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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::collections::HashMap;
    use std::sync::Arc;

    use backtrack::{self, Backtrack};
    use compile::Compiler;
    use dfa::{self, Dfa};
    use input::{ByteInput, CharInput};
    use literals::BuildPrefixes;
    use nfa::Nfa;
    use prog::{Program, InstPtr};
    use syntax;

    use {Regex, Error};

    pub type CaptureSlots<'a> = &'a mut [CaptureSlot];

    pub type CaptureSlot = Option<usize>;

    /// The parameters to running one of the four match engines.
    #[derive(Debug)]
    pub struct Search<'caps, 'matches> {
        /// The matching engine writes capture locations to this slice.
        ///
        /// Note that some matching engines, like the DFA, have limited support
        /// for this. The DFA can only fill in one capture location (the end
        /// location of the match).
        pub captures: CaptureSlots<'caps>,
        /// The matching engine indicates which match instructions were executed
        /// when searching stopped.
        ///
        /// In standard searches, there is exactly one value in this slice and it
        /// should be initialized to `false`. When executing sets of regexes,
        /// there should be a location for each regex.
        pub matches: &'matches mut [bool],
    }

    impl<'caps, 'matches> Search<'caps, 'matches> {
        pub fn quit_after_first_match(&self) -> bool {
            self.captures.is_empty() && self.matches.len() == 1
        }

        pub fn all_matched(&self) -> bool {
            self.matches.iter().all(|m| *m)
        }

        pub fn copy_captures_from(&mut self, caps: &[Option<usize>]) {
            for (slot, val) in self.captures.iter_mut().zip(caps.iter()) {
                *slot = *val;
            }
        }

        pub fn set_match(&mut self, match_slot: usize) {
            if let Some(old) = self.matches.get_mut(match_slot) {
                *old = true;
            }
        }

        pub fn set_start(&mut self, pos: Option<usize>) {
            self.set_capture(0, pos);
        }

        pub fn set_end(&mut self, pos: Option<usize>) {
            self.set_capture(1, pos);
        }

        fn set_capture(&mut self, i: usize, pos: Option<usize>) {
            if let Some(old_pos) = self.captures.get_mut(i) {
                *old_pos = pos;
            }
        }
    }

    /// Exec manages the execution of a regular expression.
    ///
    /// In particular, this manages the various compiled forms of a single regular
    /// expression and the choice of which matching engine to use to execute a
    /// regular expression.
    #[derive(Clone, Debug)]
    pub struct Exec {
        /// The original regular expressions given by the caller to compile.
        res: Vec<String>,
        /// A compiled program that is used in the NFA simulation and backtracking.
        /// It can be byte-based or Unicode codepoint based.
        ///
        /// N.B. It is not possibly to make this byte-based from the public API.
        /// It is only used for testing byte based programs in the NFA simulations.
        prog: Program,
        /// A compiled byte based program for DFA execution. This is only used
        /// if a DFA can be executed. (Currently, only word boundary assertions are
        /// not supported.) Note that this program contains an embedded `.*?`
        /// preceding the first capture group, unless the regex is anchored at the
        /// beginning.
        dfa: Program,
        /// The same as above, except the program is reversed (and there is no
        /// preceding `.*?`). This is used by the DFA to find the starting location
        /// of matches.
        dfa_reverse: Program,
        /// Set to true if and only if the DFA can be executed.
        can_dfa: bool,
        /// A preference for matching engine selection.
        ///
        /// This defaults to Automatic, which means the matching engine is selected
        /// based on heuristics (such as the nature and size of the compiled
        /// program, in addition to the size of the search text).
        ///
        /// If either Nfa or Backtrack is set, then it is always used because
        /// either is capable of executing every compiled program on any input
        /// size.
        match_engine: MatchEngine,
    }

    /// Facilitates the construction of an executor by exposing various knobs
    /// to control how a regex is executed and what kinds of resources it's
    /// permitted to use.
    pub struct ExecBuilder {
        res: Vec<String>,
        match_engine: MatchEngine,
        size_limit: usize,
        bytes: bool,
    }

    impl ExecBuilder {
        /// Create a regex execution builder.
        ///
        /// This uses default settings for everything except the regex itself,
        /// which must be provided. Further knobs can be set by calling methods,
        /// and then finally, `build` to actually create the executor.
        pub fn new(re: &str) -> Self {
            Self::new_many(&[re])
        }

