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    // Copyright 2013-2014 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.
    //
    // ignore-lexer-test FIXME #15679

    //! Hex binary-to-text encoding

    pub use self::FromHexError::*;

    use std::fmt;
    use std::error;

    /// A trait for converting a value to hexadecimal encoding
    pub trait ToHex {
        /// Converts the value of `self` to a hex value, returning the owned
        /// string.
        fn to_hex(&self) -> String;
    }

    static CHARS: &'static[u8] = b"0123456789abcdef";

    impl ToHex for [u8] {
        /// Turn a vector of `u8` bytes into a hexadecimal string.
        ///
        /// # Example
        ///
        /// ```rust
        /// extern crate rustc_serialize;
        /// use rustc_serialize::hex::ToHex;
        ///
        /// fn main () {
        ///     let str = [52,32].to_hex();
        ///     println!("{}", str);
        /// }
        /// ```
        fn to_hex(&self) -> String {
            let mut v = Vec::with_capacity(self.len() * 2);
            for &byte in self.iter() {
                v.push(CHARS[(byte >> 4) as usize]);
                v.push(CHARS[(byte & 0xf) as usize]);
            }

            unsafe {
                String::from_utf8_unchecked(v)
            }
        }
    }

    impl<'a, T: ?Sized + ToHex> ToHex for &'a T {
        fn to_hex(&self) -> String {
            (**self).to_hex()
        }
    }

    /// A trait for converting hexadecimal encoded values
    pub trait FromHex {
        /// Converts the value of `self`, interpreted as hexadecimal encoded data,
        /// into an owned vector of bytes, returning the vector.
        fn from_hex(&self) -> Result<Vec<u8>, FromHexError>;
    }

    /// Errors that can occur when decoding a hex encoded string
    #[derive(Clone, Copy)]
    pub enum FromHexError {
        /// The input contained a character not part of the hex format
        InvalidHexCharacter(char, usize),
        /// The input had an invalid length
        InvalidHexLength,
    }

    impl fmt::Debug for FromHexError {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            match *self {
                InvalidHexCharacter(ch, idx) =>
                    write!(f, "Invalid character '{}' at position {}", ch, idx),
                InvalidHexLength => write!(f, "Invalid input length"),
            }
        }
    }

    impl error::Error for FromHexError {
        fn description(&self) -> &str {
            match *self {
                InvalidHexCharacter(_, _) => "invalid character",
                InvalidHexLength => "invalid length",
            }
        }
    }

    impl fmt::Display for FromHexError {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            fmt::Debug::fmt(&self, f)
        }
    }

    impl FromHex for str {
        /// Convert any hexadecimal encoded string (literal, `@`, `&`, or `~`)
        /// to the byte values it encodes.
        ///
        /// You can use the `String::from_utf8` function to turn a
        /// `Vec<u8>` into a string with characters corresponding to those values.
        ///
        /// # Example
        ///
        /// This converts a string literal to hexadecimal and back.
        ///
        /// ```rust
        /// extern crate rustc_serialize;
        /// use rustc_serialize::hex::{FromHex, ToHex};
        ///
        /// fn main () {
        ///     let hello_str = "Hello, World".as_bytes().to_hex();
        ///     println!("{}", hello_str);
        ///     let bytes = hello_str.from_hex().unwrap();
        ///     println!("{:?}", bytes);
        ///     let result_str = String::from_utf8(bytes).unwrap();
        ///     println!("{}", result_str);
        /// }
        /// ```
        fn from_hex(&self) -> Result<Vec<u8>, FromHexError> {
            // This may be an overestimate if there is any whitespace
            let mut b = Vec::with_capacity(self.len() / 2);
            let mut modulus = 0;
            let mut buf = 0;

            for (idx, byte) in self.bytes().enumerate() {
                buf <<= 4;

                match byte {
                    b'A'...b'F' => buf |= byte - b'A' + 10,
                    b'a'...b'f' => buf |= byte - b'a' + 10,
                    b'0'...b'9' => buf |= byte - b'0',
                    b' '|b'\r'|b'\n'|b'\t' => {
                        buf >>= 4;
                        continue
                    }
                    _ => {
                        let ch = self[idx..].chars().next().unwrap();
                        return Err(InvalidHexCharacter(ch, idx))
                    }
                }

                modulus += 1;
                if modulus == 2 {
                    modulus = 0;
                    b.push(buf);
                }
            }

            match modulus {
                0 => Ok(b.into_iter().collect()),
                _ => Err(InvalidHexLength),
            }
        }
    }

    impl<'a, T: ?Sized + FromHex> FromHex for &'a T {
        fn from_hex(&self) -> Result<Vec<u8>, FromHexError> {
            (**self).from_hex()
        }
    }

    #[cfg(test)]
    mod tests {
        use hex::{FromHex, ToHex};

        #[test]
        pub fn test_to_hex() {
            assert_eq!("foobar".as_bytes().to_hex(), "666f6f626172");
        }

        #[test]
        pub fn test_from_hex_okay() {
            assert_eq!("666f6f626172".from_hex().unwrap(),
                       b"foobar");
            assert_eq!("666F6F626172".from_hex().unwrap(),
                       b"foobar");
        }

        #[test]
        pub fn test_from_hex_odd_len() {
            assert!("666".from_hex().is_err());
            assert!("66 6".from_hex().is_err());
        }

        #[test]
        pub fn test_from_hex_invalid_char() {
            assert!("66y6".from_hex().is_err());
        }

        #[test]
        pub fn test_from_hex_ignores_whitespace() {
            assert_eq!("666f 6f6\r\n26172 ".from_hex().unwrap(),
                       b"foobar");
        }

        #[test]
        pub fn test_to_hex_all_bytes() {
            for i in 0..256 {
                assert_eq!([i as u8].to_hex(), format!("{:02x}", i));
            }
        }

        #[test]
        pub fn test_from_hex_all_bytes() {
            for i in 0..256 {
                let ii: &[u8] = &[i as u8];
                assert_eq!(format!("{:02x}", i).from_hex().unwrap(),
                           ii);
                assert_eq!(format!("{:02X}", i).from_hex().unwrap(),
                           ii);
            }
        }
    }

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