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    //! A private parser implementation of IPv4, IPv6, and socket addresses.
    //!
    //! This module is "publicly exported" through the `FromStr` implementations
    //! below.

    use error::Error;
    use fmt;
    use net::{IpAddr, Ipv4Addr, Ipv6Addr, SocketAddr, SocketAddrV4, SocketAddrV6};
    use str::FromStr;

    struct Parser<'a> {
        // parsing as ASCII, so can use byte array
        s: &'a [u8],
        pos: usize,
    }

    impl<'a> Parser<'a> {
        fn new(s: &'a str) -> Parser<'a> {
            Parser {
                s: s.as_bytes(),
                pos: 0,
            }
        }

        fn is_eof(&self) -> bool {
            self.pos == self.s.len()
        }

        // Commit only if parser returns Some
        fn read_atomically<T, F>(&mut self, cb: F) -> Option<T> where
            F: FnOnce(&mut Parser) -> Option<T>,
        {
            let pos = self.pos;
            let r = cb(self);
            if r.is_none() {
                self.pos = pos;
            }
            r
        }

        // Commit only if parser read till EOF
        fn read_till_eof<T, F>(&mut self, cb: F) -> Option<T> where
            F: FnOnce(&mut Parser) -> Option<T>,
        {
            self.read_atomically(move |p| {
                cb(p).filter(|_| p.is_eof())
            })
        }

        // Return result of first successful parser
        fn read_or<T>(&mut self, parsers: &mut [Box<dyn FnMut(&mut Parser) -> Option<T> + 'static>])
                   -> Option<T> {
            for pf in parsers {
                if let Some(r) = self.read_atomically(|p: &mut Parser| pf(p)) {
                    return Some(r);
                }
            }
            None
        }

        // Apply 3 parsers sequentially
        fn read_seq_3<A, B, C, PA, PB, PC>(&mut self,
                                           pa: PA,
                                           pb: PB,
                                           pc: PC)
                                           -> Option<(A, B, C)> where
            PA: FnOnce(&mut Parser) -> Option<A>,
            PB: FnOnce(&mut Parser) -> Option<B>,
            PC: FnOnce(&mut Parser) -> Option<C>,
        {
            self.read_atomically(move |p| {
                let a = pa(p);
                let b = if a.is_some() { pb(p) } else { None };
                let c = if b.is_some() { pc(p) } else { None };
                match (a, b, c) {
                    (Some(a), Some(b), Some(c)) => Some((a, b, c)),
                    _ => None
                }
            })
        }

        // Read next char
        fn read_char(&mut self) -> Option<char> {
            if self.is_eof() {
                None
            } else {
                let r = self.s[self.pos] as char;
                self.pos += 1;
                Some(r)
            }
        }

        // Return char and advance iff next char is equal to requested
        fn read_given_char(&mut self, c: char) -> Option<char> {
            self.read_atomically(|p| {
                match p.read_char() {
                    Some(next) if next == c => Some(next),
                    _ => None,
                }
            })
        }

        // Read digit
        fn read_digit(&mut self, radix: u8) -> Option<u8> {
            fn parse_digit(c: char, radix: u8) -> Option<u8> {
                let c = c as u8;
                // assuming radix is either 10 or 16
                if c >= b'0' && c <= b'9' {
                    Some(c - b'0')
                } else if radix > 10 && c >= b'a' && c < b'a' + (radix - 10) {
                    Some(c - b'a' + 10)
                } else if radix > 10 && c >= b'A' && c < b'A' + (radix - 10) {
                    Some(c - b'A' + 10)
                } else {
                    None
                }
            }

            self.read_atomically(|p| {
                p.read_char().and_then(|c| parse_digit(c, radix))
            })
        }

        fn read_number_impl(&mut self, radix: u8, max_digits: u32, upto: u32) -> Option<u32> {
            let mut r = 0;
            let mut digit_count = 0;
            loop {
                match self.read_digit(radix) {
                    Some(d) => {
                        r = r * (radix as u32) + (d as u32);
                        digit_count += 1;
                        if digit_count > max_digits || r >= upto {
                            return None
                        }
                    }
                    None => {
                        if digit_count == 0 {
                            return None
                        } else {
                            return Some(r)
                        }
                    }
                };
            }
        }

        // Read number, failing if max_digits of number value exceeded
        fn read_number(&mut self, radix: u8, max_digits: u32, upto: u32) -> Option<u32> {
            self.read_atomically(|p| p.read_number_impl(radix, max_digits, upto))
        }

        fn read_ipv4_addr_impl(&mut self) -> Option<Ipv4Addr> {
            let mut bs = [0; 4];
            let mut i = 0;
            while i < 4 {
                if i != 0 && self.read_given_char('.').is_none() {
                    return None;
                }

                bs[i] = self.read_number(10, 3, 0x100).map(|n| n as u8)?;
                i += 1;
            }
            Some(Ipv4Addr::new(bs[0], bs[1], bs[2], bs[3]))
        }