        /// Like new, but compiles the union of the given regular expressions.
        ///
        /// Note that when compiling 2 or more regular expressions, capture groups
        /// are completely unsupported. (This means both `find` and `captures`
        /// wont work.)
        pub fn new_many<I, S>(res: I) -> Self
                where S: AsRef<str>, I: IntoIterator<Item=S> {
            ExecBuilder {
                res: res.into_iter().map(|s| s.as_ref().to_owned()).collect(),
                match_engine: MatchEngine::Automatic,
                size_limit: 10 * (1 << 20),
                bytes: false,
            }
        }

        /// Set the matching engine to be automatically determined.
        ///
        /// This is the default state and will apply whatever optimizations are
        /// possible, such as running a DFA.
        ///
        /// This overrides whatever was previously set via the `nfa` or
        /// `bounded_backtracking` methods.
        pub fn automatic(mut self) -> Self {
            self.match_engine = MatchEngine::Automatic;
            self
        }

        /// Sets the matching engine to use the NFA algorithm no matter what
        /// optimizations are possible.
        ///
        /// This overrides whatever was previously set via the `automatic` or
        /// `bounded_backtracking` methods.
        pub fn nfa(mut self) -> Self {
            self.match_engine = MatchEngine::Nfa;
            self
        }

        /// Sets the matching engine to use a bounded backtracking engine no
        /// matter what optimizations are possible.
        ///
        /// One must use this with care, since the bounded backtracking engine
        /// uses memory proportion to `len(regex) * len(text)`.
        ///
        /// This overrides whatever was previously set via the `automatic` or
        /// `nfa` methods.
        pub fn bounded_backtracking(mut self) -> Self {
            self.match_engine = MatchEngine::Backtrack;
            self
        }

        /// Sets the size limit on a single compiled regular expression program.
        ///
        /// The default is ~10MB.
        ///
        /// N.B. Typically, multiple programs are compiled for every regular
        /// expression and this limit applies to *each* of them.
        pub fn size_limit(mut self, bytes: usize) -> Self {
            self.size_limit = bytes;
            self
        }

        /// Compiles byte based programs for use with the NFA matching engines.
        ///
        /// By default, the NFA engines match on Unicode scalar values. They can
        /// be made to use byte based programs instead. In general, the byte based
        /// programs are slower because of a less efficient encoding of character
        /// classes. However, it may be useful (some day) for matching on raw
        /// bytes that may not be UTF-8.
        ///
        /// Note that this does not impact DFA matching engines, which always
        /// execute on bytes.
        pub fn bytes(mut self, yes: bool) -> Self {
            self.bytes = yes;
            self
        }

        /// Build an executor that can run a regular expression.
        pub fn build(self) -> Result<Exec, Error> {
            if self.res.is_empty() {
                return Err(Error::InvalidSet);
            }
            let mut exprs = vec![];
            for re in &self.res {
                exprs.push(try!(syntax::Expr::parse(re)));
            }
            let mut prog = try!(
                Compiler::new()
                         .size_limit(self.size_limit)
                         .bytes(self.bytes)
                         .compile(&exprs));
            let mut dfa = try!(
                Compiler::new()
                         .size_limit(self.size_limit)
                         .dfa(true)
                         .compile(&exprs));
            let dfa_reverse = try!(
                Compiler::new()
                         .size_limit(self.size_limit)
                         .dfa(true)
                         .reverse(true)
                         .compile(&exprs));

