// Copyright 2018 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 provides the implementation of the encoding and decoding of
// network protocol headers.
package header

import (
	"encoding/binary"

	"github.com/google/netstack/tcpip"
	"github.com/google/netstack/tcpip/buffer"
)

func calculateChecksum(buf []byte, initial uint32) uint16 {
	v := initial

	l := len(buf)
	if l&1 != 0 {
		l--
		v += uint32(buf[l]) << 8
	}

	for i := 0; i < l; i += 2 {
		v += (uint32(buf[i]) << 8) + uint32(buf[i+1])
	}

	return ChecksumCombine(uint16(v), uint16(v>>16))
}

// Checksum calculates the checksum (as defined in RFC 1071) of the bytes in the
// given byte array.
//
// The initial checksum must have been computed on an even number of bytes.
func Checksum(buf []byte, initial uint16) uint16 {
	return calculateChecksum(buf, uint32(initial))
}

// ChecksumVV calculates the checksum (as defined in RFC 1071) of the bytes in
// the given VectorizedView.
//
// The initial checksum must have been computed on an even number of bytes.
func ChecksumVV(vv buffer.VectorisedView, initial uint16) uint16 {
	var odd bool
	sum := initial
	for _, v := range vv.Views() {
		if len(v) == 0 {
			continue
		}
		s := uint32(sum)
		if odd {
			s += uint32(v[0])
			v = v[1:]
		}
		odd = len(v)&1 != 0
		sum = calculateChecksum(v, s)
	}
	return sum
}

// ChecksumCombine combines the two uint16 to form their checksum. This is done
// by adding them and the carry.
//
// Note that checksum a must have been computed on an even number of bytes.
func ChecksumCombine(a, b uint16) uint16 {
	v := uint32(a) + uint32(b)
	return uint16(v + v>>16)
}

// PseudoHeaderChecksum calculates the pseudo-header checksum for the given
// destination protocol and network address. Pseudo-headers are needed by
// transport layers when calculating their own checksum.
func PseudoHeaderChecksum(protocol tcpip.TransportProtocolNumber, srcAddr tcpip.Address, dstAddr tcpip.Address, totalLen uint16) uint16 {
	xsum := Checksum([]byte(srcAddr), 0)
	xsum = Checksum([]byte(dstAddr), xsum)

	// Add the length portion of the checksum to the pseudo-checksum.
	tmp := make([]byte, 2)
	binary.BigEndian.PutUint16(tmp, totalLen)
	xsum = Checksum(tmp, xsum)

	return Checksum([]byte{0, uint8(protocol)}, xsum)
}