        // Read IPv4 address
        fn read_ipv4_addr(&mut self) -> Option<Ipv4Addr> {
            self.read_atomically(|p| p.read_ipv4_addr_impl())
        }

        fn read_ipv6_addr_impl(&mut self) -> Option<Ipv6Addr> {
            fn ipv6_addr_from_head_tail(head: &[u16], tail: &[u16]) -> Ipv6Addr {
                assert!(head.len() + tail.len() <= 8);
                let mut gs = [0; 8];
                gs[..head.len()].copy_from_slice(head);
                gs[(8 - tail.len()) .. 8].copy_from_slice(tail);
                Ipv6Addr::new(gs[0], gs[1], gs[2], gs[3], gs[4], gs[5], gs[6], gs[7])
            }

            fn read_groups(p: &mut Parser, groups: &mut [u16; 8], limit: usize)
                           -> (usize, bool) {
                let mut i = 0;
                while i < limit {
                    if i < limit - 1 {
                        let ipv4 = p.read_atomically(|p| {
                            if i == 0 || p.read_given_char(':').is_some() {
                                p.read_ipv4_addr()
                            } else {
                                None
                            }
                        });
                        if let Some(v4_addr) = ipv4 {
                            let octets = v4_addr.octets();
                            groups[i + 0] = ((octets[0] as u16) << 8) | (octets[1] as u16);
                            groups[i + 1] = ((octets[2] as u16) << 8) | (octets[3] as u16);
                            return (i + 2, true);
                        }
                    }

                    let group = p.read_atomically(|p| {
                        if i == 0 || p.read_given_char(':').is_some() {
                            p.read_number(16, 4, 0x10000).map(|n| n as u16)
                        } else {
                            None
                        }
                    });
                    match group {
                        Some(g) => groups[i] = g,
                        None => return (i, false)
                    }
                    i += 1;
                }
                (i, false)
            }

            let mut head = [0; 8];
            let (head_size, head_ipv4) = read_groups(self, &mut head, 8);

            if head_size == 8 {
                return Some(Ipv6Addr::new(
                    head[0], head[1], head[2], head[3],
                    head[4], head[5], head[6], head[7]))
            }

            // IPv4 part is not allowed before `::`
            if head_ipv4 {
                return None
            }

            // read `::` if previous code parsed less than 8 groups
            if !self.read_given_char(':').is_some() || !self.read_given_char(':').is_some() {
                return None;
            }

            let mut tail = [0; 8];
            // `::` indicates one or more groups of 16 bits of zeros
            let limit = 8 - (head_size + 1);
            let (tail_size, _) = read_groups(self, &mut tail, limit);
            Some(ipv6_addr_from_head_tail(&head[..head_size], &tail[..tail_size]))
        }

        fn read_ipv6_addr(&mut self) -> Option<Ipv6Addr> {
            self.read_atomically(|p| p.read_ipv6_addr_impl())
        }

        fn read_ip_addr(&mut self) -> Option<IpAddr> {
            let ipv4_addr = |p: &mut Parser| p.read_ipv4_addr().map(IpAddr::V4);
            let ipv6_addr = |p: &mut Parser| p.read_ipv6_addr().map(IpAddr::V6);
            self.read_or(&mut [Box::new(ipv4_addr), Box::new(ipv6_addr)])
        }

        fn read_socket_addr_v4(&mut self) -> Option<SocketAddrV4> {
            let ip_addr = |p: &mut Parser| p.read_ipv4_addr();
            let colon = |p: &mut Parser| p.read_given_char(':');
            let port = |p: &mut Parser| {
                p.read_number(10, 5, 0x10000).map(|n| n as u16)
            };

            self.read_seq_3(ip_addr, colon, port).map(|t| {
                let (ip, _, port): (Ipv4Addr, char, u16) = t;
                SocketAddrV4::new(ip, port)
            })
        }

        fn read_socket_addr_v6(&mut self) -> Option<SocketAddrV6> {
            let ip_addr = |p: &mut Parser| {
                let open_br = |p: &mut Parser| p.read_given_char('[');
                let ip_addr = |p: &mut Parser| p.read_ipv6_addr();
                let clos_br = |p: &mut Parser| p.read_given_char(']');
                p.read_seq_3(open_br, ip_addr, clos_br).map(|t| t.1)
            };
            let colon = |p: &mut Parser| p.read_given_char(':');
            let port = |p: &mut Parser| {
                p.read_number(10, 5, 0x10000).map(|n| n as u16)
            };

            self.read_seq_3(ip_addr, colon, port).map(|t| {
                let (ip, _, port): (Ipv6Addr, char, u16) = t;
                SocketAddrV6::new(ip, port, 0, 0)
            })
        }

        fn read_socket_addr(&mut self) -> Option<SocketAddr> {
            let v4 = |p: &mut Parser| p.read_socket_addr_v4().map(SocketAddr::V4);
            let v6 = |p: &mut Parser| p.read_socket_addr_v6().map(SocketAddr::V6);
            self.read_or(&mut [Box::new(v4), Box::new(v6)])
        }
    }