            // Compute literal prefixes for only `prog`, which is likely a Unicode
            // based program. Literal prefix extract currently works better on
            // Unicode programs.
            prog.prefixes = BuildPrefixes::new(&prog).literals().into_matcher();
            // And give it to the DFA too, which can use Unicode prefixes even
            // though the program itself is byte based.
            dfa.prefixes = prog.prefixes.clone();
            let can_dfa = dfa::can_exec(&dfa);
            Ok(Exec {
                res: self.res,
                prog: prog,
                dfa: dfa,
                dfa_reverse: dfa_reverse,
                can_dfa: can_dfa,
                match_engine: self.match_engine,
            })
        }
    }

    impl Exec {
        /// The main entry point for execution of a regular expression on text.
        ///
        /// caps represents the capture locations that the caller wants. Generally,
        /// there are three varieties: no captures requested (e.g., `is_match`),
        /// one capture requested (e.g., `find` or `find_iter`) or multiple
        /// captures requested (e.g., `captures` or `captures_iter` along with
        /// at least one capturing group in the regex). Each of these three cases
        /// provokes different behavior from the matching engines, where fewer
        /// captures generally means faster matching.
        ///
        /// text should be the search text and start should be the position in
        /// the text to start searching. Note that passing a simple slice here
        /// isn't sufficient, since look-behind assertions sometimes need to
        /// inspect the character immediately preceding the start location.
        ///
        /// Note that this method takes self.match_engine into account when
        /// choosing the engine to use. If self.match_engine is Nfa or Backtrack,
        /// then that engine is always used. Otherwise, one is selected
        /// automatically.
        pub fn exec<'c, 'm>(
            &self,
            search: &mut Search<'c, 'm>,
            text: &str,
            start: usize,
        ) -> bool {
            // Why isn't the DFA or literal engine checked for here? Well, it's
            // only possible to execute those engines in exec_auto. See comment on
            // MatchEngine below for more details.
            match self.match_engine {
                MatchEngine::Automatic => self.exec_auto(search, text, start),
                MatchEngine::Backtrack => self.exec_backtrack(search, text, start),
                MatchEngine::Nfa => self.exec_nfa(search, text, start),
            }
        }

        /// Like exec, but always selects the engine automatically.
        fn exec_auto<'c, 'm>(
            &self,
            search: &mut Search<'c, 'm>,
            text: &str,
            start: usize,
        ) -> bool {
            if search.captures.len() <= 2 && self.prog.prefixes.at_match() {
                // We should be able to execute the literal engine even if there
                // are more captures by falling back to the NFA engine after a
                // match. However, that's effectively what the NFA engine does
                // already (since it will use the literal engine if it exists).
                self.exec_literals(search, text, start)
            } else if self.can_dfa {
                self.exec_dfa(search, text, start)
            } else {
                self.exec_auto_nfa(search, text, start)
            }
        }

        /// Like exec, but always tries to execute the lazy DFA.
        ///
        /// Note that self.can_dfa must be true. This will panic otherwise.
        fn exec_dfa<'a, 'c, 'm>(
            &self,
            search: &'a mut Search<'c, 'm>,
            text: &str,
            start: usize,
        ) -> bool {
            debug_assert!(self.can_dfa);
            let btext = text.as_bytes();
            if !Dfa::exec(&self.dfa, search, btext, start) {
                return false;
            }
            let match_end = match search.captures.get(1) {
                Some(&Some(i)) => i,
                // The DFA returned true for a match, but did not set any capture
                // location because the caller didn't ask for them. Therefore, we
                // can quit immediately.
                _ => return true,
            };
            // invariant: caps.len() >= 2 && caps.len() % 2 == 0
            // If the reported end of the match is the same as the start, then we
            // have an empty match and we can quit now.
            if start == match_end {
                // Be careful... If the caller wants sub-captures, than we are
                // obliged to run the NFA to get them.
                if search.captures.len() == 2 {
                    // The caller only needs the start/end, so we can avoid the
                    // NFA here.
                    search.captures[0] = Some(start);
                    search.captures[1] = Some(start);
                    return true;
                }
                return self.exec_auto_nfa(search, text, start);
            }
            // OK, now we find the start of the match by running the DFA backwards
            // on the text. We *start* the search at the end of the match.
            let matched = Dfa::exec(
                &self.dfa_reverse, search, &btext[start..], match_end - start);
            if !matched {
                panic!("BUG: forward match implies backward match");
            }
            let match_start = match search.captures.get(0) {
                Some(&Some(i)) => start + i,
                _ => panic!("BUG: early match can't happen on reverse search"),
            };
            if search.captures.len() == 2 {
                // If the caller doesn't care about capture locations, then we can
                // avoid running the NFA to fill them in.
                search.captures[0] = Some(match_start);
                search.captures[1] = Some(match_end);
                return true;
            }
            self.exec_auto_nfa(search, text, match_start)
        }

