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

    #[cfg(test)]
    mod tests;

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

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

    impl<'a> Parser<'a> {
        fn new(input: &'a str) -> Parser<'a> {
            Parser { state: input.as_bytes() }
        }

        fn is_eof(&self) -> bool {
            self.state.is_empty()
        }

        /// Run a parser, and restore the pre-parse state if it fails
        fn read_atomically<T, F>(&mut self, inner: F) -> Option<T>
        where
            F: FnOnce(&mut Parser<'_>) -> Option<T>,
        {
            let state = self.state;
            let result = inner(self);
            if result.is_none() {
                self.state = state;
            }
            result
        }

        /// Run a parser, but fail if the entire input wasn't consumed.
        /// Doesn't run atomically.
        fn read_till_eof<T, F>(&mut self, inner: F) -> Option<T>
        where
            F: FnOnce(&mut Parser<'_>) -> Option<T>,
        {
            inner(self).filter(|_| self.is_eof())
        }

        /// Same as read_till_eof, but returns a Result<AddrParseError> on failure
        fn parse_with<T, F>(&mut self, inner: F) -> Result<T, AddrParseError>
        where
            F: FnOnce(&mut Parser<'_>) -> Option<T>,
        {
            self.read_till_eof(inner).ok_or(AddrParseError(()))
        }

        /// Read the next character from the input
        fn read_char(&mut self) -> Option<char> {
            self.state.split_first().map(|(&b, tail)| {
                self.state = tail;
                b as char
            })
        }

        /// Read the next character from the input if it matches the target
        fn read_given_char(&mut self, target: char) -> Option<char> {
            self.read_atomically(|p| p.read_char().filter(|&c| c == target))
        }

        /// Helper for reading separators in an indexed loop. Reads the separator
        /// character iff index > 0, then runs the parser. When used in a loop,
        /// the separator character will only be read on index > 0 (see
        /// read_ipv4_addr for an example)
        fn read_separator<T, F>(&mut self, sep: char, index: usize, inner: F) -> Option<T>
        where
            F: FnOnce(&mut Parser<'_>) -> Option<T>,
        {
            self.read_atomically(move |p| {
                if index > 0 {
                    let _ = p.read_given_char(sep)?;
                }
                inner(p)
            })
        }

        // Read a single digit in the given radix. For instance, 0-9 in radix 10;
        // 0-9A-F in radix 16.
        fn read_digit(&mut self, radix: u32) -> Option<u32> {
            self.read_atomically(move |p| p.read_char()?.to_digit(radix))
        }

        // Read a number off the front of the input in the given radix, stopping
        // at the first non-digit character or eof. Fails if the number has more
        // digits than max_digits, or the value is >= upto, or if there is no number.
        fn read_number(&mut self, radix: u32, max_digits: u32, upto: u32) -> Option<u32> {
            self.read_atomically(move |p| {
                let mut result = 0;
                let mut digit_count = 0;

                while let Some(digit) = p.read_digit(radix) {
                    result = (result * radix) + digit;
                    digit_count += 1;
                    if digit_count > max_digits || result >= upto {
                        return None;
                    }
                }

                if digit_count == 0 { None } else { Some(result) }
            })
        }

        /// Read an IPv4 address
        fn read_ipv4_addr(&mut self) -> Option<Ipv4Addr> {
            self.read_atomically(|p| {
                let mut groups = [0; 4];

                for (i, slot) in groups.iter_mut().enumerate() {
                    *slot = p.read_separator('.', i, |p| p.read_number(10, 3, 0x100))? as u8;
                }

                Some(groups.into())
            })
        }

        /// Read an IPV6 Address
        fn read_ipv6_addr(&mut self) -> Option<Ipv6Addr> {
            /// Read a chunk of an ipv6 address into `groups`. Returns the number
            /// of groups read, along with a bool indicating if an embedded
            /// trailing ipv4 address was read. Specifically, read a series of
            /// colon-separated ipv6 groups (0x0000 - 0xFFFF), with an optional
            /// trailing embedded ipv4 address.
            fn read_groups(p: &mut Parser<'_>, groups: &mut [u16]) -> (usize, bool) {
                let limit = groups.len();

                for (i, slot) in groups.iter_mut().enumerate() {
                    // Try to read a trailing embedded ipv4 address. There must be
                    // at least two groups left.
                    if i < limit - 1 {
                        let ipv4 = p.read_separator(':', i, |p| p.read_ipv4_addr());

