kashida 0.3.1

Insert Kashidas/Tatweel into Arabic text, e.g. for justification purposes.
Documentation 
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#![allow(unused)]

use alloc::{boxed::Box, vec::Vec};
use core::iter;
use hashbrown::{HashMap, hash_map::Entry};
use itertools::Itertools;
use unicode_joining_type::{JoiningGroup, get_joining_group};

use crate::KashidaCandidate;
use crate::global::*;

fn is_alaph(c: char) -> bool {
    matches!(get_joining_group(c), JoiningGroup::Alaph)
}
fn is_lamadh(c: char) -> bool {
    matches!(get_joining_group(c), JoiningGroup::Lamadh)
}

// Useful resources: https://www.unicode.org/versions/Unicode15.0.0/ch09.pdf
//                   https://bug-attachments.documentfoundation.org/attachment.cgi?id=182206
#[must_use]
pub(crate) fn find_kashidas(input: &str) -> Box<[usize]> {
    let mut candidates: HashMap<_, KashidaCandidate> =
        HashMap::with_capacity(input.split_whitespace().count());

    let (word_segmenter, grapheme_segmenter) = super::get_segmenters();

    let words =
        word_segmenter.as_borrowed().segment_str(input).tuple_windows().filter_map(|(wb1, wb2)| {
            Some((&input[wb1..wb2], wb1)).filter(|s| !s.0.trim().is_empty())
        });

    for (word_idx, (word, word_start)) in words.enumerate() {
        let graphemes = grapheme_segmenter
            .as_borrowed()
            .segment_str(word)
            .tuple_windows()
            .map(|(gb1, gb2)| Some(&word[gb1..gb2]))
            .pad_using(2, |_| None)
            .tuple_windows();

        for glyph_window in graphemes {
            find_kashidas_in_glyph_run(glyph_window, input, |kc| {
                match candidates.entry(word_idx) {
                    Entry::Occupied(mut e)
                        if kc.bp_priority <= e.get().bp_priority
                            && kc.breakpoint > (word.len() / 2 + word_start) =>
                    {
                        e.insert(kc)
                    }
                    Entry::Occupied(_) => kc,
                    Entry::Vacant(e) => *e.insert(kc),
                };
            });
        }
    }

    let mut ret = candidates.into_values().collect::<Vec<_>>();
    ret.sort_by_key(|a| a.breakpoint);
    ret.sort_by_key(|a| a.bp_priority);
    ret.into_iter().map(|kc| kc.breakpoint).collect()
}

fn find_kashidas_in_glyph_run(
    (g1, g2): (Option<&str>, Option<&str>),
    input: &str,
    mut insert_candidate: impl FnMut(KashidaCandidate),
) {
    let breakpoint = |g: &str| g.as_ptr() as usize - input.as_ptr() as usize;
    match (g1, g2) {
        // If Input contains Kashida, that's the place
        (Some(g), _) if g.chars().all(is_kashida) => {
            insert_candidate(KashidaCandidate::new(breakpoint(g) + g.len(), 0));
        }

        // deal with لا early
        (Some(lam), Some(alef)) if lam.contains(is_lamadh) && alef.contains(is_alaph) => {}

        (Some(preceding), Some(g))
            if preceding.contains(joins_following) && g.contains(joins_preceding) =>
        {
            insert_candidate(KashidaCandidate::new(breakpoint(g), 1));
        }
        _ => {}
    }
}

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

    #[test]
    fn trisagion_candidates() {
        let input = "ܒܫܸܡ ܐܲܒ݂ܵܐ ܘܲܒ݂ܪܵܐ ܘܪܘܼܚܵܐ ܕܩܘܼܕ݂ܫܵܐ";
        let candidates = find_kashidas(input);

        assert_eq!(candidates, vec![6, 19, 30, 49, 68].into_boxed_slice());
    }
}