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    // Copyright 2014-2015 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.

    use std::char;
    use std::cmp::Ordering;
    use std::fmt;
    use std::ops;
    use std::u32;

    use syntax;

    use utf8::{decode_utf8, decode_last_utf8};
    use literals::LiteralSearcher;

    /// Represents a location in the input.
    #[derive(Clone, Copy, Debug)]
    pub struct InputAt {
        pos: usize,
        c: Char,
        byte: Option<u8>,
        len: usize,
    }

    impl InputAt {
        /// Returns true iff this position is at the beginning of the input.
        pub fn is_start(&self) -> bool {
            self.pos == 0
        }

        /// Returns true iff this position is past the end of the input.
        pub fn is_end(&self) -> bool {
            self.c.is_none() && self.byte.is_none()
        }

        /// Returns the character at this position.
        ///
        /// If this position is just before or after the input, then an absent
        /// character is returned.
        pub fn char(&self) -> Char {
            self.c
        }

        /// Returns the byte at this position.
        pub fn byte(&self) -> Option<u8> {
            self.byte
        }

        /// Returns the UTF-8 width of the character at this position.
        pub fn len(&self) -> usize {
            self.len
        }

        /// Returns the byte offset of this position.
        pub fn pos(&self) -> usize {
            self.pos
        }

        /// Returns the byte offset of the next position in the input.
        pub fn next_pos(&self) -> usize {
            self.pos + self.len
        }
    }

    /// An abstraction over input used in the matching engines.
    pub trait Input {
        /// Return an encoding of the position at byte offset `i`.
        fn at(&self, i: usize) -> InputAt;

        /// Return the Unicode character occurring next to `at`.
        ///
        /// If no such character could be decoded, then `Char` is absent.
        fn next_char(&self, at: InputAt) -> Char;

        /// Return the Unicode character occurring previous to `at`.
        ///
        /// If no such character could be decoded, then `Char` is absent.
        fn previous_char(&self, at: InputAt) -> Char;

        /// Scan the input for a matching prefix.
        fn prefix_at(
            &self,
            prefixes: &LiteralSearcher,
            at: InputAt,
        ) -> Option<InputAt>;

        /// The number of bytes in the input.
        fn len(&self) -> usize;

        /// Return the given input as a sequence of bytes.
        fn as_bytes(&self) -> &[u8];
    }

    impl<'a, T: Input> Input for &'a T {
        fn at(&self, i: usize) -> InputAt { (**self).at(i) }

        fn next_char(&self, at: InputAt) -> Char { (**self).next_char(at) }

        fn previous_char(&self, at: InputAt) -> Char { (**self).previous_char(at) }

        fn prefix_at(
            &self,
            prefixes: &LiteralSearcher,
            at: InputAt,
        ) -> Option<InputAt> {
            (**self).prefix_at(prefixes, at)
        }

        fn len(&self) -> usize { (**self).len() }

        fn as_bytes(&self) -> &[u8] { (**self).as_bytes() }
    }

    /// An input reader over characters.
    #[derive(Clone, Copy, Debug)]
    pub struct CharInput<'t>(&'t [u8]);

    impl<'t> CharInput<'t> {
        /// Return a new character input reader for the given string.
        pub fn new(s: &'t [u8]) -> CharInput<'t> {
            CharInput(s)
        }
    }

    impl<'t> ops::Deref for CharInput<'t> {
        type Target = [u8];

        fn deref(&self) -> &[u8] {
            self.0
        }
    }

    impl<'t> Input for CharInput<'t> {
        fn at(&self, i: usize) -> InputAt {
            let c = decode_utf8(&self[i..]).map(|(c, _)| c).into();
            InputAt {
                pos: i,
                c: c,
                byte: None,
                len: c.len_utf8(),
            }
        }

        fn next_char(&self, at: InputAt) -> Char {
            at.char()
        }

        fn previous_char(&self, at: InputAt) -> Char {
            decode_last_utf8(&self[..at.pos()]).map(|(c, _)| c).into()
        }

        fn prefix_at(
            &self,
            prefixes: &LiteralSearcher,
            at: InputAt,
        ) -> Option<InputAt> {
            prefixes.find(&self[at.pos()..]).map(|(s, _)| self.at(at.pos() + s))
        }

