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    use std::collections::HashMap;
    use std::cmp::Ordering;
    use std::fmt;
    use std::ops::Deref;
    use std::mem;
    use std::slice;
    use std::sync::Arc;

    use input::Char;
    use literals::LiteralSearcher;

    /// InstPtr represents the index of an instruction in a regex program.
    pub type InstPtr = usize;

    /// Program is a sequence of instructions and various facts about thos
    /// instructions.
    #[derive(Clone)]
    pub struct Program {
        /// A sequence of instructions that represents an NFA.
        pub insts: Vec<Inst>,
        /// Pointers to each Match instruction in the sequence.
        ///
        /// This is always length 1 unless this program represents a regex set.
        pub matches: Vec<InstPtr>,
        /// The ordered sequence of all capture groups extracted from the AST.
        /// Unnamed groups are `None`.
        pub captures: Vec<Option<String>>,
        /// Pointers to all named capture groups into `captures`.
        pub capture_name_idx: Arc<HashMap<String, usize>>,
        /// A pointer to the start instruction. This can vary depending on how
        /// the program was compiled. For example, programs for use with the DFA
        /// engine have a `.*?` inserted at the beginning of unanchored regular
        /// expressions. The actual starting point of the program is after the
        /// `.*?`.
        pub start: InstPtr,
        /// A set of equivalence classes for discriminating bytes in the compiled
        /// program.
        pub byte_classes: Vec<u8>,
        /// When true, this program can only match valid UTF-8.
        pub only_utf8: bool,
        /// When true, this program uses byte range instructions instead of Unicode
        /// range instructions.
        pub is_bytes: bool,
        /// When true, the program is compiled for DFA matching. For example, this
        /// implies `is_bytes` and also inserts a preceding `.*?` for unanchored
        /// regexes.
        pub is_dfa: bool,
        /// When true, the program matches text in reverse (for use only in the
        /// DFA).
        pub is_reverse: bool,
        /// Whether the regex must match from the start of the input.
        pub is_anchored_start: bool,
        /// Whether the regex must match at the end of the input.
        pub is_anchored_end: bool,
        /// Whether this program contains a Unicode word boundary instruction.
        pub has_unicode_word_boundary: bool,
        /// A possibly empty machine for very quickly matching prefix literals.
        pub prefixes: LiteralSearcher,
        /// A limit on the size of the cache that the DFA is allowed to use while
        /// matching.
        ///
        /// The cache limit specifies approximately how much space we're willing to
        /// give to the state cache. Once the state cache exceeds the size, it is
        /// wiped and all states must be re-computed.
        ///
        /// Note that this value does not impact correctness. It can be set to 0
        /// and the DFA will run just fine. (It will only ever store exactly one
        /// state in the cache, and will likely run very slowly, but it will work.)
        ///
        /// Also note that this limit is *per thread of execution*. That is,
        /// if the same regex is used to search text across multiple threads
        /// simultaneously, then the DFA cache is not shared. Instead, copies are
        /// made.
        pub dfa_size_limit: usize,
    }

    impl Program {
        /// Creates an empty instruction sequence. Fields are given default
        /// values.
        pub fn new() -> Self {
            Program {
                insts: vec![],
                matches: vec![],
                captures: vec![],
                capture_name_idx: Arc::new(HashMap::new()),
                start: 0,
                byte_classes: vec![0; 256],
                only_utf8: true,
                is_bytes: false,
                is_dfa: false,
                is_reverse: false,
                is_anchored_start: false,
                is_anchored_end: false,
                has_unicode_word_boundary: false,
                prefixes: LiteralSearcher::empty(),
                dfa_size_limit: 2 * (1<<20),
            }
        }

        /// If pc is an index to a no-op instruction (like Save), then return the
        /// next pc that is not a no-op instruction.
        pub fn skip(&self, mut pc: usize) -> usize {
            loop {
                match self[pc] {
                    Inst::Save(ref i) => pc = i.goto,
                    _ => return pc,
                }
            }
        }

        /// Return true if and only if an execution engine at instruction `pc` will
        /// always lead to a match.
        pub fn leads_to_match(&self, pc: usize) -> bool {
            if self.matches.len() > 1 {
                // If we have a regex set, then we have more than one ending
                // state, so leading to one of those states is generally
                // meaningless.
                return false;
            }
            match self[self.skip(pc)] {
                Inst::Match(_) => true,
                _ => false,
            }
        }

        /// Returns true if the current configuration demands that an implicit
        /// `.*?` be prepended to the instruction sequence.
        pub fn needs_dotstar(&self) -> bool {
            self.is_dfa && !self.is_reverse && !self.is_anchored_start
        }

        /// Returns true if this program uses Byte instructions instead of
        /// Char/Range instructions.
        pub fn uses_bytes(&self) -> bool {
            self.is_bytes || self.is_dfa || !self.only_utf8
        }

        /// Returns true if this program exclusively matches valid UTF-8 bytes.
        ///
        /// That is, if an invalid UTF-8 byte is seen, then no match is possible.
        pub fn only_utf8(&self) -> bool {
            self.only_utf8
        }

