[gostls13.git] / src / net / netip / netip.go

// Copyright 2020 The Go 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 netip defines an IP address type that's a small value type.
// Building on that [Addr] type, the package also defines [AddrPort] (an
// IP address and a port) and [Prefix] (an IP address and a bit length
// prefix).
//
// Compared to the [net.IP] type, [Addr] type takes less memory, is immutable,
// and is comparable (supports == and being a map key).
package netip

import (
        "errors"
        "math"
        "strconv"

        "internal/bytealg"
        "internal/intern"
        "internal/itoa"
)

// Sizes: (64-bit)
//   net.IP:     24 byte slice header + {4, 16} = 28 to 40 bytes
//   net.IPAddr: 40 byte slice header + {4, 16} = 44 to 56 bytes + zone length
//   netip.Addr: 24 bytes (zone is per-name singleton, shared across all users)

// Addr represents an IPv4 or IPv6 address (with or without a scoped
// addressing zone), similar to [net.IP] or [net.IPAddr].
//
// Unlike [net.IP] or [net.IPAddr], Addr is a comparable value
// type (it supports == and can be a map key) and is immutable.
//
// The zero Addr is not a valid IP address.
// Addr{} is distinct from both 0.0.0.0 and ::.
type Addr struct {
        // addr is the hi and lo bits of an IPv6 address. If z==z4,
        // hi and lo contain the IPv4-mapped IPv6 address.
        //
        // hi and lo are constructed by interpreting a 16-byte IPv6
        // address as a big-endian 128-bit number. The most significant
        // bits of that number go into hi, the rest into lo.
        //
        // For example, 0011:2233:4455:6677:8899:aabb:ccdd:eeff is stored as:
        //  addr.hi = 0x0011223344556677
        //  addr.lo = 0x8899aabbccddeeff
        //
        // We store IPs like this, rather than as [16]byte, because it
        // turns most operations on IPs into arithmetic and bit-twiddling
        // operations on 64-bit registers, which is much faster than
        // bytewise processing.
        addr uint128

        // z is a combination of the address family and the IPv6 zone.
        //
        // nil means invalid IP address (for a zero Addr).
        // z4 means an IPv4 address.
        // z6noz means an IPv6 address without a zone.
        //
        // Otherwise it's the interned zone name string.
        z *intern.Value
}

// z0, z4, and z6noz are sentinel Addr.z values.
// See the Addr type's field docs.
var (
        z0    = (*intern.Value)(nil)
        z4    = new(intern.Value)
        z6noz = new(intern.Value)
)

// IPv6LinkLocalAllNodes returns the IPv6 link-local all nodes multicast
// address ff02::1.
func IPv6LinkLocalAllNodes() Addr { return AddrFrom16([16]byte{0: 0xff, 1: 0x02, 15: 0x01}) }

// IPv6LinkLocalAllRouters returns the IPv6 link-local all routers multicast
// address ff02::2.
func IPv6LinkLocalAllRouters() Addr { return AddrFrom16([16]byte{0: 0xff, 1: 0x02, 15: 0x02}) }

// IPv6Loopback returns the IPv6 loopback address ::1.
func IPv6Loopback() Addr { return AddrFrom16([16]byte{15: 0x01}) }

// IPv6Unspecified returns the IPv6 unspecified address "::".
func IPv6Unspecified() Addr { return Addr{z: z6noz} }

// IPv4Unspecified returns the IPv4 unspecified address "0.0.0.0".
func IPv4Unspecified() Addr { return AddrFrom4([4]byte{}) }

// AddrFrom4 returns the address of the IPv4 address given by the bytes in addr.
func AddrFrom4(addr [4]byte) Addr {
        return Addr{
                addr: uint128{0, 0xffff00000000 | uint64(addr[0])<<24 | uint64(addr[1])<<16 | uint64(addr[2])<<8 | uint64(addr[3])},
                z:    z4,
        }
}

// AddrFrom16 returns the IPv6 address given by the bytes in addr.
// An IPv4-mapped IPv6 address is left as an IPv6 address.
// (Use Unmap to convert them if needed.)
func AddrFrom16(addr [16]byte) Addr {
        return Addr{
                addr: uint128{
                        beUint64(addr[:8]),
                        beUint64(addr[8:]),
                },
                z: z6noz,
        }
}

// ParseAddr parses s as an IP address, returning the result. The string
// s can be in dotted decimal ("192.0.2.1"), IPv6 ("2001:db8::68"),
// or IPv6 with a scoped addressing zone ("fe80::1cc0:3e8c:119f:c2e1%ens18").
func ParseAddr(s string) (Addr, error) {
        for i := 0; i < len(s); i++ {
                switch s[i] {
                case '.':
                        return parseIPv4(s)
                case ':':
                        return parseIPv6(s)
                case '%':
                        // Assume that this was trying to be an IPv6 address with
                        // a zone specifier, but the address is missing.
                        return Addr{}, parseAddrError{in: s, msg: "missing IPv6 address"}
                }
        }
        return Addr{}, parseAddrError{in: s, msg: "unable to parse IP"}
}

// MustParseAddr calls ParseAddr(s) and panics on error.
// It is intended for use in tests with hard-coded strings.
func MustParseAddr(s string) Addr {
        ip, err := ParseAddr(s)
        if err != nil {
                panic(err)
        }
        return ip
}

type parseAddrError struct {
        in  string // the string given to ParseAddr
        msg string // an explanation of the parse failure
        at  string // optionally, the unparsed portion of in at which the error occurred.
}

func (err parseAddrError) Error() string {
        q := strconv.Quote
        if err.at != "" {
                return "ParseAddr(" + q(err.in) + "): " + err.msg + " (at " + q(err.at) + ")"
        }
        return "ParseAddr(" + q(err.in) + "): " + err.msg
}

