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    // pest. The Elegant Parser
    // Copyright (c) 2018 Dragoș Tiselice
    //
    // 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. All files in the project carrying such notice may not be copied,
    // modified, or distributed except according to those terms.

    use alloc::borrow::ToOwned;
    use alloc::boxed::Box;
    use alloc::rc::Rc;
    use alloc::vec;
    use alloc::vec::Vec;
    use core::num::NonZeroUsize;
    use core::ops::Range;
    use core::sync::atomic::{AtomicUsize, Ordering};

    use crate::error::{Error, ErrorVariant};
    use crate::iterators::{pairs, QueueableToken};
    use crate::position::{self, Position};
    use crate::span::Span;
    use crate::stack::Stack;
    use crate::RuleType;

    /// The current lookahead status of a [`ParserState`].
    ///
    /// [`ParserState`]: struct.ParserState.html
    #[derive(Clone, Copy, Debug, Eq, PartialEq)]
    pub enum Lookahead {
        /// The positive predicate, written as an ampersand &,
        /// attempts to match its inner expression.
        /// If the inner expression succeeds, parsing continues,
        /// but at the same position as the predicate —
        /// &foo ~ bar is thus a kind of "AND" statement:
        /// "the input string must match foo AND bar".
        /// If the inner expression fails,
        /// the whole expression fails too.
        Positive,
        /// The negative predicate, written as an exclamation mark !,
        /// attempts to match its inner expression.
        /// If the inner expression fails, the predicate succeeds
        /// and parsing continues at the same position as the predicate.
        /// If the inner expression succeeds, the predicate fails —
        /// !foo ~ bar is thus a kind of "NOT" statement:
        /// "the input string must match bar but NOT foo".
        Negative,
        /// No lookahead (i.e. it will consume input).
        None,
    }

    /// The current atomicity of a [`ParserState`].
    ///
    /// [`ParserState`]: struct.ParserState.html
    #[derive(Clone, Copy, Debug, Eq, PartialEq)]
    pub enum Atomicity {
        /// prevents implicit whitespace: inside an atomic rule,
        /// the tilde ~ means "immediately followed by",
        /// and repetition operators (asterisk * and plus sign +)
        /// have no implicit separation. In addition, all other rules
        /// called from an atomic rule are also treated as atomic.
        /// (interior matching rules are silent)
        Atomic,
        /// The same as atomic, but inner tokens are produced as normal.
        CompoundAtomic,
        /// implicit whitespace is enabled
        NonAtomic,
    }

    /// Type alias to simplify specifying the return value of chained closures.
    pub type ParseResult<S> = Result<S, S>;

    /// Match direction for the stack. Used in `PEEK[a..b]`/`stack_match_peek_slice`.
    #[derive(Clone, Copy, Debug, Eq, PartialEq)]
    pub enum MatchDir {
        /// from the bottom to the top of the stack
        BottomToTop,
        /// from the top to the bottom of the stack
        TopToBottom,
    }

    static CALL_LIMIT: AtomicUsize = AtomicUsize::new(0);

    /// Sets the maximum call limit for the parser state
    /// to prevent stack overflows or excessive execution times
    /// in some grammars.
    /// If set, the calls are tracked as a running total
    /// over all non-terminal rules that can nest closures
    /// (which are passed to transform the parser state).
    ///
    /// # Arguments
    ///
    /// * `limit` - The maximum number of calls. If None,
    ///             the number of calls is unlimited.
    pub fn set_call_limit(limit: Option<NonZeroUsize>) {
        CALL_LIMIT.store(limit.map(|f| f.get()).unwrap_or(0), Ordering::Relaxed);
    }

    #[derive(Debug)]
    struct CallLimitTracker {
        current_call_limit: Option<(usize, usize)>,
    }

    impl Default for CallLimitTracker {
        fn default() -> Self {
            let limit = CALL_LIMIT.load(Ordering::Relaxed);
            let current_call_limit = if limit > 0 { Some((0, limit)) } else { None };
            Self { current_call_limit }
        }
    }

    impl CallLimitTracker {
        fn limit_reached(&self) -> bool {
            self.current_call_limit
                .map_or(false, |(current, limit)| current >= limit)
        }

        fn increment_depth(&mut self) {
            if let Some((current, _)) = &mut self.current_call_limit {
                *current += 1;
            }
        }
    }

    /// The complete state of a [`Parser`].
    ///
    /// [`Parser`]: trait.Parser.html
    #[derive(Debug)]
    pub struct ParserState<'i, R: RuleType> {
        position: Position<'i>,
        queue: Vec<QueueableToken<R>>,
        lookahead: Lookahead,
        pos_attempts: Vec<R>,
        neg_attempts: Vec<R>,
        attempt_pos: usize,
        atomicity: Atomicity,
        stack: Stack<Span<'i>>,
        call_tracker: CallLimitTracker,
    }