        /// Return the approximate heap usage of this instruction sequence in
        /// bytes.
        pub fn approximate_size(&self) -> usize {
            // The only instruction that uses heap space is Ranges (for
            // Unicode codepoint programs) to store non-overlapping codepoint
            // ranges. To keep this operation constant time, we ignore them.
            (self.len() * mem::size_of::<Inst>())
            + (self.matches.len() * mem::size_of::<InstPtr>())
            + (self.captures.len() * mem::size_of::<Option<String>>())
            + (self.capture_name_idx.len() *
               (mem::size_of::<String>() + mem::size_of::<usize>()))
            + (self.byte_classes.len() * mem::size_of::<u8>())
            + self.prefixes.approximate_size()
        }
    }

    impl Deref for Program {
        type Target = [Inst];

        fn deref(&self) -> &Self::Target {
            &*self.insts
        }
    }

    impl fmt::Debug for Program {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            use self::Inst::*;

            fn with_goto(cur: usize, goto: usize, fmtd: String) -> String {
                if goto == cur + 1 {
                    fmtd
                } else {
                    format!("{} (goto: {})", fmtd, goto)
                }
            }

            fn visible_byte(b: u8) -> String {
                use std::ascii::escape_default;
                let escaped = escape_default(b).collect::<Vec<u8>>();
                String::from_utf8_lossy(&escaped).into_owned()
            }

            for (pc, inst) in self.iter().enumerate() {
                match *inst {
                    Match(slot) => {
                        try!(write!(f, "{:04} Match({:?})", pc, slot))
                    }
                    Save(ref inst) => {
                        let s = format!("{:04} Save({})", pc, inst.slot);
                        try!(write!(f, "{}", with_goto(pc, inst.goto, s)));
                    }
                    Split(ref inst) => {
                        try!(write!(f, "{:04} Split({}, {})",
                                    pc, inst.goto1, inst.goto2));
                    }
                    EmptyLook(ref inst) => {
                        let s = format!("{:?}", inst.look);
                        try!(write!(f, "{:04} {}",
                                    pc, with_goto(pc, inst.goto, s)));
                    }
                    Char(ref inst) => {
                        let s = format!("{:?}", inst.c);
                        try!(write!(f, "{:04} {}",
                                    pc, with_goto(pc, inst.goto, s)));
                    }
                    Ranges(ref inst) => {
                        let ranges = inst.ranges
                            .iter()
                            .map(|r| format!("{:?}-{:?}", r.0, r.1))
                            .collect::<Vec<String>>()
                            .join(", ");
                        let s = format!("{}", ranges);
                        try!(write!(f, "{:04} {}",
                                    pc, with_goto(pc, inst.goto, s)));
                    }
                    Bytes(ref inst) => {
                        let s = format!(
                            "Bytes({}, {})",
                            visible_byte(inst.start),
                            visible_byte(inst.end));
                        try!(write!(f, "{:04} {}",
                                    pc, with_goto(pc, inst.goto, s)));
                    }
                }
                if pc == self.start {
                    try!(write!(f, " (start)"));
                }
                try!(write!(f, "\n"));
            }
            Ok(())
        }
    }

    impl<'a> IntoIterator for &'a Program {
        type Item = &'a Inst;
        type IntoIter = slice::Iter<'a, Inst>;
        fn into_iter(self) -> Self::IntoIter { self.iter() }
    }

    /// Inst is an instruction code in a Regex program.
    ///
    /// Regrettably, a regex program either contains Unicode codepoint
    /// instructions (Char and Ranges) or it contains byte instructions (Bytes).
    /// A regex program can never contain both.
    ///
    /// It would be worth investigating splitting this into two distinct types and
    /// then figuring out how to make the matching engines polymorphic over those
    /// types without sacrificing performance.
    ///
    /// Other than the benefit of moving invariants into the type system, another
    /// benefit is the decreased size. If we remove the `Char` and `Ranges`
    /// instructions from the `Inst` enum, then its size shrinks from 40 bytes to
    /// 24 bytes. (This is because of the removal of a `Vec` in the `Ranges`
    /// variant.) Given that byte based machines are typically much bigger than
    /// their Unicode analogues (because they can decode UTF-8 directly), this ends
    /// up being a pretty significant savings.
    #[derive(Clone, Debug)]
    pub enum Inst {
        /// Match indicates that the program has reached a match state.
        ///
        /// The number in the match corresponds to the Nth logical regular
        /// expression in this program. This index is always 0 for normal regex
        /// programs. Values greater than 0 appear when compiling regex sets, and
        /// each match instruction gets its own unique value. The value corresponds
        /// to the Nth regex in the set.
        Match(usize),
        /// Save causes the program to save the current location of the input in
        /// the slot indicated by InstSave.
        Save(InstSave),
        /// Split causes the program to diverge to one of two paths in the
        /// program, preferring goto1 in InstSplit.
        Split(InstSplit),
        /// EmptyLook represents a zero-width assertion in a regex program. A
        /// zero-width assertion does not consume any of the input text.
        EmptyLook(InstEmptyLook),
        /// Char requires the regex program to match the character in InstChar at
        /// the current position in the input.
        Char(InstChar),
        /// Ranges requires the regex program to match the character at the current
        /// position in the input with one of the ranges specified in InstRanges.
        Ranges(InstRanges),
        /// Bytes is like Ranges, except it expresses a single byte range. It is
        /// used in conjunction with Split instructions to implement multi-byte
        /// character classes.
        Bytes(InstBytes),
    }