// parseIPv4 parses s as an IPv4 address (in form "192.168.0.1").
func parseIPv4(s string) (ip Addr, err error) {
        var fields [4]uint8
        var val, pos int
        var digLen int // number of digits in current octet
        for i := 0; i < len(s); i++ {
                if s[i] >= '0' && s[i] <= '9' {
                        if digLen == 1 && val == 0 {
                                return Addr{}, parseAddrError{in: s, msg: "IPv4 field has octet with leading zero"}
                        }
                        val = val*10 + int(s[i]) - '0'
                        digLen++
                        if val > 255 {
                                return Addr{}, parseAddrError{in: s, msg: "IPv4 field has value >255"}
                        }
                } else if s[i] == '.' {
                        // .1.2.3
                        // 1.2.3.
                        // 1..2.3
                        if i == 0 || i == len(s)-1 || s[i-1] == '.' {
                                return Addr{}, parseAddrError{in: s, msg: "IPv4 field must have at least one digit", at: s[i:]}
                        }
                        // 1.2.3.4.5
                        if pos == 3 {
                                return Addr{}, parseAddrError{in: s, msg: "IPv4 address too long"}
                        }
                        fields[pos] = uint8(val)
                        pos++
                        val = 0
                        digLen = 0
                } else {
                        return Addr{}, parseAddrError{in: s, msg: "unexpected character", at: s[i:]}
                }
        }
        if pos < 3 {
                return Addr{}, parseAddrError{in: s, msg: "IPv4 address too short"}
        }
        fields[3] = uint8(val)
        return AddrFrom4(fields), nil
}

// parseIPv6 parses s as an IPv6 address (in form "2001:db8::68").
func parseIPv6(in string) (Addr, error) {
        s := in

        // Split off the zone right from the start. Yes it's a second scan
        // of the string, but trying to handle it inline makes a bunch of
        // other inner loop conditionals more expensive, and it ends up
        // being slower.
        zone := ""
        i := bytealg.IndexByteString(s, '%')
        if i != -1 {
                s, zone = s[:i], s[i+1:]
                if zone == "" {
                        // Not allowed to have an empty zone if explicitly specified.
                        return Addr{}, parseAddrError{in: in, msg: "zone must be a non-empty string"}
                }
        }

        var ip [16]byte
        ellipsis := -1 // position of ellipsis in ip

        // Might have leading ellipsis
        if len(s) >= 2 && s[0] == ':' && s[1] == ':' {
                ellipsis = 0
                s = s[2:]
                // Might be only ellipsis
                if len(s) == 0 {
                        return IPv6Unspecified().WithZone(zone), nil
                }
        }

        // Loop, parsing hex numbers followed by colon.
        i = 0
        for i < 16 {
                // Hex number. Similar to parseIPv4, inlining the hex number
                // parsing yields a significant performance increase.
                off := 0
                acc := uint32(0)
                for ; off < len(s); off++ {
                        c := s[off]
                        if c >= '0' && c <= '9' {
                                acc = (acc << 4) + uint32(c-'0')
                        } else if c >= 'a' && c <= 'f' {
                                acc = (acc << 4) + uint32(c-'a'+10)
                        } else if c >= 'A' && c <= 'F' {
                                acc = (acc << 4) + uint32(c-'A'+10)
                        } else {
                                break
                        }
                        if acc > math.MaxUint16 {
                                // Overflow, fail.
                                return Addr{}, parseAddrError{in: in, msg: "IPv6 field has value >=2^16", at: s}
                        }
                }
                if off == 0 {
                        // No digits found, fail.
                        return Addr{}, parseAddrError{in: in, msg: "each colon-separated field must have at least one digit", at: s}
                }

                // If followed by dot, might be in trailing IPv4.
                if off < len(s) && s[off] == '.' {
                        if ellipsis < 0 && i != 12 {
                                // Not the right place.
                                return Addr{}, parseAddrError{in: in, msg: "embedded IPv4 address must replace the final 2 fields of the address", at: s}
                        }
                        if i+4 > 16 {
                                // Not enough room.
                                return Addr{}, parseAddrError{in: in, msg: "too many hex fields to fit an embedded IPv4 at the end of the address", at: s}
                        }
                        // TODO: could make this a bit faster by having a helper
                        // that parses to a [4]byte, and have both parseIPv4 and
                        // parseIPv6 use it.
                        ip4, err := parseIPv4(s)
                        if err != nil {
                                return Addr{}, parseAddrError{in: in, msg: err.Error(), at: s}
                        }
                        ip[i] = ip4.v4(0)
                        ip[i+1] = ip4.v4(1)
                        ip[i+2] = ip4.v4(2)
                        ip[i+3] = ip4.v4(3)
                        s = ""
                        i += 4
                        break
                }

                // Save this 16-bit chunk.
                ip[i] = byte(acc >> 8)
                ip[i+1] = byte(acc)
                i += 2

                // Stop at end of string.
                s = s[off:]
                if len(s) == 0 {
                        break
                }

                // Otherwise must be followed by colon and more.
                if s[0] != ':' {
                        return Addr{}, parseAddrError{in: in, msg: "unexpected character, want colon", at: s}
                } else if len(s) == 1 {
                        return Addr{}, parseAddrError{in: in, msg: "colon must be followed by more characters", at: s}
                }
                s = s[1:]

                // Look for ellipsis.
                if s[0] == ':' {
                        if ellipsis >= 0 { // already have one
                                return Addr{}, parseAddrError{in: in, msg: "multiple :: in address", at: s}
                        }
                        ellipsis = i
                        s = s[1:]
                        if len(s) == 0 { // can be at end
                                break
                        }
                }
        }

        // Must have used entire string.
        if len(s) != 0 {
                return Addr{}, parseAddrError{in: in, msg: "trailing garbage after address", at: s}
        }

        // If didn't parse enough, expand ellipsis.
        if i < 16 {
                if ellipsis < 0 {
                        return Addr{}, parseAddrError{in: in, msg: "address string too short"}
                }
                n := 16 - i
                for j := i - 1; j >= ellipsis; j-- {
                        ip[j+n] = ip[j]
                }
                for j := ellipsis + n - 1; j >= ellipsis; j-- {
                        ip[j] = 0
                }
        } else if ellipsis >= 0 {
                // Ellipsis must represent at least one 0 group.
                return Addr{}, parseAddrError{in: in, msg: "the :: must expand to at least one field of zeros"}
        }
        return AddrFrom16(ip).WithZone(zone), nil
}

// AddrFromSlice parses the 4- or 16-byte byte slice as an IPv4 or IPv6 address.
// Note that a net.IP can be passed directly as the []byte argument.
// If slice's length is not 4 or 16, AddrFromSlice returns Addr{}, false.
func AddrFromSlice(slice []byte) (ip Addr, ok bool) {
        switch len(slice) {
        case 4:
                return AddrFrom4([4]byte(slice)), true
        case 16:
                return AddrFrom16([16]byte(slice)), true
        }
        return Addr{}, false
}