    /// Creates a `ParserState` from a `&str`, supplying it to a closure `f`.
    ///
    /// # Examples
    ///
    /// ```
    /// # use pest;
    /// let input = "";
    /// pest::state::<(), _>(input, |s| Ok(s)).unwrap();
    /// ```
    #[allow(clippy::perf)]
    pub fn state<'i, R: RuleType, F>(input: &'i str, f: F) -> Result<pairs::Pairs<'i, R>, Error<R>>
    where
        F: FnOnce(Box<ParserState<'i, R>>) -> ParseResult<Box<ParserState<'i, R>>>,
    {
        let state = ParserState::new(input);

        match f(state) {
            Ok(state) => {
                let len = state.queue.len();
                Ok(pairs::new(Rc::new(state.queue), input, 0, len))
            }
            Err(mut state) => {
                let variant = if state.reached_call_limit() {
                    ErrorVariant::CustomError {
                        message: "call limit reached".to_owned(),
                    }
                } else {
                    state.pos_attempts.sort();
                    state.pos_attempts.dedup();
                    state.neg_attempts.sort();
                    state.neg_attempts.dedup();
                    ErrorVariant::ParsingError {
                        positives: state.pos_attempts.clone(),
                        negatives: state.neg_attempts.clone(),
                    }
                };

                Err(Error::new_from_pos(
                    variant,
                    // TODO(performance): Guarantee state.attempt_pos is a valid position
                    position::Position::new(input, state.attempt_pos).unwrap(),
                ))
            }
        }
    }

    impl<'i, R: RuleType> ParserState<'i, R> {
        /// Allocates a fresh `ParserState` object to the heap and returns the owned `Box`. This `Box`
        /// will be passed from closure to closure based on the needs of the specified `Parser`.
        ///
        /// # Examples
        ///
        /// ```
        /// # use pest;
        /// let input = "";
        /// let state: Box<pest::ParserState<&str>> = pest::ParserState::new(input);
        /// ```
        pub fn new(input: &'i str) -> Box<Self> {
            Box::new(ParserState {
                position: Position::from_start(input),
                queue: vec![],
                lookahead: Lookahead::None,
                pos_attempts: vec![],
                neg_attempts: vec![],
                attempt_pos: 0,
                atomicity: Atomicity::NonAtomic,
                stack: Stack::new(),
                call_tracker: Default::default(),
            })
        }

        /// Returns a reference to the current `Position` of the `ParserState`.
        ///
        /// # Examples
        ///
        /// ```
        /// # use pest;
        /// # #[allow(non_camel_case_types)]
        /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
        /// enum Rule {
        ///     ab
        /// }
        ///
        /// let input = "ab";
        /// let mut state: Box<pest::ParserState<'_, Rule>> = pest::ParserState::new(input);
        /// let position = state.position();
        /// assert_eq!(position.pos(), 0);
        /// ```
        pub fn position(&self) -> &Position<'i> {
            &self.position
        }

        /// Returns the current atomicity of the `ParserState`.
        ///
        /// # Examples
        ///
        /// ```
        /// # use pest;
        /// # use pest::Atomicity;
        /// # #[allow(non_camel_case_types)]
        /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
        /// enum Rule {
        ///     ab
        /// }
        ///
        /// let input = "ab";
        /// let mut state: Box<pest::ParserState<'_, Rule>> = pest::ParserState::new(input);
        /// let atomicity = state.atomicity();
        /// assert_eq!(atomicity, Atomicity::NonAtomic);
        /// ```
        pub fn atomicity(&self) -> Atomicity {
            self.atomicity
        }

        #[inline]
        fn inc_call_check_limit(mut self: Box<Self>) -> ParseResult<Box<Self>> {
            if self.call_tracker.limit_reached() {
                return Err(self);
            }
            self.call_tracker.increment_depth();
            Ok(self)
        }

        #[inline]
        fn reached_call_limit(&self) -> bool {
            self.call_tracker.limit_reached()
        }

        /// Wrapper needed to generate tokens. This will associate the `R` type rule to the closure
        /// meant to match the rule.
        ///
        /// # Examples
        ///
        /// ```
        /// # use pest;
        /// # #[allow(non_camel_case_types)]
        /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
        /// enum Rule {
        ///     a
        /// }
        ///
        /// let input = "a";
        /// let pairs: Vec<_> = pest::state(input, |state| {
        ///     state.rule(Rule::a, |s| Ok(s))
        /// }).unwrap().collect();
        ///
        /// assert_eq!(pairs.len(), 1);
        /// ```
        #[inline]
        pub fn rule<F>(mut self: Box<Self>, rule: R, f: F) -> ParseResult<Box<Self>>
        where
            F: FnOnce(Box<Self>) -> ParseResult<Box<Self>>,
        {
            self = self.inc_call_check_limit()?;
            let actual_pos = self.position.pos();
            let index = self.queue.len();

            let (pos_attempts_index, neg_attempts_index) = if actual_pos == self.attempt_pos {
                (self.pos_attempts.len(), self.neg_attempts.len())
            } else {
                // Attempts have not been cleared yet since the attempt_pos is older.
                (0, 0)
            };

            if self.lookahead == Lookahead::None && self.atomicity != Atomicity::Atomic {
                // Pair's position will only be known after running the closure.
                self.queue.push(QueueableToken::Start {
                    end_token_index: 0,
                    input_pos: actual_pos,
                });
            }

