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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::rc::Rc;
    use alloc::vec::Vec;
    use core::fmt;
    use core::str;

    use super::queueable_token::QueueableToken;
    use crate::position;
    use crate::token::Token;
    use crate::RuleType;

    /// An iterator over [`Token`]s. It is created by [`Pair::tokens`] and [`Pairs::tokens`].
    ///
    /// [`Token`]: ../enum.Token.html
    /// [`Pair::tokens`]: struct.Pair.html#method.tokens
    /// [`Pairs::tokens`]: struct.Pairs.html#method.tokens
    #[derive(Clone)]
    pub struct Tokens<'i, R> {
        /// # Safety:
        ///
        /// All `QueueableToken`s' `input_pos` must be valid character boundary indices into `input`.
        queue: Rc<Vec<QueueableToken<R>>>,
        input: &'i str,
        start: usize,
        end: usize,
    }

    // TODO(safety): QueueableTokens must be valid indices into input.
    pub fn new<R: RuleType>(
        queue: Rc<Vec<QueueableToken<R>>>,
        input: &str,
        start: usize,
        end: usize,
    ) -> Tokens<'_, R> {
        if cfg!(debug_assertions) {
            for tok in queue.iter() {
                match *tok {
                    QueueableToken::Start { input_pos, .. } | QueueableToken::End { input_pos, .. } => {
                        assert!(
                            input.get(input_pos..).is_some(),
                            "💥 UNSAFE `Tokens` CREATED 💥"
                        )
                    }
                }
            }
        }

        Tokens {
            queue,
            input,
            start,
            end,
        }
    }

    impl<'i, R: RuleType> Tokens<'i, R> {
        fn create_token(&self, index: usize) -> Token<'i, R> {
            match self.queue[index] {
                QueueableToken::Start {
                    end_token_index,
                    input_pos,
                } => {
                    let rule = match self.queue[end_token_index] {
                        QueueableToken::End { rule, .. } => rule,
                        _ => unreachable!(),
                    };

                    Token::Start {
                        rule,
                        // QueueableTokens are safely created.
                        pos: unsafe { position::Position::new_unchecked(self.input, input_pos) },
                    }
                }
                QueueableToken::End {
                    rule, input_pos, ..
                } => {
                    Token::End {
                        rule,
                        // QueueableTokens are safely created.
                        pos: unsafe { position::Position::new_unchecked(self.input, input_pos) },
                    }
                }
            }
        }
    }

    impl<'i, R: RuleType> Iterator for Tokens<'i, R> {
        type Item = Token<'i, R>;

        fn next(&mut self) -> Option<Self::Item> {
            if self.start >= self.end {
                return None;
            }

            let token = self.create_token(self.start);

            self.start += 1;

            Some(token)
        }
    }

    impl<'i, R: RuleType> DoubleEndedIterator for Tokens<'i, R> {
        fn next_back(&mut self) -> Option<Self::Item> {
            if self.end <= self.start {
                return None;
            }

            let token = self.create_token(self.end - 1);

            self.end -= 1;

            Some(token)
        }
    }

    impl<'i, R: RuleType> fmt::Debug for Tokens<'i, R> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_list().entries(self.clone()).finish()
        }
    }

    #[cfg(test)]
    mod tests {
        use super::super::super::macros::tests::*;
        use super::super::super::Parser;
        use super::Token;
        use alloc::vec::Vec;

        #[test]
        fn double_ended_iter_for_tokens() {
            let pairs = AbcParser::parse(Rule::a, "abcde").unwrap();
            let mut tokens = pairs.clone().tokens().collect::<Vec<Token<'_, Rule>>>();
            tokens.reverse();
            let reverse_tokens = pairs.tokens().rev().collect::<Vec<Token<'_, Rule>>>();
            assert_eq!(tokens, reverse_tokens);
        }
    }