// v4 returns the i'th byte of ip. If ip is not an IPv4, v4 returns
// unspecified garbage.
func (ip Addr) v4(i uint8) uint8 {
        return uint8(ip.addr.lo >> ((3 - i) * 8))
}

// v6 returns the i'th byte of ip. If ip is an IPv4 address, this
// accesses the IPv4-mapped IPv6 address form of the IP.
func (ip Addr) v6(i uint8) uint8 {
        return uint8(*(ip.addr.halves()[(i/8)%2]) >> ((7 - i%8) * 8))
}

// v6u16 returns the i'th 16-bit word of ip. If ip is an IPv4 address,
// this accesses the IPv4-mapped IPv6 address form of the IP.
func (ip Addr) v6u16(i uint8) uint16 {
        return uint16(*(ip.addr.halves()[(i/4)%2]) >> ((3 - i%4) * 16))
}

// isZero reports whether ip is the zero value of the IP type.
// The zero value is not a valid IP address of any type.
//
// Note that "0.0.0.0" and "::" are not the zero value. Use IsUnspecified to
// check for these values instead.
func (ip Addr) isZero() bool {
        // Faster than comparing ip == Addr{}, but effectively equivalent,
        // as there's no way to make an IP with a nil z from this package.
        return ip.z == z0
}

// IsValid reports whether the Addr is an initialized address (not the zero Addr).
//
// Note that "0.0.0.0" and "::" are both valid values.
func (ip Addr) IsValid() bool { return ip.z != z0 }

// BitLen returns the number of bits in the IP address:
// 128 for IPv6, 32 for IPv4, and 0 for the zero Addr.
//
// Note that IPv4-mapped IPv6 addresses are considered IPv6 addresses
// and therefore have bit length 128.
func (ip Addr) BitLen() int {
        switch ip.z {
        case z0:
                return 0
        case z4:
                return 32
        }
        return 128
}

// Zone returns ip's IPv6 scoped addressing zone, if any.
func (ip Addr) Zone() string {
        if ip.z == nil {
                return ""
        }
        zone, _ := ip.z.Get().(string)
        return zone
}

// Compare returns an integer comparing two IPs.
// The result will be 0 if ip == ip2, -1 if ip < ip2, and +1 if ip > ip2.
// The definition of "less than" is the same as the Less method.
func (ip Addr) Compare(ip2 Addr) int {
        f1, f2 := ip.BitLen(), ip2.BitLen()
        if f1 < f2 {
                return -1
        }
        if f1 > f2 {
                return 1
        }
        hi1, hi2 := ip.addr.hi, ip2.addr.hi
        if hi1 < hi2 {
                return -1
        }
        if hi1 > hi2 {
                return 1
        }
        lo1, lo2 := ip.addr.lo, ip2.addr.lo
        if lo1 < lo2 {
                return -1
        }
        if lo1 > lo2 {
                return 1
        }
        if ip.Is6() {
                za, zb := ip.Zone(), ip2.Zone()
                if za < zb {
                        return -1
                }
                if za > zb {
                        return 1
                }
        }
        return 0
}

// Less reports whether ip sorts before ip2.
// IP addresses sort first by length, then their address.
// IPv6 addresses with zones sort just after the same address without a zone.
func (ip Addr) Less(ip2 Addr) bool { return ip.Compare(ip2) == -1 }

// Is4 reports whether ip is an IPv4 address.
//
// It returns false for IPv4-mapped IPv6 addresses. See Addr.Unmap.
func (ip Addr) Is4() bool {
        return ip.z == z4
}

// Is4In6 reports whether ip is an IPv4-mapped IPv6 address.
func (ip Addr) Is4In6() bool {
        return ip.Is6() && ip.addr.hi == 0 && ip.addr.lo>>32 == 0xffff
}

// Is6 reports whether ip is an IPv6 address, including IPv4-mapped
// IPv6 addresses.
func (ip Addr) Is6() bool {
        return ip.z != z0 && ip.z != z4
}

// Unmap returns ip with any IPv4-mapped IPv6 address prefix removed.
//
// That is, if ip is an IPv6 address wrapping an IPv4 address, it
// returns the wrapped IPv4 address. Otherwise it returns ip unmodified.
func (ip Addr) Unmap() Addr {
        if ip.Is4In6() {
                ip.z = z4
        }
        return ip
}

// WithZone returns an IP that's the same as ip but with the provided
// zone. If zone is empty, the zone is removed. If ip is an IPv4
// address, WithZone is a no-op and returns ip unchanged.
func (ip Addr) WithZone(zone string) Addr {
        if !ip.Is6() {
                return ip
        }
        if zone == "" {
                ip.z = z6noz
                return ip
        }
        ip.z = intern.GetByString(zone)
        return ip
}

// withoutZone unconditionally strips the zone from ip.
// It's similar to WithZone, but small enough to be inlinable.
func (ip Addr) withoutZone() Addr {
        if !ip.Is6() {
                return ip
        }
        ip.z = z6noz
        return ip
}

// hasZone reports whether ip has an IPv6 zone.
func (ip Addr) hasZone() bool {
        return ip.z != z0 && ip.z != z4 && ip.z != z6noz
}

// IsLinkLocalUnicast reports whether ip is a link-local unicast address.
func (ip Addr) IsLinkLocalUnicast() bool {
        // Dynamic Configuration of IPv4 Link-Local Addresses
        // https://datatracker.ietf.org/doc/html/rfc3927#section-2.1
        if ip.Is4() {
                return ip.v4(0) == 169 && ip.v4(1) == 254
        }
        // IP Version 6 Addressing Architecture (2.4 Address Type Identification)
        // https://datatracker.ietf.org/doc/html/rfc4291#section-2.4
        if ip.Is6() {
                return ip.v6u16(0)&0xffc0 == 0xfe80
        }
        return false // zero value
}

// IsLoopback reports whether ip is a loopback address.
func (ip Addr) IsLoopback() bool {
        // Requirements for Internet Hosts -- Communication Layers (3.2.1.3 Addressing)
        // https://datatracker.ietf.org/doc/html/rfc1122#section-3.2.1.3
        if ip.Is4() {
                return ip.v4(0) == 127
        }
        // IP Version 6 Addressing Architecture (2.4 Address Type Identification)
        // https://datatracker.ietf.org/doc/html/rfc4291#section-2.4
        if ip.Is6() {
                return ip.addr.hi == 0 && ip.addr.lo == 1
        }
        return false // zero value
}

