// Copyright 2021 The gVisor Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

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 (
	GREHeaderSize = 4

	// 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))
}