            let attempts = self.attempts_at(actual_pos);

            let result = f(self);

            match result {
                Ok(mut new_state) => {
                    if new_state.lookahead == Lookahead::Negative {
                        new_state.track(
                            rule,
                            actual_pos,
                            pos_attempts_index,
                            neg_attempts_index,
                            attempts,
                        );
                    }

                    if new_state.lookahead == Lookahead::None
                        && new_state.atomicity != Atomicity::Atomic
                    {
                        // Storing the pair's index in the first token that was added before the closure was
                        // run.
                        let new_index = new_state.queue.len();
                        match new_state.queue[index] {
                            QueueableToken::Start {
                                ref mut end_token_index,
                                ..
                            } => *end_token_index = new_index,
                            _ => unreachable!(),
                        };

                        let new_pos = new_state.position.pos();

                        new_state.queue.push(QueueableToken::End {
                            start_token_index: index,
                            rule,
                            input_pos: new_pos,
                        });
                    }

                    Ok(new_state)
                }
                Err(mut new_state) => {
                    if new_state.lookahead != Lookahead::Negative {
                        new_state.track(
                            rule,
                            actual_pos,
                            pos_attempts_index,
                            neg_attempts_index,
                            attempts,
                        );
                    }

                    if new_state.lookahead == Lookahead::None
                        && new_state.atomicity != Atomicity::Atomic
                    {
                        new_state.queue.truncate(index);
                    }

                    Err(new_state)
                }
            }
        }

        fn attempts_at(&self, pos: usize) -> usize {
            if self.attempt_pos == pos {
                self.pos_attempts.len() + self.neg_attempts.len()
            } else {
                0
            }
        }

        fn track(
            &mut self,
            rule: R,
            pos: usize,
            pos_attempts_index: usize,
            neg_attempts_index: usize,
            prev_attempts: usize,
        ) {
            if self.atomicity == Atomicity::Atomic {
                return;
            }

            // If nested rules made no progress, there is no use to report them; it's only useful to
            // track the current rule, the exception being when only one attempt has been made during
            // the children rules.
            let curr_attempts = self.attempts_at(pos);
            if curr_attempts > prev_attempts && curr_attempts - prev_attempts == 1 {
                return;
            }

            if pos == self.attempt_pos {
                self.pos_attempts.truncate(pos_attempts_index);
                self.neg_attempts.truncate(neg_attempts_index);
            }

            if pos > self.attempt_pos {
                self.pos_attempts.clear();
                self.neg_attempts.clear();
                self.attempt_pos = pos;
            }

            let attempts = if self.lookahead != Lookahead::Negative {
                &mut self.pos_attempts
            } else {
                &mut self.neg_attempts
            };

            if pos == self.attempt_pos {
                attempts.push(rule);
            }
        }

        /// Starts a sequence of transformations provided by `f` from the `Box<ParserState>`. Returns
        /// the same `Result` returned by `f` in the case of an `Ok`, or `Err` with the current
        /// `Box<ParserState>` otherwise.
        ///
        /// This method is useful to parse sequences that only match together which usually come in the
        /// form of chained `Result`s with
        /// [`Result::and_then`](https://doc.rust-lang.org/std/result/enum.Result.html#method.and_then).
        ///
        ///
        /// # Examples
        ///
        /// ```
        /// # use pest;
        /// # #[allow(non_camel_case_types)]
        /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
        /// enum Rule {
        ///     a
        /// }
        ///
        /// let input = "a";
        /// let pairs: Vec<_> = pest::state(input, |state| {
        ///     state.sequence(|s| {
        ///         s.rule(Rule::a, |s| Ok(s)).and_then(|s| {
        ///             s.match_string("b")
        ///         })
        ///     }).or_else(|s| {
        ///         Ok(s)
        ///     })
        /// }).unwrap().collect();
        ///
        /// assert_eq!(pairs.len(), 0);
        /// ```
        #[inline]
        pub fn sequence<F>(mut self: Box<Self>, f: F) -> ParseResult<Box<Self>>
        where
            F: FnOnce(Box<Self>) -> ParseResult<Box<Self>>,
        {
            self = self.inc_call_check_limit()?;
            let token_index = self.queue.len();
            let initial_pos = self.position;

            let result = f(self);

            match result {
                Ok(new_state) => Ok(new_state),
                Err(mut new_state) => {
                    // Restore the initial position and truncate the token queue.
                    new_state.position = initial_pos;
                    new_state.queue.truncate(token_index);
                    Err(new_state)
                }
            }
        }