// IsMulticast reports whether ip is a multicast address.
func (ip Addr) IsMulticast() bool {
        // Host Extensions for IP Multicasting (4. HOST GROUP ADDRESSES)
        // https://datatracker.ietf.org/doc/html/rfc1112#section-4
        if ip.Is4() {
                return ip.v4(0)&0xf0 == 0xe0
        }
        // IP Version 6 Addressing Architecture (2.4 Address Type Identification)
        // https://datatracker.ietf.org/doc/html/rfc4291#section-2.4
        if ip.Is6() {
                return ip.addr.hi>>(64-8) == 0xff // ip.v6(0) == 0xff
        }
        return false // zero value
}

// IsInterfaceLocalMulticast reports whether ip is an IPv6 interface-local
// multicast address.
func (ip Addr) IsInterfaceLocalMulticast() bool {
        // IPv6 Addressing Architecture (2.7.1. Pre-Defined Multicast Addresses)
        // https://datatracker.ietf.org/doc/html/rfc4291#section-2.7.1
        if ip.Is6() {
                return ip.v6u16(0)&0xff0f == 0xff01
        }
        return false // zero value
}

// IsLinkLocalMulticast reports whether ip is a link-local multicast address.
func (ip Addr) IsLinkLocalMulticast() bool {
        // IPv4 Multicast Guidelines (4. Local Network Control Block (224.0.0/24))
        // https://datatracker.ietf.org/doc/html/rfc5771#section-4
        if ip.Is4() {
                return ip.v4(0) == 224 && ip.v4(1) == 0 && ip.v4(2) == 0
        }
        // IPv6 Addressing Architecture (2.7.1. Pre-Defined Multicast Addresses)
        // https://datatracker.ietf.org/doc/html/rfc4291#section-2.7.1
        if ip.Is6() {
                return ip.v6u16(0)&0xff0f == 0xff02
        }
        return false // zero value
}

// IsGlobalUnicast reports whether ip is a global unicast address.
//
// It returns true for IPv6 addresses which fall outside of the current
// IANA-allocated 2000::/3 global unicast space, with the exception of the
// link-local address space. It also returns true even if ip is in the IPv4
// private address space or IPv6 unique local address space.
// It returns false for the zero Addr.
//
// For reference, see RFC 1122, RFC 4291, and RFC 4632.
func (ip Addr) IsGlobalUnicast() bool {
        if ip.z == z0 {
                // Invalid or zero-value.
                return false
        }

        // Match package net's IsGlobalUnicast logic. Notably private IPv4 addresses
        // and ULA IPv6 addresses are still considered "global unicast".
        if ip.Is4() && (ip == IPv4Unspecified() || ip == AddrFrom4([4]byte{255, 255, 255, 255})) {
                return false
        }

        return ip != IPv6Unspecified() &&
                !ip.IsLoopback() &&
                !ip.IsMulticast() &&
                !ip.IsLinkLocalUnicast()
}

// IsPrivate reports whether ip is a private address, according to RFC 1918
// (IPv4 addresses) and RFC 4193 (IPv6 addresses). That is, it reports whether
// ip is in 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, or fc00::/7. This is the
// same as net.IP.IsPrivate.
func (ip Addr) IsPrivate() bool {
        // Match the stdlib's IsPrivate logic.
        if ip.Is4() {
                // RFC 1918 allocates 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 as
                // private IPv4 address subnets.
                return ip.v4(0) == 10 ||
                        (ip.v4(0) == 172 && ip.v4(1)&0xf0 == 16) ||
                        (ip.v4(0) == 192 && ip.v4(1) == 168)
        }

        if ip.Is6() {
                // RFC 4193 allocates fc00::/7 as the unique local unicast IPv6 address
                // subnet.
                return ip.v6(0)&0xfe == 0xfc
        }

        return false // zero value
}

// IsUnspecified reports whether ip is an unspecified address, either the IPv4
// address "0.0.0.0" or the IPv6 address "::".
//
// Note that the zero Addr is not an unspecified address.
func (ip Addr) IsUnspecified() bool {
        return ip == IPv4Unspecified() || ip == IPv6Unspecified()
}

// Prefix keeps only the top b bits of IP, producing a Prefix
// of the specified length.
// If ip is a zero Addr, Prefix always returns a zero Prefix and a nil error.
// Otherwise, if bits is less than zero or greater than ip.BitLen(),
// Prefix returns an error.
func (ip Addr) Prefix(b int) (Prefix, error) {
        if b < 0 {
                return Prefix{}, errors.New("negative Prefix bits")
        }
        effectiveBits := b
        switch ip.z {
        case z0:
                return Prefix{}, nil
        case z4:
                if b > 32 {
                        return Prefix{}, errors.New("prefix length " + itoa.Itoa(b) + " too large for IPv4")
                }
                effectiveBits += 96
        default:
                if b > 128 {
                        return Prefix{}, errors.New("prefix length " + itoa.Itoa(b) + " too large for IPv6")
                }
        }
        ip.addr = ip.addr.and(mask6(effectiveBits))
        return PrefixFrom(ip, b), nil
}

// As16 returns the IP address in its 16-byte representation.
// IPv4 addresses are returned as IPv4-mapped IPv6 addresses.
// IPv6 addresses with zones are returned without their zone (use the
// Zone method to get it).
// The ip zero value returns all zeroes.
func (ip Addr) As16() (a16 [16]byte) {
        bePutUint64(a16[:8], ip.addr.hi)
        bePutUint64(a16[8:], ip.addr.lo)
        return a16
}

// As4 returns an IPv4 or IPv4-in-IPv6 address in its 4-byte representation.
// If ip is the zero Addr or an IPv6 address, As4 panics.
// Note that 0.0.0.0 is not the zero Addr.
func (ip Addr) As4() (a4 [4]byte) {
        if ip.z == z4 || ip.Is4In6() {
                bePutUint32(a4[:], uint32(ip.addr.lo))
                return a4
        }
        if ip.z == z0 {
                panic("As4 called on IP zero value")
        }
        panic("As4 called on IPv6 address")
}

// AsSlice returns an IPv4 or IPv6 address in its respective 4-byte or 16-byte representation.
func (ip Addr) AsSlice() []byte {
        switch ip.z {
        case z0:
                return nil
        case z4:
                var ret [4]byte
                bePutUint32(ret[:], uint32(ip.addr.lo))
                return ret[:]
        default:
                var ret [16]byte
                bePutUint64(ret[:8], ip.addr.hi)
                bePutUint64(ret[8:], ip.addr.lo)
                return ret[:]
        }
}

