package inet

import (
	"encoding/binary"
	"fmt"

	"pcap-web-tools/internal/pcap"
)

const (
	etherTypeIPv4 = 0x0800
	etherTypeIPv6 = 0x86dd
	etherTypeVLAN = 0x8100
	etherTypeQinQ = 0x88a8
)

// FindIP returns the offset of the IP header within a link-layer frame of the
// given pcap link type, by parsing the link-layer header.
func FindIP(frame []byte, linkType uint32) (int, error) {
	switch linkType {
	case pcap.LinkTypeRaw:
		return 0, nil

	case pcap.LinkTypeEthernet:
		off := 12
		for {
			if off+2 > len(frame) {
				return 0, fmt.Errorf("Ethernet header truncated")
			}
			et := binary.BigEndian.Uint16(frame[off:])
			if et == etherTypeVLAN || et == etherTypeQinQ {
				off += 4
				continue
			}
			return etherIP(et, off+2)
		}

	case pcap.LinkTypeLinuxSLL:
		if len(frame) < 16 {
			return 0, fmt.Errorf("Linux cooked (SLL) header truncated")
		}
		return etherIP(binary.BigEndian.Uint16(frame[14:]), 16)

	case pcap.LinkTypeLinuxSLL2:
		if len(frame) < 20 {
			return 0, fmt.Errorf("Linux cooked v2 (SLL2) header truncated")
		}
		return etherIP(binary.BigEndian.Uint16(frame[0:]), 20)
	}
	return 0, fmt.Errorf("cannot locate the IP header for link type %s", pcap.LinkTypeName(linkType))
}

func etherIP(etherType uint16, off int) (int, error) {
	if etherType != etherTypeIPv4 && etherType != etherTypeIPv6 {
		return 0, fmt.Errorf("link-layer payload is not IP (EtherType 0x%04x)", etherType)
	}
	return off, nil
}