        /// Repeatedly applies the transformation provided by `f` from the `Box<ParserState>`. Returns
        /// `Ok` with the updated `Box<ParserState>` returned by `f` wrapped up in an `Err`.
        ///
        /// # Examples
        ///
        /// ```
        /// # use pest;
        /// # #[allow(non_camel_case_types)]
        /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
        /// enum Rule {
        ///     ab
        /// }
        ///
        /// let input = "aab";
        /// let mut state: Box<pest::ParserState<'_, Rule>> = pest::ParserState::new(input);
        /// let mut result = state.repeat(|s| {
        ///     s.match_string("a")
        /// });
        /// assert!(result.is_ok());
        /// assert_eq!(result.unwrap().position().pos(), 2);
        ///
        /// state = pest::ParserState::new(input);
        /// result = state.repeat(|s| {
        ///     s.match_string("b")
        /// });
        /// assert!(result.is_ok());
        /// assert_eq!(result.unwrap().position().pos(), 0);
        /// ```
        #[inline]
        pub fn repeat<F>(mut self: Box<Self>, mut f: F) -> ParseResult<Box<Self>>
        where
            F: FnMut(Box<Self>) -> ParseResult<Box<Self>>,
        {
            self = self.inc_call_check_limit()?;
            let mut result = f(self);

            loop {
                match result {
                    Ok(state) => result = f(state),
                    Err(state) => return Ok(state),
                };
            }
        }

        /// Optionally applies the transformation provided by `f` from the `Box<ParserState>`. Returns
        /// `Ok` with the updated `Box<ParserState>` returned by `f` regardless of the `Result`.
        ///
        /// # Examples
        ///
        /// ```
        /// # use pest;
        /// # #[allow(non_camel_case_types)]
        /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
        /// enum Rule {
        ///     ab
        /// }
        ///
        /// let input = "ab";
        /// let mut state: Box<pest::ParserState<'_, Rule>> = pest::ParserState::new(input);
        /// let result = state.optional(|s| {
        ///     s.match_string("ab")
        /// });
        /// assert!(result.is_ok());
        ///
        /// state = pest::ParserState::new(input);
        /// let result = state.optional(|s| {
        ///     s.match_string("ac")
        /// });
        /// assert!(result.is_ok());
        /// ```
        #[inline]
        pub fn optional<F>(mut self: Box<Self>, f: F) -> ParseResult<Box<Self>>
        where
            F: FnOnce(Box<Self>) -> ParseResult<Box<Self>>,
        {
            self = self.inc_call_check_limit()?;
            match f(self) {
                Ok(state) | Err(state) => Ok(state),
            }
        }

        /// Attempts to match a single character based on a filter function. Returns `Ok` with the
        /// updated `Box<ParserState>` if successful, or `Err` with the updated `Box<ParserState>`
        /// otherwise.
        ///
        /// # Examples
        ///
        /// ```
        /// # use pest;
        /// # #[allow(non_camel_case_types)]
        /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
        /// enum Rule {}
        ///
        /// let input = "ab";
        /// let mut state: Box<pest::ParserState<'_, Rule>> = pest::ParserState::new(input);
        /// let result = state.match_char_by(|c| c.is_ascii());
        /// assert!(result.is_ok());
        /// assert_eq!(result.unwrap().position().pos(), 1);
        ///
        /// let input = "❤";
        /// let mut state: Box<pest::ParserState<'_, Rule>> = pest::ParserState::new(input);
        /// let result = state.match_char_by(|c| c.is_ascii());
        /// assert!(result.is_err());
        /// assert_eq!(result.unwrap_err().position().pos(), 0);
        /// ```
        #[inline]
        pub fn match_char_by<F>(mut self: Box<Self>, f: F) -> ParseResult<Box<Self>>
        where
            F: FnOnce(char) -> bool,
        {
            if self.position.match_char_by(f) {
                Ok(self)
            } else {
                Err(self)
            }
        }

        /// Attempts to match the given string. Returns `Ok` with the updated `Box<ParserState>` if
        /// successful, or `Err` with the updated `Box<ParserState>` otherwise.
        ///
        /// # Examples
        ///
        /// ```
        /// # use pest;
        /// # #[allow(non_camel_case_types)]
        /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
        /// enum Rule {}
        ///
        /// let input = "ab";
        /// let mut state: Box<pest::ParserState<'_, Rule>> = pest::ParserState::new(input);
        /// let mut result = state.match_string("ab");
        /// assert!(result.is_ok());
        /// assert_eq!(result.unwrap().position().pos(), 2);
        ///
        /// state = pest::ParserState::new(input);
        /// result = state.match_string("ac");
        /// assert!(result.is_err());
        /// assert_eq!(result.unwrap_err().position().pos(), 0);
        /// ```
        #[inline]
        pub fn match_string(mut self: Box<Self>, string: &str) -> ParseResult<Box<Self>> {
            if self.position.match_string(string) {
                Ok(self)
            } else {
                Err(self)
            }
        }

        /// Attempts to case-insensitively match the given string. Returns `Ok` with the updated
        /// `Box<ParserState>` if successful, or `Err` with the updated `Box<ParserState>` otherwise.
        ///
        /// # Examples
        ///
        /// ```
        /// # use pest;
        /// # #[allow(non_camel_case_types)]
        /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
        /// enum Rule {}
        ///
        /// let input = "ab";
        /// let mut state: Box<pest::ParserState<'_, Rule>> = pest::ParserState::new(input);
        /// let mut result = state.match_insensitive("AB");
        /// assert!(result.is_ok());
        /// assert_eq!(result.unwrap().position().pos(), 2);
        ///
        /// state = pest::ParserState::new(input);
        /// result = state.match_insensitive("AC");
        /// assert!(result.is_err());
        /// assert_eq!(result.unwrap_err().position().pos(), 0);
        /// ```
        #[inline]
        pub fn match_insensitive(mut self: Box<Self>, string: &str) -> ParseResult<Box<Self>> {
            if self.position.match_insensitive(string) {
                Ok(self)
            } else {
                Err(self)
            }
        }