// Next returns the address following ip.
// If there is none, it returns the zero Addr.
func (ip Addr) Next() Addr {
        ip.addr = ip.addr.addOne()
        if ip.Is4() {
                if uint32(ip.addr.lo) == 0 {
                        // Overflowed.
                        return Addr{}
                }
        } else {
                if ip.addr.isZero() {
                        // Overflowed
                        return Addr{}
                }
        }
        return ip
}

// Prev returns the IP before ip.
// If there is none, it returns the IP zero value.
func (ip Addr) Prev() Addr {
        if ip.Is4() {
                if uint32(ip.addr.lo) == 0 {
                        return Addr{}
                }
        } else if ip.addr.isZero() {
                return Addr{}
        }
        ip.addr = ip.addr.subOne()
        return ip
}

// String returns the string form of the IP address ip.
// It returns one of 5 forms:
//
//   - "invalid IP", if ip is the zero Addr
//   - IPv4 dotted decimal ("192.0.2.1")
//   - IPv6 ("2001:db8::1")
//   - "::ffff:1.2.3.4" (if Is4In6)
//   - IPv6 with zone ("fe80:db8::1%eth0")
//
// Note that unlike package net's IP.String method,
// IPv4-mapped IPv6 addresses format with a "::ffff:"
// prefix before the dotted quad.
func (ip Addr) String() string {
        switch ip.z {
        case z0:
                return "invalid IP"
        case z4:
                return ip.string4()
        default:
                if ip.Is4In6() {
                        if z := ip.Zone(); z != "" {
                                return "::ffff:" + ip.Unmap().string4() + "%" + z
                        } else {
                                return "::ffff:" + ip.Unmap().string4()
                        }
                }
                return ip.string6()
        }
}

// AppendTo appends a text encoding of ip,
// as generated by MarshalText,
// to b and returns the extended buffer.
func (ip Addr) AppendTo(b []byte) []byte {
        switch ip.z {
        case z0:
                return b
        case z4:
                return ip.appendTo4(b)
        default:
                if ip.Is4In6() {
                        b = append(b, "::ffff:"...)
                        b = ip.Unmap().appendTo4(b)
                        if z := ip.Zone(); z != "" {
                                b = append(b, '%')
                                b = append(b, z...)
                        }
                        return b
                }
                return ip.appendTo6(b)
        }
}

// digits is a string of the hex digits from 0 to f. It's used in
// appendDecimal and appendHex to format IP addresses.
const digits = "0123456789abcdef"

// appendDecimal appends the decimal string representation of x to b.
func appendDecimal(b []byte, x uint8) []byte {
        // Using this function rather than strconv.AppendUint makes IPv4
        // string building 2x faster.

        if x >= 100 {
                b = append(b, digits[x/100])
        }
        if x >= 10 {
                b = append(b, digits[x/10%10])
        }
        return append(b, digits[x%10])
}

// appendHex appends the hex string representation of x to b.
func appendHex(b []byte, x uint16) []byte {
        // Using this function rather than strconv.AppendUint makes IPv6
        // string building 2x faster.

        if x >= 0x1000 {
                b = append(b, digits[x>>12])
        }
        if x >= 0x100 {
                b = append(b, digits[x>>8&0xf])
        }
        if x >= 0x10 {
                b = append(b, digits[x>>4&0xf])
        }
        return append(b, digits[x&0xf])
}

// appendHexPad appends the fully padded hex string representation of x to b.
func appendHexPad(b []byte, x uint16) []byte {
        return append(b, digits[x>>12], digits[x>>8&0xf], digits[x>>4&0xf], digits[x&0xf])
}

func (ip Addr) string4() string {
        const max = len("255.255.255.255")
        ret := make([]byte, 0, max)
        ret = ip.appendTo4(ret)
        return string(ret)
}

func (ip Addr) appendTo4(ret []byte) []byte {
        ret = appendDecimal(ret, ip.v4(0))
        ret = append(ret, '.')
        ret = appendDecimal(ret, ip.v4(1))
        ret = append(ret, '.')
        ret = appendDecimal(ret, ip.v4(2))
        ret = append(ret, '.')
        ret = appendDecimal(ret, ip.v4(3))
        return ret
}

// string6 formats ip in IPv6 textual representation. It follows the
// guidelines in section 4 of RFC 5952
// (https://tools.ietf.org/html/rfc5952#section-4): no unnecessary
// zeros, use :: to elide the longest run of zeros, and don't use ::
// to compact a single zero field.
func (ip Addr) string6() string {
        // Use a zone with a "plausibly long" name, so that most zone-ful
        // IP addresses won't require additional allocation.
        //
        // The compiler does a cool optimization here, where ret ends up
        // stack-allocated and so the only allocation this function does
        // is to construct the returned string. As such, it's okay to be a
        // bit greedy here, size-wise.
        const max = len("ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff%enp5s0")
        ret := make([]byte, 0, max)
        ret = ip.appendTo6(ret)
        return string(ret)
}

func (ip Addr) appendTo6(ret []byte) []byte {
        zeroStart, zeroEnd := uint8(255), uint8(255)
        for i := uint8(0); i < 8; i++ {
                j := i
                for j < 8 && ip.v6u16(j) == 0 {
                        j++
                }
                if l := j - i; l >= 2 && l > zeroEnd-zeroStart {
                        zeroStart, zeroEnd = i, j
                }
        }

        for i := uint8(0); i < 8; i++ {
                if i == zeroStart {
                        ret = append(ret, ':', ':')
                        i = zeroEnd
                        if i >= 8 {
                                break
                        }
                } else if i > 0 {
                        ret = append(ret, ':')
                }

                ret = appendHex(ret, ip.v6u16(i))
        }

        if ip.z != z6noz {
                ret = append(ret, '%')
                ret = append(ret, ip.Zone()...)
        }
        return ret
}