    /// Representation of the Save instruction.
    #[derive(Clone, Debug)]
    pub struct InstSave {
        /// The next location to execute in the program.
        pub goto: InstPtr,
        /// The capture slot (there are two slots for every capture in a regex,
        /// including the zeroth capture for the entire match).
        pub slot: usize,
    }

    /// Representation of the Split instruction.
    #[derive(Clone, Debug)]
    pub struct InstSplit {
        /// The first instruction to try. A match resulting from following goto1
        /// has precedence over a match resulting from following goto2.
        pub goto1: InstPtr,
        /// The second instruction to try. A match resulting from following goto1
        /// has precedence over a match resulting from following goto2.
        pub goto2: InstPtr,
    }

    /// Representation of the EmptyLook instruction.
    #[derive(Clone, Debug)]
    pub struct InstEmptyLook {
        /// The next location to execute in the program if this instruction
        /// succeeds.
        pub goto: InstPtr,
        /// The type of zero-width assertion to check.
        pub look: EmptyLook,
    }

    /// The set of zero-width match instructions.
    #[derive(Clone, Copy, Debug, PartialEq, Eq)]
    pub enum EmptyLook {
        /// Start of line or input.
        StartLine,
        /// End of line or input.
        EndLine,
        /// Start of input.
        StartText,
        /// End of input.
        EndText,
        /// Word character on one side and non-word character on other.
        WordBoundary,
        /// Word character on both sides or non-word character on both sides.
        NotWordBoundary,
        /// ASCII word boundary.
        WordBoundaryAscii,
        /// Not ASCII word boundary.
        NotWordBoundaryAscii,
    }

    impl InstEmptyLook {
        /// Tests whether the pair of characters matches this zero-width
        /// instruction.
        pub fn matches(&self, c1: Char, c2: Char) -> bool {
            use self::EmptyLook::*;
            match self.look {
                StartLine => c1.is_none() || c1 == '\n',
                EndLine => c2.is_none() || c2 == '\n',
                StartText => c1.is_none(),
                EndText => c2.is_none(),
                WordBoundary => {
                    let (w1, w2) = (c1.is_word_char(), c2.is_word_char());
                    w1 ^ w2
                }
                NotWordBoundary => {
                    let (w1, w2) = (c1.is_word_char(), c2.is_word_char());
                    !(w1 ^ w2)
                }
                WordBoundaryAscii => {
                    let (w1, w2) = (c1.is_word_byte(), c2.is_word_byte());
                    w1 ^ w2
                }
                NotWordBoundaryAscii => {
                    let (w1, w2) = (c1.is_word_byte(), c2.is_word_byte());
                    !(w1 ^ w2)
                }
            }
        }
    }

    /// Representation of the Char instruction.
    #[derive(Clone, Debug)]
    pub struct InstChar {
        /// The next location to execute in the program if this instruction
        /// succeeds.
        pub goto: InstPtr,
        /// The character to test.
        pub c: char,
    }

    /// Representation of the Ranges instruction.
    #[derive(Clone, Debug)]
    pub struct InstRanges {
        /// The next location to execute in the program if this instruction
        /// succeeds.
        pub goto: InstPtr,
        /// The set of Unicode scalar value ranges to test.
        pub ranges: Vec<(char, char)>,
    }

    impl InstRanges {
        /// Tests whether the given input character matches this instruction.
        pub fn matches(&self, c: Char) -> bool {
            // This speeds up the `match_class_unicode` benchmark by checking
            // some common cases quickly without binary search. e.g., Matching
            // a Unicode class on predominantly ASCII text.
            for r in self.ranges.iter().take(4) {
                if c < r.0 {
                    return false;
                }
                if c <= r.1 {
                    return true;
                }
            }
            self.ranges.binary_search_by(|r| {
                if r.1 < c {
                    Ordering::Less
                } else if r.0 > c {
                    Ordering::Greater
                } else {
                    Ordering::Equal
                }
            }).is_ok()
        }

        /// Return the number of distinct characters represented by all of the
        /// ranges.
        pub fn num_chars(&self) -> usize {
            self.ranges.iter()
                .map(|&(s, e)| 1 + (e as u32) - (s as u32))
                .fold(0, |acc, len| acc + len)
                as usize
        }
    }

    /// Representation of the Bytes instruction.
    #[derive(Clone, Debug)]
    pub struct InstBytes {
        /// The next location to execute in the program if this instruction
        /// succeeds.
        pub goto: InstPtr,
        /// The start (inclusive) of this byte range.
        pub start: u8,
        /// The end (inclusive) of this byte range.
        pub end: u8,
    }

    impl InstBytes {
        /// Returns true if and only if the given byte is in this range.
        pub fn matches(&self, byte: u8) -> bool {
            self.start <= byte && byte <= self.end
        }
    }

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