        /// This is like exec_auto, except it always chooses between either the
        /// full NFA simulation or the bounded backtracking engine.
        fn exec_auto_nfa<'c, 'm>(
            &self,
            search: &mut Search<'c, 'm>,
            text: &str,
            start: usize,
        ) -> bool {
            if backtrack::should_exec(self.prog.len(), text.len()) {
                self.exec_backtrack(search, text, start)
            } else {
                self.exec_nfa(search, text, start)
            }
        }

        /// Always run the NFA algorithm.
        fn exec_nfa<'c, 'm>(
            &self,
            search: &mut Search<'c, 'm>,
            text: &str,
            start: usize,
        ) -> bool {
            if self.prog.is_bytes {
                Nfa::exec(&self.prog, search, ByteInput::new(text), start)
            } else {
                Nfa::exec(&self.prog, search, CharInput::new(text), start)
            }
        }

        /// Always runs the NFA using bounded backtracking.
        fn exec_backtrack<'c, 'm>(
            &self,
            search: &mut Search<'c, 'm>,
            text: &str,
            start: usize,
        ) -> bool {
            if self.prog.is_bytes {
                Backtrack::exec(&self.prog, search, ByteInput::new(text), start)
            } else {
                Backtrack::exec(&self.prog, search, CharInput::new(text), start)
            }
        }

        /// Executes the special literal matching engine.
        ///
        /// When a regular expression is small and can be expanded to a finite set
        /// of literals that all result in matches, then we can avoid all of the
        /// regex machinery and use specialized DFAs.
        ///
        /// This panics if the set of literals do not correspond to matches.
        fn exec_literals<'c, 'm>(
            &self,
            search: &mut Search<'c, 'm>,
            text: &str,
            start: usize,
        ) -> bool {
            debug_assert!(self.prog.prefixes.at_match());
            match self.prog.prefixes.find(&text.as_bytes()[start..]) {
                None => false,
                Some((s, e)) => {
                    if search.captures.len() == 2 {
                        search.captures[0] = Some(start + s);
                        search.captures[1] = Some(start + e);
                    }
                    true
                }
            }
        }

        /// Build a dynamic Regex from this executor.
        pub fn into_regex(self) -> Regex {
            Regex::Dynamic(self)
        }

        /// The original regular expressions given by the caller that were
        /// compiled.
        pub fn regex_strings(&self) -> &[String] {
            &self.res
        }

        /// Return a slice of instruction pointers to match slots.
        ///
        /// There is a match slot for every regular expression in this executor.
        pub fn matches(&self) -> &[InstPtr] {
            &self.prog.matches
        }

        /// Return a slice of capture names.
        ///
        /// Any capture that isn't named is None.
        pub fn captures(&self) -> &[Option<String>] {
            &self.prog.captures
        }

        /// Return a reference to named groups mapping (from group name to
        /// group position).
        pub fn capture_name_idx(&self) -> &Arc<HashMap<String, usize>> {
            &self.prog.capture_name_idx
        }
    }

    /// Some of the matching engines offered by this regex implementation.
    ///
    /// This is exported for use in testing.
    ///
    /// Note that only engines that can be used on *every* regex are exposed here.
    /// For example, it is useful for testing purposes to say, "always execute
    /// the backtracking engine" or "always execute the full NFA simulation."
    /// However, we cannot say things like, "always execute the pure literals
    /// engine" or "always execute the DFA" because they only work on a subset of
    /// regexes supported by this crate. Specifically, the only way to run the
    /// DFA or literal engines is to use Automatic.
    #[doc(hidden)]
    #[derive(Clone, Copy, Debug)]
    enum MatchEngine {
        /// Automatically choose the best matching engine based on heuristics.
        Automatic,
        /// A bounded backtracking implementation. About twice as fast as the
        /// NFA, but can only work on small regexes and small input.
        Backtrack,
        /// A full NFA simulation. Can always be employed but almost always the
        /// slowest choice.
        Nfa,
    }

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