// StringExpanded is like String but IPv6 addresses are expanded with leading
// zeroes and no "::" compression. For example, "2001:db8::1" becomes
// "2001:0db8:0000:0000:0000:0000:0000:0001".
func (ip Addr) StringExpanded() string {
        switch ip.z {
        case z0, z4:
                return ip.String()
        }

        const size = len("ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff")
        ret := make([]byte, 0, size)
        for i := uint8(0); i < 8; i++ {
                if i > 0 {
                        ret = append(ret, ':')
                }

                ret = appendHexPad(ret, ip.v6u16(i))
        }

        if ip.z != z6noz {
                // The addition of a zone will cause a second allocation, but when there
                // is no zone the ret slice will be stack allocated.
                ret = append(ret, '%')
                ret = append(ret, ip.Zone()...)
        }
        return string(ret)
}

// MarshalText implements the encoding.TextMarshaler interface,
// The encoding is the same as returned by String, with one exception:
// If ip is the zero Addr, the encoding is the empty string.
func (ip Addr) MarshalText() ([]byte, error) {
        switch ip.z {
        case z0:
                return []byte(""), nil
        case z4:
                max := len("255.255.255.255")
                b := make([]byte, 0, max)
                return ip.appendTo4(b), nil
        default:
                max := len("ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff%enp5s0")
                b := make([]byte, 0, max)
                if ip.Is4In6() {
                        b = append(b, "::ffff:"...)
                        b = ip.Unmap().appendTo4(b)
                        if z := ip.Zone(); z != "" {
                                b = append(b, '%')
                                b = append(b, z...)
                        }
                        return b, nil
                }
                return ip.appendTo6(b), nil
        }

}

// UnmarshalText implements the encoding.TextUnmarshaler interface.
// The IP address is expected in a form accepted by ParseAddr.
//
// If text is empty, UnmarshalText sets *ip to the zero Addr and
// returns no error.
func (ip *Addr) UnmarshalText(text []byte) error {
        if len(text) == 0 {
                *ip = Addr{}
                return nil
        }
        var err error
        *ip, err = ParseAddr(string(text))
        return err
}

func (ip Addr) marshalBinaryWithTrailingBytes(trailingBytes int) []byte {
        var b []byte
        switch ip.z {
        case z0:
                b = make([]byte, trailingBytes)
        case z4:
                b = make([]byte, 4+trailingBytes)
                bePutUint32(b, uint32(ip.addr.lo))
        default:
                z := ip.Zone()
                b = make([]byte, 16+len(z)+trailingBytes)
                bePutUint64(b[:8], ip.addr.hi)
                bePutUint64(b[8:], ip.addr.lo)
                copy(b[16:], z)
        }
        return b
}

// MarshalBinary implements the encoding.BinaryMarshaler interface.
// It returns a zero-length slice for the zero Addr,
// the 4-byte form for an IPv4 address,
// and the 16-byte form with zone appended for an IPv6 address.
func (ip Addr) MarshalBinary() ([]byte, error) {
        return ip.marshalBinaryWithTrailingBytes(0), nil
}

// UnmarshalBinary implements the encoding.BinaryUnmarshaler interface.
// It expects data in the form generated by MarshalBinary.
func (ip *Addr) UnmarshalBinary(b []byte) error {
        n := len(b)
        switch {
        case n == 0:
                *ip = Addr{}
                return nil
        case n == 4:
                *ip = AddrFrom4([4]byte(b))
                return nil
        case n == 16:
                *ip = AddrFrom16([16]byte(b))
                return nil
        case n > 16:
                *ip = AddrFrom16([16]byte(b[:16])).WithZone(string(b[16:]))
                return nil
        }
        return errors.New("unexpected slice size")
}

// AddrPort is an IP and a port number.
type AddrPort struct {
        ip   Addr
        port uint16
}

// AddrPortFrom returns an AddrPort with the provided IP and port.
// It does not allocate.
func AddrPortFrom(ip Addr, port uint16) AddrPort { return AddrPort{ip: ip, port: port} }

// Addr returns p's IP address.
func (p AddrPort) Addr() Addr { return p.ip }

// Port returns p's port.
func (p AddrPort) Port() uint16 { return p.port }

// splitAddrPort splits s into an IP address string and a port
// string. It splits strings shaped like "foo:bar" or "[foo]:bar",
// without further validating the substrings. v6 indicates whether the
// ip string should parse as an IPv6 address or an IPv4 address, in
// order for s to be a valid ip:port string.
func splitAddrPort(s string) (ip, port string, v6 bool, err error) {
        i := bytealg.LastIndexByteString(s, ':')
        if i == -1 {
                return "", "", false, errors.New("not an ip:port")
        }

        ip, port = s[:i], s[i+1:]
        if len(ip) == 0 {
                return "", "", false, errors.New("no IP")
        }
        if len(port) == 0 {
                return "", "", false, errors.New("no port")
        }
        if ip[0] == '[' {
                if len(ip) < 2 || ip[len(ip)-1] != ']' {
                        return "", "", false, errors.New("missing ]")
                }
                ip = ip[1 : len(ip)-1]
                v6 = true
        }

        return ip, port, v6, nil
}

// ParseAddrPort parses s as an AddrPort.
//
// It doesn't do any name resolution: both the address and the port
// must be numeric.
func ParseAddrPort(s string) (AddrPort, error) {
        var ipp AddrPort
        ip, port, v6, err := splitAddrPort(s)
        if err != nil {
                return ipp, err
        }
        port16, err := strconv.ParseUint(port, 10, 16)
        if err != nil {
                return ipp, errors.New("invalid port " + strconv.Quote(port) + " parsing " + strconv.Quote(s))
        }
        ipp.port = uint16(port16)
        ipp.ip, err = ParseAddr(ip)
        if err != nil {
                return AddrPort{}, err
        }
        if v6 && ipp.ip.Is4() {
                return AddrPort{}, errors.New("invalid ip:port " + strconv.Quote(s) + ", square brackets can only be used with IPv6 addresses")
        } else if !v6 && ipp.ip.Is6() {
                return AddrPort{}, errors.New("invalid ip:port " + strconv.Quote(s) + ", IPv6 addresses must be surrounded by square brackets")
        }
        return ipp, nil
}

// MustParseAddrPort calls ParseAddrPort(s) and panics on error.
// It is intended for use in tests with hard-coded strings.
func MustParseAddrPort(s string) AddrPort {
        ip, err := ParseAddrPort(s)
        if err != nil {
                panic(err)
        }
        return ip
}