        /// Attempts to match a single character from the given range. Returns `Ok` with the updated
        /// `Box<ParserState>` if successful, or `Err` with the updated `Box<ParserState>` otherwise.
        ///
        /// # Caution
        /// The provided `range` is intepreted as inclusive.
        ///
        /// # Examples
        ///
        /// ```
        /// # use pest;
        /// # #[allow(non_camel_case_types)]
        /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
        /// enum Rule {}
        ///
        /// let input = "ab";
        /// let mut state: Box<pest::ParserState<'_, Rule>> = pest::ParserState::new(input);
        /// let mut result = state.match_range('a'..'z');
        /// assert!(result.is_ok());
        /// assert_eq!(result.unwrap().position().pos(), 1);
        ///
        /// state = pest::ParserState::new(input);
        /// result = state.match_range('A'..'Z');
        /// assert!(result.is_err());
        /// assert_eq!(result.unwrap_err().position().pos(), 0);
        /// ```
        #[inline]
        pub fn match_range(mut self: Box<Self>, range: Range<char>) -> ParseResult<Box<Self>> {
            if self.position.match_range(range) {
                Ok(self)
            } else {
                Err(self)
            }
        }

        /// Attempts to skip `n` characters forward. Returns `Ok` with the updated `Box<ParserState>`
        /// if successful, or `Err` with the updated `Box<ParserState>` otherwise.
        ///
        /// # Examples
        ///
        /// ```
        /// # use pest;
        /// # #[allow(non_camel_case_types)]
        /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
        /// enum Rule {}
        ///
        /// let input = "ab";
        /// let mut state: Box<pest::ParserState<'_, Rule>> = pest::ParserState::new(input);
        /// let mut result = state.skip(1);
        /// assert!(result.is_ok());
        /// assert_eq!(result.unwrap().position().pos(), 1);
        ///
        /// state = pest::ParserState::new(input);
        /// result = state.skip(3);
        /// assert!(result.is_err());
        /// assert_eq!(result.unwrap_err().position().pos(), 0);
        /// ```
        #[inline]
        pub fn skip(mut self: Box<Self>, n: usize) -> ParseResult<Box<Self>> {
            if self.position.skip(n) {
                Ok(self)
            } else {
                Err(self)
            }
        }

        /// Attempts to skip forward until one of the given strings is found. Returns `Ok` with the
        /// updated `Box<ParserState>` whether or not one of the strings is found.
        ///
        /// # Examples
        ///
        /// ```
        /// # use pest;
        /// # #[allow(non_camel_case_types)]
        /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
        /// enum Rule {}
        ///
        /// let input = "abcd";
        /// let mut state: Box<pest::ParserState<'_, Rule>> = pest::ParserState::new(input);
        /// let mut result = state.skip_until(&["c", "d"]);
        /// assert!(result.is_ok());
        /// assert_eq!(result.unwrap().position().pos(), 2);
        /// ```
        #[inline]
        pub fn skip_until(mut self: Box<Self>, strings: &[&str]) -> ParseResult<Box<Self>> {
            self.position.skip_until(strings);
            Ok(self)
        }

        /// Attempts to match the start of the input. Returns `Ok` with the current `Box<ParserState>`
        /// if the parser has not yet advanced, or `Err` with the current `Box<ParserState>` otherwise.
        ///
        /// # Examples
        ///
        /// ```
        /// # use pest;
        /// # #[allow(non_camel_case_types)]
        /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
        /// enum Rule {}
        ///
        /// let input = "ab";
        /// let mut state: Box<pest::ParserState<'_, Rule>> = pest::ParserState::new(input);
        /// let mut result = state.start_of_input();
        /// assert!(result.is_ok());
        ///
        /// state = pest::ParserState::new(input);
        /// state = state.match_string("ab").unwrap();
        /// result = state.start_of_input();
        /// assert!(result.is_err());
        /// ```
        #[inline]
        pub fn start_of_input(self: Box<Self>) -> ParseResult<Box<Self>> {
            if self.position.at_start() {
                Ok(self)
            } else {
                Err(self)
            }
        }

        /// Attempts to match the end of the input. Returns `Ok` with the current `Box<ParserState>` if
        /// there is no input remaining, or `Err` with the current `Box<ParserState>` otherwise.
        ///
        /// # Examples
        ///
        /// ```
        /// # use pest;
        /// # #[allow(non_camel_case_types)]
        /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
        /// enum Rule {}
        ///
        /// let input = "ab";
        /// let mut state: Box<pest::ParserState<'_, Rule>> = pest::ParserState::new(input);
        /// let mut result = state.end_of_input();
        /// assert!(result.is_err());
        ///
        /// state = pest::ParserState::new(input);
        /// state = state.match_string("ab").unwrap();
        /// result = state.end_of_input();
        /// assert!(result.is_ok());
        /// ```
        #[inline]
        pub fn end_of_input(self: Box<Self>) -> ParseResult<Box<Self>> {
            if self.position.at_end() {
                Ok(self)
            } else {
                Err(self)
            }
        }

