package header

import (
	"encoding/binary"

	"gvisor.dev/gvisor/pkg/tcpip"
)

const (
	greProtocolType  = 2
)

// GRE represents a GRE header stored in a byte array.
type GRE []byte

const (
	// GREProtocolNumber is GRE's transport protocol number.
	GREProtocolNumber tcpip.TransportProtocolNumber = 47
)

func (b GRE) ProtocolType() tcpip.NetworkProtocolNumber {
	proto := binary.BigEndian.Uint16(b[greProtocolType:])
	return tcpip.NetworkProtocolNumber(proto)
}

// SetLength sets the "length" field of the udp header.
func (b GRE) SetProtocolType(protocol tcpip.NetworkProtocolNumber) {
	binary.BigEndian.PutUint16(b[greProtocolType:], uint16(protocol))
}