= 6 {
		writeGB32960Time(frame[bodyStart:bodyStart+6], time.Now().Add(time.Duration(frameIndex)*time.Second))
	}
	frame[len(frame)-1] = gb32960BCC(frame[2 : len(frame)-1])
	return frame, nil
}

func encodeBCD(value string, size int) []byte {
	out := make([]byte, size)
	if len(value)%2 == 1 {
		value = "0" + value
	}
	if len(value) > size*2 {
		value = value[len(value)-size*2:]
	}
	for len(value) < size*2 {
		value = "0" + value
	}
	for i := 0; i < size; i++ {
		out[i] = (value[i*2]-'0')<<4 | (value[i*2+1] - '0')
	}
	return out
}

func jt808Checksum(data []byte) byte {
	var out byte
	for _, value := range data {
		out ^= value
	}
	return out
}

func jt808Escape(payload []byte) []byte {
	out := make([]byte, 0, len(payload))
	for _, value := range payload {
		switch value {
		case 0x7e:
			out = append(out, 0x7d, 0x02)
		case 0x7d:
			out = append(out, 0x7d, 0x01)
		default:
			out = append(out, value)
		}
	}
	return out
}

func jt808Unescape(payload []byte) ([]byte, error) {
	out := make([]byte, 0, len(payload))
	for i := 0; i < len(payload); i++ {
		if payload[i] != 0x7d {
			out = append(out, payload[i])
			continue
		}
		if i+1 >= len(payload) {
			return nil, fmt.Errorf("invalid jt808 escape at end")
		}
		i++
		switch payload[i] {
		case 0x01:
			out = append(out, 0x7d)
		case 0x02:
			out = append(out, 0x7e)
		default:
			return nil, fmt.Errorf("invalid jt808 escape sequence 0x%02x", payload[i])
		}
	}
	return out, nil
}

func writeJT808Time(out []byte, value time.Time) {
	value = value.In(time.FixedZone("CST", 8*3600))
	year := value.Year() % 100
	parts := []int{year, int(value.Month()), value.Day(), value.Hour(), value.Minute(), value.Second()}
	for i, part := range parts {
		out[i] = byte(part/10)<<4 | byte(part%10)
	}
}

func writeGB32960Time(out []byte, value time.Time) {
	value = value.In(time.FixedZone("CST", 8*3600))
	out[0] = byte(value.Year() % 100)
	out[1] = byte(value.Month())
	out[2] = byte(value.Day())
	out[3] = byte(value.Hour())
	out[4] = byte(value.Minute())
	out[5] = byte(value.Second())
}

func gb32960BCC(data []byte) byte {
	var out byte
	for _, value := range data {
		out ^= value
	}
	return out
}