        /// Starts a lookahead transformation provided by `f` from the `Box<ParserState>`. It returns
        /// `Ok` with the current `Box<ParserState>` if `f` also returns an `Ok`, or `Err` with the current
        /// `Box<ParserState>` otherwise. If `is_positive` is `false`, it swaps the `Ok` and `Err`
        /// together, negating the `Result`.
        ///
        /// # Examples
        ///
        /// ```
        /// # use pest;
        /// # #[allow(non_camel_case_types)]
        /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
        /// enum Rule {
        ///     a
        /// }
        ///
        /// let input = "a";
        /// let pairs: Vec<_> = pest::state(input, |state| {
        ///     state.lookahead(true, |state| {
        ///         state.rule(Rule::a, |s| Ok(s))
        ///     })
        /// }).unwrap().collect();
        ///
        /// assert_eq!(pairs.len(), 0);
        /// ```
        #[inline]
        pub fn lookahead<F>(mut self: Box<Self>, is_positive: bool, f: F) -> ParseResult<Box<Self>>
        where
            F: FnOnce(Box<Self>) -> ParseResult<Box<Self>>,
        {
            self = self.inc_call_check_limit()?;
            let initial_lookahead = self.lookahead;

            self.lookahead = if is_positive {
                match initial_lookahead {
                    Lookahead::None | Lookahead::Positive => Lookahead::Positive,
                    Lookahead::Negative => Lookahead::Negative,
                }
            } else {
                match initial_lookahead {
                    Lookahead::None | Lookahead::Positive => Lookahead::Negative,
                    Lookahead::Negative => Lookahead::Positive,
                }
            };

            let initial_pos = self.position;

            let result = f(self.checkpoint());

            let result_state = match result {
                Ok(mut new_state) => {
                    new_state.position = initial_pos;
                    new_state.lookahead = initial_lookahead;
                    Ok(new_state.restore())
                }
                Err(mut new_state) => {
                    new_state.position = initial_pos;
                    new_state.lookahead = initial_lookahead;
                    Err(new_state.restore())
                }
            };

            if is_positive {
                result_state
            } else {
                match result_state {
                    Ok(state) => Err(state),
                    Err(state) => Ok(state),
                }
            }
        }

        /// Transformation which stops `Token`s from being generated according to `is_atomic`.
        ///
        /// # Examples
        ///
        /// ```
        /// # use pest::{self, Atomicity};
        /// # #[allow(non_camel_case_types)]
        /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
        /// enum Rule {
        ///     a
        /// }
        ///
        /// let input = "a";
        /// let pairs: Vec<_> = pest::state(input, |state| {
        ///     state.atomic(Atomicity::Atomic, |s| {
        ///         s.rule(Rule::a, |s| Ok(s))
        ///     })
        /// }).unwrap().collect();
        ///
        /// assert_eq!(pairs.len(), 0);
        /// ```
        #[inline]
        pub fn atomic<F>(mut self: Box<Self>, atomicity: Atomicity, f: F) -> ParseResult<Box<Self>>
        where
            F: FnOnce(Box<Self>) -> ParseResult<Box<Self>>,
        {
            self = self.inc_call_check_limit()?;
            let initial_atomicity = self.atomicity;
            let should_toggle = self.atomicity != atomicity;

            if should_toggle {
                self.atomicity = atomicity;
            }

            let result = f(self);

            match result {
                Ok(mut new_state) => {
                    if should_toggle {
                        new_state.atomicity = initial_atomicity;
                    }
                    Ok(new_state)
                }
                Err(mut new_state) => {
                    if should_toggle {
                        new_state.atomicity = initial_atomicity;
                    }
                    Err(new_state)
                }
            }
        }

        /// Evaluates the result of closure `f` and pushes the span of the input consumed from before
        /// `f` is called to after `f` is called to the stack. Returns `Ok(Box<ParserState>)` if `f` is
        /// called successfully, or `Err(Box<ParserState>)` otherwise.
        ///
        /// # Examples
        ///
        /// ```
        /// # use pest;
        /// # #[allow(non_camel_case_types)]
        /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
        /// enum Rule {}
        ///
        /// let input = "ab";
        /// let mut state: Box<pest::ParserState<'_, Rule>> = pest::ParserState::new(input);
        /// let mut result = state.stack_push(|state| state.match_string("a"));
        /// assert!(result.is_ok());
        /// assert_eq!(result.unwrap().position().pos(), 1);
        /// ```
        #[inline]
        pub fn stack_push<F>(mut self: Box<Self>, f: F) -> ParseResult<Box<Self>>
        where
            F: FnOnce(Box<Self>) -> ParseResult<Box<Self>>,
        {
            self = self.inc_call_check_limit()?;
            let start = self.position;

            let result = f(self);

            match result {
                Ok(mut state) => {
                    let end = state.position;
                    state.stack.push(start.span(&end));
                    Ok(state)
                }
                Err(state) => Err(state),
            }
        }