    #[stable(feature = "ip_addr", since = "1.7.0")]
    impl FromStr for IpAddr {
        type Err = AddrParseError;
        fn from_str(s: &str) -> Result<IpAddr, AddrParseError> {
            match Parser::new(s).read_till_eof(|p| p.read_ip_addr()) {
                Some(s) => Ok(s),
                None => Err(AddrParseError(()))
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl FromStr for Ipv4Addr {
        type Err = AddrParseError;
        fn from_str(s: &str) -> Result<Ipv4Addr, AddrParseError> {
            match Parser::new(s).read_till_eof(|p| p.read_ipv4_addr()) {
                Some(s) => Ok(s),
                None => Err(AddrParseError(()))
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl FromStr for Ipv6Addr {
        type Err = AddrParseError;
        fn from_str(s: &str) -> Result<Ipv6Addr, AddrParseError> {
            match Parser::new(s).read_till_eof(|p| p.read_ipv6_addr()) {
                Some(s) => Ok(s),
                None => Err(AddrParseError(()))
            }
        }
    }

    #[stable(feature = "socket_addr_from_str", since = "1.5.0")]
    impl FromStr for SocketAddrV4 {
        type Err = AddrParseError;
        fn from_str(s: &str) -> Result<SocketAddrV4, AddrParseError> {
            match Parser::new(s).read_till_eof(|p| p.read_socket_addr_v4()) {
                Some(s) => Ok(s),
                None => Err(AddrParseError(())),
            }
        }
    }

    #[stable(feature = "socket_addr_from_str", since = "1.5.0")]
    impl FromStr for SocketAddrV6 {
        type Err = AddrParseError;
        fn from_str(s: &str) -> Result<SocketAddrV6, AddrParseError> {
            match Parser::new(s).read_till_eof(|p| p.read_socket_addr_v6()) {
                Some(s) => Ok(s),
                None => Err(AddrParseError(())),
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl FromStr for SocketAddr {
        type Err = AddrParseError;
        fn from_str(s: &str) -> Result<SocketAddr, AddrParseError> {
            match Parser::new(s).read_till_eof(|p| p.read_socket_addr()) {
                Some(s) => Ok(s),
                None => Err(AddrParseError(())),
            }
        }
    }

    /// An error which can be returned when parsing an IP address or a socket address.
    ///
    /// This error is used as the error type for the [`FromStr`] implementation for
    /// [`IpAddr`], [`Ipv4Addr`], [`Ipv6Addr`], [`SocketAddr`], [`SocketAddrV4`], and
    /// [`SocketAddrV6`].
    ///
    /// # Potential causes
    ///
    /// `AddrParseError` may be thrown because the provided string does not parse as the given type,
    /// often because it includes information only handled by a different address type.
    ///
    /// ```should_panic
    /// use std::net::IpAddr;
    /// let _foo: IpAddr = "127.0.0.1:8080".parse().expect("Cannot handle the socket port");
    /// ```
    ///
    /// [`IpAddr`] doesn't handle the port. Use [`SocketAddr`] instead.
    ///
    /// ```
    /// use std::net::SocketAddr;
    ///
    /// // No problem, the `panic!` message has disappeared.
    /// let _foo: SocketAddr = "127.0.0.1:8080".parse().expect("unreachable panic");
    /// ```
    ///
    /// [`FromStr`]: ../../std/str/trait.FromStr.html
    /// [`IpAddr`]: ../../std/net/enum.IpAddr.html
    /// [`Ipv4Addr`]: ../../std/net/struct.Ipv4Addr.html
    /// [`Ipv6Addr`]: ../../std/net/struct.Ipv6Addr.html
    /// [`SocketAddr`]: ../../std/net/enum.SocketAddr.html
    /// [`SocketAddrV4`]: ../../std/net/struct.SocketAddrV4.html
    /// [`SocketAddrV6`]: ../../std/net/struct.SocketAddrV6.html
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(Debug, Clone, PartialEq, Eq)]
    pub struct AddrParseError(());

    #[stable(feature = "addr_parse_error_error", since = "1.4.0")]
    impl fmt::Display for AddrParseError {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            fmt.write_str(self.description())
        }
    }

    #[stable(feature = "addr_parse_error_error", since = "1.4.0")]
    impl Error for AddrParseError {
        fn description(&self) -> &str {
            "invalid IP address syntax"
        }
    }

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