// Copyright 2016 The Netstack Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package header

import (
	"encoding/binary"

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

const (
	versTCFL   = 0
	payloadLen = 4
	nextHdr    = 6
	hopLimit   = 7
	v6SrcAddr  = 8
	v6DstAddr  = 24
)

// IPv6Fields contains the fields of an IPv6 packet. It is used to describe the
// fields of a packet that needs to be encoded.
type IPv6Fields struct {
	// TrafficClass is the "traffic class" field of an IPv6 packet.
	TrafficClass uint8

	// FlowLabel is the "flow label" field of an IPv6 packet.
	FlowLabel uint32

	// PayloadLength is the "payload length" field of an IPv6 packet.
	PayloadLength uint16

	// NextHeader is the "next header" field of an IPv6 packet.
	NextHeader uint8

	// HopLimit is the "hop limit" field of an IPv6 packet.
	HopLimit uint8

	// SrcAddr is the "source ip address" of an IPv6 packet.
	SrcAddr tcpip.Address

	// DstAddr is the "destination ip address" of an IPv6 packet.
	DstAddr tcpip.Address
}

// IPv6 represents an ipv6 header stored in a byte array.
// Most of the methods of IPv6 access to the underlying slice without
// checking the boundaries and could panic because of 'index out of range'.
// Always call IsValid() to validate an instance of IPv6 before using other methods.
type IPv6 []byte

const (
	// IPv6MinimumSize is the minimum size of a valid IPv6 packet.
	IPv6MinimumSize = 40

	// IPv6AddressSize is the size, in bytes, of an IPv6 address.
	IPv6AddressSize = 16

	// IPv6ProtocolNumber is IPv6's network protocol number.
	IPv6ProtocolNumber tcpip.NetworkProtocolNumber = 0x86dd

	// IPv6Version is the version of the ipv6 procotol.
	IPv6Version = 6

	// IPv6DefaultHopLimit is the default hop limit (or TTL) value for
	// sent packets.
	IPv6DefaultHopLimit = 255

	// IPv6MinimumMTU is the minimum MTU required by IPv6, per RFC 2460,
	// section 5.
	IPv6MinimumMTU = 1280

	// IPv6Loopback is the loopback address of the IPv6 procotol.
	IPv6Loopback tcpip.Address = "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01"
)

// PayloadLength returns the value of the "payload length" field of the ipv6
// header.
func (b IPv6) PayloadLength() uint16 {
	return binary.BigEndian.Uint16(b[payloadLen:])
}

// HopLimit returns the value of the "hop limit" field of the ipv6 header.
func (b IPv6) HopLimit() uint8 {
	return b[hopLimit]
}

// NextHeader returns the value of the "next header" field of the ipv6 header.
func (b IPv6) NextHeader() uint8 {
	return b[nextHdr]
}

// TransportProtocol implements Network.TransportProtocol.
func (b IPv6) TransportProtocol() tcpip.TransportProtocolNumber {
	return tcpip.TransportProtocolNumber(b.NextHeader())
}

// Payload implements Network.Payload.
func (b IPv6) Payload() []byte {
	return b[IPv6MinimumSize:][:b.PayloadLength()]
}

// SourceAddress returns the "source address" field of the ipv6 header.
func (b IPv6) SourceAddress() tcpip.Address {
	return tcpip.Address(b[v6SrcAddr : v6SrcAddr+IPv6AddressSize])
}

// DestinationAddress returns the "destination address" field of the ipv6
// header.
func (b IPv6) DestinationAddress() tcpip.Address {
	return tcpip.Address(b[v6DstAddr : v6DstAddr+IPv6AddressSize])
}

// Checksum implements Network.Checksum. Given that IPv6 doesn't have a
// checksum, it just returns 0.
func (IPv6) Checksum() uint16 {
	return 0
}

// TOS returns the "traffic class" and "flow label" fields of the ipv6 header.
func (b IPv6) TOS() (uint8, uint32) {
	v := binary.BigEndian.Uint32(b[versTCFL:])
	return uint8(v >> 20), v & 0xfffff
}

// SetTOS sets the "traffic class" and "flow label" fields of the ipv6 header.
func (b IPv6) SetTOS(t uint8, l uint32) {
	vtf := (6 << 28) | (uint32(t) << 20) | (l & 0xfffff)
	binary.BigEndian.PutUint32(b[versTCFL:], vtf)
}

// SetPayloadLength sets the "payload length" field of the ipv6 header.
func (b IPv6) SetPayloadLength(payloadLength uint16) {
	binary.BigEndian.PutUint16(b[payloadLen:], payloadLength)
}

// SetSourceAddress sets the "source address" field of the ipv6 header.
func (b IPv6) SetSourceAddress(addr tcpip.Address) {
	copy(b[v6SrcAddr:v6SrcAddr+IPv6AddressSize], addr)
}

// SetDestinationAddress sets the "destination address" field of the ipv6
// header.
func (b IPv6) SetDestinationAddress(addr tcpip.Address) {
	copy(b[v6DstAddr:v6DstAddr+IPv6AddressSize], addr)
}

// SetChecksum implements Network.SetChecksum. Given that IPv6 doesn't have a
// checksum, it is empty.
func (IPv6) SetChecksum(uint16) {
}

// Encode encodes all the fields of the ipv6 header.
func (b IPv6) Encode(i *IPv6Fields) {
	b.SetTOS(i.TrafficClass, i.FlowLabel)
	b.SetPayloadLength(i.PayloadLength)
	b[nextHdr] = i.NextHeader
	b[hopLimit] = i.HopLimit
	copy(b[v6SrcAddr:v6SrcAddr+IPv6AddressSize], i.SrcAddr)
	copy(b[v6DstAddr:v6DstAddr+IPv6AddressSize], i.DstAddr)
}

// IsValid performs basic validation on the packet.
func (b IPv6) IsValid(pktSize int) bool {
	if len(b) < IPv6MinimumSize {
		return false
	}

	dlen := int(b.PayloadLength())
	if dlen > pktSize-IPv6MinimumSize {
		return false
	}

	return true
}

// IsV4MappedAddress determines if the provided address is an IPv4 mapped
// address by checking if its prefix is 0:0:0:0:0:ffff::/96.
func IsV4MappedAddress(addr tcpip.Address) bool {
	if len(addr) != IPv6AddressSize {
		return false
	}

	const prefix = "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xff\xff"
	for i := 0; i < len(prefix); i++ {
		if prefix[i] != addr[i] {
			return false
		}
	}

	return true
}

// IsV6MulticastAddress determines if the provided address is an IPv6
// multicast address (anything starting with FF).
func IsV6MulticastAddress(addr tcpip.Address) bool {
	if len(addr) != IPv6AddressSize {
		return false
	}
	return addr[0] == 0xff
}