        /// Peeks the top of the stack and attempts to match the string. Returns `Ok(Box<ParserState>)`
        /// if the string is matched successfully, or `Err(Box<ParserState>)` otherwise.
        ///
        /// # Examples
        ///
        /// ```
        /// # use pest;
        /// # #[allow(non_camel_case_types)]
        /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
        /// enum Rule {}
        ///
        /// let input = "aa";
        /// let mut state: Box<pest::ParserState<'_, Rule>> = pest::ParserState::new(input);
        /// let mut result = state.stack_push(|state| state.match_string("a")).and_then(
        ///     |state| state.stack_peek()
        /// );
        /// assert!(result.is_ok());
        /// assert_eq!(result.unwrap().position().pos(), 2);
        /// ```
        #[inline]
        pub fn stack_peek(self: Box<Self>) -> ParseResult<Box<Self>> {
            let string = self
                .stack
                .peek()
                .expect("peek was called on empty stack")
                .as_str();
            self.match_string(string)
        }

        /// Pops the top of the stack and attempts to match the string. Returns `Ok(Box<ParserState>)`
        /// if the string is matched successfully, or `Err(Box<ParserState>)` otherwise.
        ///
        /// # Examples
        ///
        /// ```
        /// # use pest;
        /// # #[allow(non_camel_case_types)]
        /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
        /// enum Rule {}
        ///
        /// let input = "aa";
        /// let mut state: Box<pest::ParserState<'_, Rule>> = pest::ParserState::new(input);
        /// let mut result = state.stack_push(|state| state.match_string("a")).and_then(
        ///     |state| state.stack_pop()
        /// );
        /// assert!(result.is_ok());
        /// assert_eq!(result.unwrap().position().pos(), 2);
        /// ```
        #[inline]
        pub fn stack_pop(mut self: Box<Self>) -> ParseResult<Box<Self>> {
            let string = self
                .stack
                .pop()
                .expect("pop was called on empty stack")
                .as_str();
            self.match_string(string)
        }

        /// Matches part of the state of the stack.
        ///
        /// # Examples
        ///
        /// ```
        /// # use pest::{self, MatchDir};
        /// # #[allow(non_camel_case_types)]
        /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
        /// enum Rule {}
        ///
        /// let input = "abcd cd cb";
        /// let mut state: Box<pest::ParserState<'_, Rule>> = pest::ParserState::new(input);
        /// let mut result = state
        ///     .stack_push(|state| state.match_string("a"))
        ///     .and_then(|state| state.stack_push(|state| state.match_string("b")))
        ///     .and_then(|state| state.stack_push(|state| state.match_string("c")))
        ///     .and_then(|state| state.stack_push(|state| state.match_string("d")))
        ///     .and_then(|state| state.match_string(" "))
        ///     .and_then(|state| state.stack_match_peek_slice(2, None, MatchDir::BottomToTop))
        ///     .and_then(|state| state.match_string(" "))
        ///     .and_then(|state| state.stack_match_peek_slice(1, Some(-1), MatchDir::TopToBottom));
        /// assert!(result.is_ok());
        /// assert_eq!(result.unwrap().position().pos(), 10);
        /// ```
        #[inline]
        pub fn stack_match_peek_slice(
            mut self: Box<Self>,
            start: i32,
            end: Option<i32>,
            match_dir: MatchDir,
        ) -> ParseResult<Box<Self>> {
            let range = match constrain_idxs(start, end, self.stack.len()) {
                Some(r) => r,
                None => return Err(self),
            };
            // return true if an empty sequence is requested
            if range.end <= range.start {
                return Ok(self);
            }

            let mut position = self.position;
            let result = {
                let mut iter_b2t = self.stack[range].iter();
                let matcher = |span: &Span<'_>| position.match_string(span.as_str());
                match match_dir {
                    MatchDir::BottomToTop => iter_b2t.all(matcher),
                    MatchDir::TopToBottom => iter_b2t.rev().all(matcher),
                }
            };
            if result {
                self.position = position;
                Ok(self)
            } else {
                Err(self)
            }
        }

        /// Matches the full state of the stack.
        ///
        /// # Examples
        ///
        /// ```
        /// # use pest;
        /// # #[allow(non_camel_case_types)]
        /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
        /// enum Rule {}
        ///
        /// let input = "abba";
        /// let mut state: Box<pest::ParserState<'_, Rule>> = pest::ParserState::new(input);
        /// let mut result = state
        ///     .stack_push(|state| state.match_string("a"))
        ///     .and_then(|state| { state.stack_push(|state| state.match_string("b")) })
        ///     .and_then(|state| state.stack_match_peek());
        /// assert!(result.is_ok());
        /// assert_eq!(result.unwrap().position().pos(), 4);
        /// ```
        #[inline]
        pub fn stack_match_peek(self: Box<Self>) -> ParseResult<Box<Self>> {
            self.stack_match_peek_slice(0, None, MatchDir::TopToBottom)
        }

