package transport

import "encoding/binary"

const kcpPacketHeaderSize = 24

func parseKCPPacketMetadata(packet []byte) (*uint32, []KCPPacketDebugSegment) {
	conv := parseKCPConversationID(packet)
	segments, ok := parseKCPPacketSegments(packet)
	if !ok {
		return conv, nil
	}
	return conv, segments
}

func parseKCPConversationID(packet []byte) *uint32 {
	if len(packet) < 4 {
		return nil
	}
	conv := binary.LittleEndian.Uint32(packet[:4])
	return &conv
}

func parseKCPPacketSegments(packet []byte) ([]KCPPacketDebugSegment, bool) {
	if len(packet) == 0 {
		return nil, false
	}

	data := packet
	segments := make([]KCPPacketDebugSegment, 0, 1)
	for len(data) > 0 {
		if len(data) < kcpPacketHeaderSize {
			return nil, false
		}

		segmentLen := binary.LittleEndian.Uint32(data[20:])
		totalLen := kcpPacketHeaderSize + int(segmentLen)
		if len(data) < totalLen {
			return nil, false
		}

		segments = append(segments, KCPPacketDebugSegment{
			Cmd: data[4],
			Frg: data[5],
			Wnd: binary.LittleEndian.Uint16(data[6:8]),
			SN:  binary.LittleEndian.Uint32(data[12:16]),
			UNA: binary.LittleEndian.Uint32(data[16:20]),
			Len: segmentLen,
		})

		data = data[totalLen:]
	}

	return segments, true
}