package fingerprint

import (
	"net"

	"github.com/google/gopacket"
	"github.com/google/gopacket/layers"
)

// BuildSYNPacket creates a minimal TCP SYN packet for the given source and destination ports.
// The packet is not sent; it is used only to demonstrate integration with gopacket.
func BuildSYNPacket(srcIP string, dstIP string, srcPort, dstPort uint16) ([]byte, error) {
	ip := &layers.IPv4{
		SrcIP:    net.ParseIP(srcIP).To4(),
		DstIP:    net.ParseIP(dstIP).To4(),
		Version:  4,
		Protocol: layers.IPProtocolTCP,
	}
	tcp := &layers.TCP{
		SrcPort: layers.TCPPort(srcPort),
		DstPort: layers.TCPPort(dstPort),
		SYN:     true,
		Seq:     1105024978,
		Window:  14600,
	}
	if err := tcp.SetNetworkLayerForChecksum(ip); err != nil {
		return nil, err
	}
	buf := gopacket.NewSerializeBuffer()
	opts := gopacket.SerializeOptions{
		FixLengths:       true,
		ComputeChecksums: true,
	}
	if err := gopacket.SerializeLayers(buf, opts, ip, tcp); err != nil {
		return nil, err
	}
	return buf.Bytes(), nil
}