        /// Matches the full state of the stack. This method will clear the stack as it evaluates.
        ///
        /// # Examples
        ///
        /// ```
        /// /// # use pest;
        /// # #[allow(non_camel_case_types)]
        /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
        /// enum Rule {}
        ///
        /// let input = "aaaa";
        /// let mut state: Box<pest::ParserState<'_, Rule>> = pest::ParserState::new(input);
        /// let mut result = state.stack_push(|state| state.match_string("a")).and_then(|state| {
        ///     state.stack_push(|state| state.match_string("a"))
        /// }).and_then(|state| state.stack_match_peek());
        /// assert!(result.is_ok());
        /// assert_eq!(result.unwrap().position().pos(), 4);
        /// ```
        #[inline]
        pub fn stack_match_pop(mut self: Box<Self>) -> ParseResult<Box<Self>> {
            let mut position = self.position;
            let mut result = true;
            while let Some(span) = self.stack.pop() {
                result = position.match_string(span.as_str());
                if !result {
                    break;
                }
            }

            if result {
                self.position = position;
                Ok(self)
            } else {
                Err(self)
            }
        }

        /// Drops the top of the stack. Returns `Ok(Box<ParserState>)` if there was a value to drop, or
        /// `Err(Box<ParserState>)` otherwise.
        ///
        /// # Examples
        ///
        /// ```
        /// # use pest;
        /// # #[allow(non_camel_case_types)]
        /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
        /// enum Rule {}
        ///
        /// let input = "aa";
        /// let mut state: Box<pest::ParserState<'_, Rule>> = pest::ParserState::new(input);
        /// let mut result = state.stack_push(|state| state.match_string("a")).and_then(
        ///     |state| state.stack_drop()
        /// );
        /// assert!(result.is_ok());
        /// assert_eq!(result.unwrap().position().pos(), 1);
        /// ```
        #[inline]
        pub fn stack_drop(mut self: Box<Self>) -> ParseResult<Box<Self>> {
            match self.stack.pop() {
                Some(_) => Ok(self),
                None => Err(self),
            }
        }

        /// Restores the original state of the `ParserState` when `f` returns an `Err`. Currently,
        /// this method only restores the stack.
        ///
        /// # Examples
        ///
        /// ```
        /// # use pest;
        /// # #[allow(non_camel_case_types)]
        /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
        /// enum Rule {}
        ///
        /// let input = "ab";
        /// let mut state: Box<pest::ParserState<'_, Rule>> = pest::ParserState::new(input);
        /// let mut result = state.restore_on_err(|state| state.stack_push(|state|
        ///     state.match_string("a")).and_then(|state| state.match_string("a"))
        /// );
        ///
        /// assert!(result.is_err());
        ///
        /// // Since the the rule doesn't match, the "a" pushed to the stack will be removed.
        /// let catch_panic = std::panic::catch_unwind(|| result.unwrap_err().stack_pop());
        /// assert!(catch_panic.is_err());
        /// ```
        #[inline]
        pub fn restore_on_err<F>(self: Box<Self>, f: F) -> ParseResult<Box<Self>>
        where
            F: FnOnce(Box<Self>) -> ParseResult<Box<Self>>,
        {
            match f(self.checkpoint()) {
                Ok(state) => Ok(state.checkpoint_ok()),
                Err(state) => Err(state.restore()),
            }
        }

        // Mark the current state as a checkpoint and return the `Box`.
        #[inline]
        pub(crate) fn checkpoint(mut self: Box<Self>) -> Box<Self> {
            self.stack.snapshot();
            self
        }

        // The checkpoint was cleared successfully
        // so remove it without touching other stack state.
        #[inline]
        pub(crate) fn checkpoint_ok(mut self: Box<Self>) -> Box<Self> {
            self.stack.clear_snapshot();
            self
        }

        // Restore the current state to the most recent checkpoint.
        #[inline]
        pub(crate) fn restore(mut self: Box<Self>) -> Box<Self> {
            self.stack.restore();
            self
        }
    }

    fn constrain_idxs(start: i32, end: Option<i32>, len: usize) -> Option<Range<usize>> {
        let start_norm = normalize_index(start, len)?;
        let end_norm = end.map_or(Some(len), |e| normalize_index(e, len))?;
        Some(start_norm..end_norm)
    }

    /// Normalizes the index using its sequence’s length.
    /// Returns `None` if the normalized index is OOB.
    fn normalize_index(i: i32, len: usize) -> Option<usize> {
        if i > len as i32 {
            None
        } else if i >= 0 {
            Some(i as usize)
        } else {
            let real_i = len as i32 + i;
            if real_i >= 0 {
                Some(real_i as usize)
            } else {
                None
            }
        }
    }

    #[cfg(test)]
    mod test {
        use super::*;

        #[test]
        fn normalize_index_pos() {
            assert_eq!(normalize_index(4, 6), Some(4));
            assert_eq!(normalize_index(5, 5), Some(5));
            assert_eq!(normalize_index(6, 3), None);
        }

        #[test]
        fn normalize_index_neg() {
            assert_eq!(normalize_index(-4, 6), Some(2));
            assert_eq!(normalize_index(-5, 5), Some(0));
            assert_eq!(normalize_index(-6, 3), None);
        }
    }