                        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_separator(':', i, |p| p.read_number(16, 4, 0x10000));

                    match group {
                        Some(g) => *slot = g as u16,
                        None => return (i, false),
                    }
                }
                (groups.len(), false)
            }

            self.read_atomically(|p| {
                // Read the front part of the address; either the whole thing, or up
                // to the first ::
                let mut head = [0; 8];
                let (head_size, head_ipv4) = read_groups(p, &mut head);

                if head_size == 8 {
                    return Some(head.into());
                }

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

                // read `::` if previous code parsed less than 8 groups
                // `::` indicates one or more groups of 16 bits of zeros
                let _ = p.read_given_char(':')?;
                let _ = p.read_given_char(':')?;

                // Read the back part of the address. The :: must contain at least one
                // set of zeroes, so our max length is 7.
                let mut tail = [0; 7];
                let limit = 8 - (head_size + 1);
                let (tail_size, _) = read_groups(p, &mut tail[..limit]);

                // Concat the head and tail of the IP address
                head[(8 - tail_size)..8].copy_from_slice(&tail[..tail_size]);

                Some(head.into())
            })
        }

        /// Read an IP Address, either IPV4 or IPV6.
        fn read_ip_addr(&mut self) -> Option<IpAddr> {
            self.read_ipv4_addr().map(IpAddr::V4).or_else(move || self.read_ipv6_addr().map(IpAddr::V6))
        }

        /// Read a : followed by a port in base 10
        fn read_port(&mut self) -> Option<u16> {
            self.read_atomically(|p| {
                let _ = p.read_given_char(':')?;
                let port = p.read_number(10, 5, 0x10000)?;
                Some(port as u16)
            })
        }

        /// Read an IPV4 address with a port
        fn read_socket_addr_v4(&mut self) -> Option<SocketAddrV4> {
            self.read_atomically(|p| {
                let ip = p.read_ipv4_addr()?;
                let port = p.read_port()?;
                Some(SocketAddrV4::new(ip, port))
            })
        }

        /// Read an IPV6 address with a port
        fn read_socket_addr_v6(&mut self) -> Option<SocketAddrV6> {
            self.read_atomically(|p| {
                let _ = p.read_given_char('[')?;
                let ip = p.read_ipv6_addr()?;
                let _ = p.read_given_char(']')?;

                let port = p.read_port()?;
                Some(SocketAddrV6::new(ip, port, 0, 0))
            })
        }

        /// Read an IP address with a port
        fn read_socket_addr(&mut self) -> Option<SocketAddr> {
            self.read_socket_addr_v4()
                .map(SocketAddr::V4)
                .or_else(|| self.read_socket_addr_v6().map(SocketAddr::V6))
        }
    }

    #[stable(feature = "ip_addr", since = "1.7.0")]
    impl FromStr for IpAddr {
        type Err = AddrParseError;
        fn from_str(s: &str) -> Result<IpAddr, AddrParseError> {
            Parser::new(s).parse_with(|p| p.read_ip_addr())
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl FromStr for Ipv4Addr {
        type Err = AddrParseError;
        fn from_str(s: &str) -> Result<Ipv4Addr, AddrParseError> {
            Parser::new(s).parse_with(|p| p.read_ipv4_addr())
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl FromStr for Ipv6Addr {
        type Err = AddrParseError;
        fn from_str(s: &str) -> Result<Ipv6Addr, AddrParseError> {
            Parser::new(s).parse_with(|p| p.read_ipv6_addr())
        }
    }

    #[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> {
            Parser::new(s).parse_with(|p| p.read_socket_addr_v4())
        }
    }

    #[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> {
            Parser::new(s).parse_with(|p| p.read_socket_addr_v6())
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl FromStr for SocketAddr {
        type Err = AddrParseError;
        fn from_str(s: &str) -> Result<SocketAddr, AddrParseError> {
            Parser::new(s).parse_with(|p| p.read_socket_addr())
        }
    }

    /// 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");
    /// ```
    #[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 {
        #[allow(deprecated, deprecated_in_future)]
        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 {
        #[allow(deprecated)]
        fn description(&self) -> &str {
            "invalid IP address syntax"
        }
    }