crop-next-prev-graphemes.rs

use crop::Rope;

#[must_use]
pub fn prev_grapheme_boundary(rope: &Rope, mut byte_offset: usize) -> Option<usize> {
    if byte_offset == 0 {
        return None;
    }
    byte_offset -= 1;
    while !rope.is_grapheme_boundary(byte_offset) {
        byte_offset -= 1;
    }
    Some(byte_offset)
}

#[must_use]
pub fn next_grapheme_boundary(rope: &Rope, mut byte_offset: usize) -> Option<usize> {
    if byte_offset == rope.byte_len() {
        return None;
    }
    byte_offset += 1;
    while !rope.is_grapheme_boundary(byte_offset) {
        byte_offset += 1;
    }
    Some(byte_offset)
}

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

    #[test]
    fn test() {
        let rope = Rope::from("abc\n👩🏻‍❤️‍💋‍👩🏻👨‍👨‍👧\tちは");

        // Prepare trusted grapheme boundaries
        let mut expected = vec![0];
        for grapheme in rope.graphemes() {
            let prev = expected.last().unwrap();
            assert!(rope.is_grapheme_boundary(*prev));
            let next = prev + grapheme.len();
            expected.push(next);
        }
        assert_eq!(expected.iter().last(), Some(&rope.byte_len()));

        // Test `next_grapheme_boundary`
        let actual = iter::successors(Some(0), |byte_offset| {
            next_grapheme_boundary(&rope, *byte_offset)
        })
        .collect::<Vec<_>>();
        assert_eq!(expected, actual);

        // Test `prev_grapheme_boundary`
        expected.reverse();
        let actual = iter::successors(Some(rope.byte_len()), |byte_offset| {
            prev_grapheme_boundary(&rope, *byte_offset)
        })
        .collect::<Vec<_>>();
        assert_eq!(expected, actual);
    }
}