// IsValid reports whether p.Addr() is valid.
// All ports are valid, including zero.
func (p AddrPort) IsValid() bool { return p.ip.IsValid() }

func (p AddrPort) String() string {
        switch p.ip.z {
        case z0:
                return "invalid AddrPort"
        case z4:
                a := p.ip.As4()
                buf := make([]byte, 0, 21)
                for i := range a {
                        buf = strconv.AppendUint(buf, uint64(a[i]), 10)
                        buf = append(buf, "...:"[i])
                }
                buf = strconv.AppendUint(buf, uint64(p.port), 10)
                return string(buf)
        default:
                // TODO: this could be more efficient allocation-wise:
                return joinHostPort(p.ip.String(), itoa.Itoa(int(p.port)))
        }
}

func joinHostPort(host, port string) string {
        // We assume that host is a literal IPv6 address if host has
        // colons.
        if bytealg.IndexByteString(host, ':') >= 0 {
                return "[" + host + "]:" + port
        }
        return host + ":" + port
}

// AppendTo appends a text encoding of p,
// as generated by MarshalText,
// to b and returns the extended buffer.
func (p AddrPort) AppendTo(b []byte) []byte {
        switch p.ip.z {
        case z0:
                return b
        case z4:
                b = p.ip.appendTo4(b)
        default:
                if p.ip.Is4In6() {
                        b = append(b, "[::ffff:"...)
                        b = p.ip.Unmap().appendTo4(b)
                        if z := p.ip.Zone(); z != "" {
                                b = append(b, '%')
                                b = append(b, z...)
                        }
                } else {
                        b = append(b, '[')
                        b = p.ip.appendTo6(b)
                }
                b = append(b, ']')
        }
        b = append(b, ':')
        b = strconv.AppendUint(b, uint64(p.port), 10)
        return b
}

// MarshalText implements the encoding.TextMarshaler interface. The
// encoding is the same as returned by String, with one exception: if
// p.Addr() is the zero Addr, the encoding is the empty string.
func (p AddrPort) MarshalText() ([]byte, error) {
        var max int
        switch p.ip.z {
        case z0:
        case z4:
                max = len("255.255.255.255:65535")
        default:
                max = len("[ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff%enp5s0]:65535")
        }
        b := make([]byte, 0, max)
        b = p.AppendTo(b)
        return b, nil
}

// UnmarshalText implements the encoding.TextUnmarshaler
// interface. The AddrPort is expected in a form
// generated by MarshalText or accepted by ParseAddrPort.
func (p *AddrPort) UnmarshalText(text []byte) error {
        if len(text) == 0 {
                *p = AddrPort{}
                return nil
        }
        var err error
        *p, err = ParseAddrPort(string(text))
        return err
}

// MarshalBinary implements the encoding.BinaryMarshaler interface.
// It returns Addr.MarshalBinary with an additional two bytes appended
// containing the port in little-endian.
func (p AddrPort) MarshalBinary() ([]byte, error) {
        b := p.Addr().marshalBinaryWithTrailingBytes(2)
        lePutUint16(b[len(b)-2:], p.Port())
        return b, nil
}

// UnmarshalBinary implements the encoding.BinaryUnmarshaler interface.
// It expects data in the form generated by MarshalBinary.
func (p *AddrPort) UnmarshalBinary(b []byte) error {
        if len(b) < 2 {
                return errors.New("unexpected slice size")
        }
        var addr Addr
        err := addr.UnmarshalBinary(b[:len(b)-2])
        if err != nil {
                return err
        }
        *p = AddrPortFrom(addr, leUint16(b[len(b)-2:]))
        return nil
}

// Prefix is an IP address prefix (CIDR) representing an IP network.
//
// The first Bits() of Addr() are specified. The remaining bits match any address.
// The range of Bits() is [0,32] for IPv4 or [0,128] for IPv6.
type Prefix struct {
        ip Addr

        // bitsPlusOne stores the prefix bit length plus one.
        // A Prefix is valid if and only if bitsPlusOne is non-zero.
        bitsPlusOne uint8
}

// PrefixFrom returns a Prefix with the provided IP address and bit
// prefix length.
//
// It does not allocate. Unlike Addr.Prefix, PrefixFrom does not mask
// off the host bits of ip.
//
// If bits is less than zero or greater than ip.BitLen, Prefix.Bits
// will return an invalid value -1.
func PrefixFrom(ip Addr, bits int) Prefix {
        var bitsPlusOne uint8
        if !ip.isZero() && bits >= 0 && bits <= ip.BitLen() {
                bitsPlusOne = uint8(bits) + 1
        }
        return Prefix{
                ip:          ip.withoutZone(),
                bitsPlusOne: bitsPlusOne,
        }
}

// Addr returns p's IP address.
func (p Prefix) Addr() Addr { return p.ip }

// Bits returns p's prefix length.
//
// It reports -1 if invalid.
func (p Prefix) Bits() int { return int(p.bitsPlusOne) - 1 }

// IsValid reports whether p.Bits() has a valid range for p.Addr().
// If p.Addr() is the zero Addr, IsValid returns false.
// Note that if p is the zero Prefix, then p.IsValid() == false.
func (p Prefix) IsValid() bool { return p.bitsPlusOne > 0 }

func (p Prefix) isZero() bool { return p == Prefix{} }

// IsSingleIP reports whether p contains exactly one IP.
func (p Prefix) IsSingleIP() bool { return p.IsValid() && p.Bits() == p.ip.BitLen() }

// ParsePrefix parses s as an IP address prefix.
// The string can be in the form "192.168.1.0/24" or "2001:db8::/32",
// the CIDR notation defined in RFC 4632 and RFC 4291.
// IPv6 zones are not permitted in prefixes, and an error will be returned if a
// zone is present.
//
// Note that masked address bits are not zeroed. Use Masked for that.
func ParsePrefix(s string) (Prefix, error) {
        i := bytealg.LastIndexByteString(s, '/')
        if i < 0 {
                return Prefix{}, errors.New("netip.ParsePrefix(" + strconv.Quote(s) + "): no '/'")
        }
        ip, err := ParseAddr(s[:i])
        if err != nil {
                return Prefix{}, errors.New("netip.ParsePrefix(" + strconv.Quote(s) + "): " + err.Error())
        }
        // IPv6 zones are not allowed: https://go.dev/issue/51899
        if ip.Is6() && ip.z != z6noz {
                return Prefix{}, errors.New("netip.ParsePrefix(" + strconv.Quote(s) + "): IPv6 zones cannot be present in a prefix")
        }

        bitsStr := s[i+1:]
        bits, err := strconv.Atoi(bitsStr)
        if err != nil {
                return Prefix{}, errors.New("netip.ParsePrefix(" + strconv.Quote(s) + "): bad bits after slash: " + strconv.Quote(bitsStr))
        }
        maxBits := 32
        if ip.Is6() {
                maxBits = 128
        }
        if bits < 0 || bits > maxBits {
                return Prefix{}, errors.New("netip.ParsePrefix(" + strconv.Quote(s) + "): prefix length out of range")
        }
        return PrefixFrom(ip, bits), nil
}

