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    // Copyright 2015 The Servo Project Developers. See the
    // COPYRIGHT file at the top-level directory of this distribution.
    //
    // 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.

    //! 3.3.2 Explicit Levels and Directions
    //!
    //! <http://www.unicode.org/reports/tr9/#Explicit_Levels_and_Directions>

    use super::char_data::{BidiClass, is_rtl};
    use super::level::Level;

    use BidiClass::*;

    /// Compute explicit embedding levels for one paragraph of text (X1-X8).
    ///
    /// `processing_classes[i]` must contain the `BidiClass` of the char at byte index `i`,
    /// for each char in `text`.
    #[cfg_attr(feature = "flame_it", flame)]
    pub fn compute(
        text: &str,
        para_level: Level,
        original_classes: &[BidiClass],
        levels: &mut [Level],
        processing_classes: &mut [BidiClass],
    ) {
        assert_eq!(text.len(), original_classes.len());

        // <http://www.unicode.org/reports/tr9/#X1>
        let mut stack = DirectionalStatusStack::new();
        stack.push(para_level, OverrideStatus::Neutral);

        let mut overflow_isolate_count = 0u32;
        let mut overflow_embedding_count = 0u32;
        let mut valid_isolate_count = 0u32;

        for (i, c) in text.char_indices() {
            match original_classes[i] {

                // Rules X2-X5c
                RLE | LRE | RLO | LRO | RLI | LRI | FSI => {
                    let last_level = stack.last().level;

                    // X5a-X5c: Isolate initiators get the level of the last entry on the stack.
                    let is_isolate = matches!(original_classes[i], RLI | LRI | FSI);
                    if is_isolate {
                        levels[i] = last_level;
                        match stack.last().status {
                            OverrideStatus::RTL => processing_classes[i] = R,
                            OverrideStatus::LTR => processing_classes[i] = L,
                            _ => {}
                        }
                    }

                    let new_level = if is_rtl(original_classes[i]) {
                        last_level.new_explicit_next_rtl()
                    } else {
                        last_level.new_explicit_next_ltr()
                    };
                    if new_level.is_ok() && overflow_isolate_count == 0 &&
                        overflow_embedding_count == 0
                    {
                        let new_level = new_level.unwrap();
                        stack.push(
                            new_level,
                            match original_classes[i] {
                                RLO => OverrideStatus::RTL,
                                LRO => OverrideStatus::LTR,
                                RLI | LRI | FSI => OverrideStatus::Isolate,
                                _ => OverrideStatus::Neutral,
                            },
                        );
                        if is_isolate {
                            valid_isolate_count += 1;
                        } else {
                            // The spec doesn't explicitly mention this step, but it is necessary.
                            // See the reference implementations for comparison.
                            levels[i] = new_level;
                        }
                    } else if is_isolate {
                        overflow_isolate_count += 1;
                    } else if overflow_isolate_count == 0 {
                        overflow_embedding_count += 1;
                    }
                }

                // <http://www.unicode.org/reports/tr9/#X6a>
                PDI => {
                    if overflow_isolate_count > 0 {
                        overflow_isolate_count -= 1;
                    } else if valid_isolate_count > 0 {
                        overflow_embedding_count = 0;
                        loop {
                            // Pop everything up to and including the last Isolate status.
                            match stack.vec.pop() {
                                None |
                                Some(Status { status: OverrideStatus::Isolate, .. }) => break,
                                _ => continue,
                            }
                        }
                        valid_isolate_count -= 1;
                    }
                    let last = stack.last();
                    levels[i] = last.level;
                    match last.status {
                        OverrideStatus::RTL => processing_classes[i] = R,
                        OverrideStatus::LTR => processing_classes[i] = L,
                        _ => {}
                    }
                }

                // <http://www.unicode.org/reports/tr9/#X7>
                PDF => {
                    if overflow_isolate_count > 0 {
                        continue;
                    }
                    if overflow_embedding_count > 0 {
                        overflow_embedding_count -= 1;
                        continue;
                    }
                    if stack.last().status != OverrideStatus::Isolate && stack.vec.len() >= 2 {
                        stack.vec.pop();
                    }
                    // The spec doesn't explicitly mention this step, but it is necessary.
                    // See the reference implementations for comparison.
                    levels[i] = stack.last().level;
                }

                // Nothing
                B | BN => {}

                // <http://www.unicode.org/reports/tr9/#X6>
                _ => {
                    let last = stack.last();
                    levels[i] = last.level;
                    match last.status {
                        OverrideStatus::RTL => processing_classes[i] = R,
                        OverrideStatus::LTR => processing_classes[i] = L,
                        _ => {}
                    }
                }
            }

            // Handle multi-byte characters.
            for j in 1..c.len_utf8() {
                levels[i + j] = levels[i];
                processing_classes[i + j] = processing_classes[i];
            }
        }
    }

    /// Entries in the directional status stack:
    struct Status {
        level: Level,
        status: OverrideStatus,
    }

    #[derive(PartialEq)]
    enum OverrideStatus {
        Neutral,
        RTL,
        LTR,
        Isolate,
    }

    struct DirectionalStatusStack {
        vec: Vec<Status>,
    }

    impl DirectionalStatusStack {
        fn new() -> Self {
            DirectionalStatusStack { vec: Vec::with_capacity(Level::max_explicit_depth() as usize + 2) }
        }

        fn push(&mut self, level: Level, status: OverrideStatus) {
            self.vec.push(Status { level, status });
        }

        fn last(&self) -> &Status {
            self.vec.last().unwrap()
        }
    }