        fn len(&self) -> usize {
            self.0.len()
        }

        fn as_bytes(&self) -> &[u8] {
            self.0
        }
    }

    /// An input reader over bytes.
    ///
    /// N.B. We represent the reader with a string for now, since that gives us
    /// easy access to necessary Unicode decoding (used for word boundary look
    /// ahead/look behind).
    #[derive(Clone, Copy, Debug)]
    pub struct ByteInput<'t>(&'t [u8]);

    impl<'t> ByteInput<'t> {
        /// Return a new byte-based input reader for the given string.
        pub fn new(s: &'t [u8]) -> ByteInput<'t> {
            ByteInput(s)
        }
    }

    impl<'t> ops::Deref for ByteInput<'t> {
        type Target = [u8];

        fn deref(&self) -> &[u8] {
            self.0
        }
    }

    impl<'t> Input for ByteInput<'t> {
        fn at(&self, i: usize) -> InputAt {
            InputAt {
                pos: i,
                c: None.into(),
                byte: self.get(i).map(|&b| b),
                len: 1,
            }
        }

        fn next_char(&self, at: InputAt) -> Char {
            decode_utf8(&self[at.pos()..]).map(|(c, _)| c).into()
        }

        fn previous_char(&self, at: InputAt) -> Char {
            decode_last_utf8(&self[..at.pos()]).map(|(c, _)| c).into()
        }

        fn prefix_at(
            &self,
            prefixes: &LiteralSearcher,
            at: InputAt,
        ) -> Option<InputAt> {
            prefixes.find(&self[at.pos()..]).map(|(s, _)| self.at(at.pos() + s))
        }

        fn len(&self) -> usize {
            self.0.len()
        }

        fn as_bytes(&self) -> &[u8] {
            self.0
        }
    }

    /// An inline representation of `Option<char>`.
    ///
    /// This eliminates the need to do case analysis on `Option<char>` to determine
    /// ordinality with other characters.
    ///
    /// (The `Option<char>` is not related to encoding. Instead, it is used in the
    /// matching engines to represent the beginning and ending boundaries of the
    /// search text.)
    #[derive(Clone, Copy, Hash, PartialEq, Eq, PartialOrd, Ord)]
    pub struct Char(u32);

    impl fmt::Debug for Char {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            match char::from_u32(self.0) {
                None => write!(f, "Empty"),
                Some(c) => write!(f, "{:?}", c),
            }
        }
    }

    impl Char {
        /// Returns true iff the character is absent.
        #[inline]
        pub fn is_none(self) -> bool { self.0 == u32::MAX }

        /// Returns the length of the character's UTF-8 encoding.
        ///
        /// If the character is absent, then `0` is returned.
        #[inline]
        pub fn len_utf8(self) -> usize {
            char::from_u32(self.0).map_or(0, |c| c.len_utf8())
        }

        /// Returns true iff the character is a word character.
        ///
        /// If the character is absent, then false is returned.
        pub fn is_word_char(self) -> bool {
            char::from_u32(self.0).map_or(false, syntax::is_word_char)
        }

        /// Returns true iff the byte is a word byte.
        ///
        /// If the byte is absent, then false is returned.
        pub fn is_word_byte(self) -> bool {
            match char::from_u32(self.0) {
                None => false,
                Some(c) if c <= '\u{FF}' => syntax::is_word_byte(c as u8),
                Some(_) => false,
            }
        }

        /// Converts the character to a real primitive `char`.
        ///
        /// If the character is absent, then `None` is returned.
        pub fn as_char(self) -> Option<char> {
            // This is only used in the `regex!` macro because it expands char
            // classes into `match` expressions (instead of binary search).
            char::from_u32(self.0)
        }
    }

    impl From<char> for Char {
        fn from(c: char) -> Char { Char(c as u32) }
    }

    impl From<Option<char>> for Char {
        fn from(c: Option<char>) -> Char {
            c.map_or(Char(u32::MAX), |c| c.into())
        }
    }

    impl PartialEq<char> for Char {
        #[inline]
        fn eq(&self, other: &char) -> bool { self.0 == *other as u32 }
    }

    impl PartialEq<Char> for char {
        #[inline]
        fn eq(&self, other: &Char) -> bool { *self as u32 == other.0 }
    }

    impl PartialOrd<char> for Char {
        #[inline]
        fn partial_cmp(&self, other: &char) -> Option<Ordering> {
            self.0.partial_cmp(&(*other as u32))
        }
    }

    impl PartialOrd<Char> for char {
        #[inline]
        fn partial_cmp(&self, other: &Char) -> Option<Ordering> {
            (*self as u32).partial_cmp(&other.0)
        }
    }

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