// MustParsePrefix calls ParsePrefix(s) and panics on error.
// It is intended for use in tests with hard-coded strings.
func MustParsePrefix(s string) Prefix {
        ip, err := ParsePrefix(s)
        if err != nil {
                panic(err)
        }
        return ip
}

// Masked returns p in its canonical form, with all but the high
// p.Bits() bits of p.Addr() masked off.
//
// If p is zero or otherwise invalid, Masked returns the zero Prefix.
func (p Prefix) Masked() Prefix {
        m, _ := p.ip.Prefix(p.Bits())
        return m
}

// Contains reports whether the network p includes ip.
//
// An IPv4 address will not match an IPv6 prefix.
// An IPv4-mapped IPv6 address will not match an IPv4 prefix.
// A zero-value IP will not match any prefix.
// If ip has an IPv6 zone, Contains returns false,
// because Prefixes strip zones.
func (p Prefix) Contains(ip Addr) bool {
        if !p.IsValid() || ip.hasZone() {
                return false
        }
        if f1, f2 := p.ip.BitLen(), ip.BitLen(); f1 == 0 || f2 == 0 || f1 != f2 {
                return false
        }
        if ip.Is4() {
                // xor the IP addresses together; mismatched bits are now ones.
                // Shift away the number of bits we don't care about.
                // Shifts in Go are more efficient if the compiler can prove
                // that the shift amount is smaller than the width of the shifted type (64 here).
                // We know that p.bits is in the range 0..32 because p is Valid;
                // the compiler doesn't know that, so mask with 63 to help it.
                // Now truncate to 32 bits, because this is IPv4.
                // If all the bits we care about are equal, the result will be zero.
                return uint32((ip.addr.lo^p.ip.addr.lo)>>((32-p.Bits())&63)) == 0
        } else {
                // xor the IP addresses together.
                // Mask away the bits we don't care about.
                // If all the bits we care about are equal, the result will be zero.
                return ip.addr.xor(p.ip.addr).and(mask6(p.Bits())).isZero()
        }
}

// Overlaps reports whether p and o contain any IP addresses in common.
//
// If p and o are of different address families or either have a zero
// IP, it reports false. Like the Contains method, a prefix with an
// IPv4-mapped IPv6 address is still treated as an IPv6 mask.
func (p Prefix) Overlaps(o Prefix) bool {
        if !p.IsValid() || !o.IsValid() {
                return false
        }
        if p == o {
                return true
        }
        if p.ip.Is4() != o.ip.Is4() {
                return false
        }
        var minBits int
        if pb, ob := p.Bits(), o.Bits(); pb < ob {
                minBits = pb
        } else {
                minBits = ob
        }
        if minBits == 0 {
                return true
        }
        // One of these Prefix calls might look redundant, but we don't require
        // that p and o values are normalized (via Prefix.Masked) first,
        // so the Prefix call on the one that's already minBits serves to zero
        // out any remaining bits in IP.
        var err error
        if p, err = p.ip.Prefix(minBits); err != nil {
                return false
        }
        if o, err = o.ip.Prefix(minBits); err != nil {
                return false
        }
        return p.ip == o.ip
}

// AppendTo appends a text encoding of p,
// as generated by MarshalText,
// to b and returns the extended buffer.
func (p Prefix) AppendTo(b []byte) []byte {
        if p.isZero() {
                return b
        }
        if !p.IsValid() {
                return append(b, "invalid Prefix"...)
        }

        // p.ip is non-nil, because p is valid.
        if p.ip.z == z4 {
                b = p.ip.appendTo4(b)
        } else {
                if p.ip.Is4In6() {
                        b = append(b, "::ffff:"...)
                        b = p.ip.Unmap().appendTo4(b)
                } else {
                        b = p.ip.appendTo6(b)
                }
        }

        b = append(b, '/')
        b = appendDecimal(b, uint8(p.Bits()))
        return b
}

// MarshalText implements the encoding.TextMarshaler interface,
// The encoding is the same as returned by String, with one exception:
// If p is the zero value, the encoding is the empty string.
func (p Prefix) MarshalText() ([]byte, error) {
        var max int
        switch p.ip.z {
        case z0:
        case z4:
                max = len("255.255.255.255/32")
        default:
                max = len("ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff%enp5s0/128")
        }
        b := make([]byte, 0, max)
        b = p.AppendTo(b)
        return b, nil
}

// UnmarshalText implements the encoding.TextUnmarshaler interface.
// The IP address is expected in a form accepted by ParsePrefix
// or generated by MarshalText.
func (p *Prefix) UnmarshalText(text []byte) error {
        if len(text) == 0 {
                *p = Prefix{}
                return nil
        }
        var err error
        *p, err = ParsePrefix(string(text))
        return err
}

// MarshalBinary implements the encoding.BinaryMarshaler interface.
// It returns Addr.MarshalBinary with an additional byte appended
// containing the prefix bits.
func (p Prefix) MarshalBinary() ([]byte, error) {
        b := p.Addr().withoutZone().marshalBinaryWithTrailingBytes(1)
        b[len(b)-1] = uint8(p.Bits())
        return b, nil
}

// UnmarshalBinary implements the encoding.BinaryUnmarshaler interface.
// It expects data in the form generated by MarshalBinary.
func (p *Prefix) UnmarshalBinary(b []byte) error {
        if len(b) < 1 {
                return errors.New("unexpected slice size")
        }
        var addr Addr
        err := addr.UnmarshalBinary(b[:len(b)-1])
        if err != nil {
                return err
        }
        *p = PrefixFrom(addr, int(b[len(b)-1]))
        return nil
}

// String returns the CIDR notation of p: "<ip>/<bits>".
func (p Prefix) String() string {
        if !p.IsValid() {
                return "invalid Prefix"
        }
        return p.ip.String() + "/" + itoa.Itoa(p.Bits())
}