jp-deinflector 0.2.2

/// Converts all katakana characters contained in the input string `kata`
/// into their hiragana counterparts. All other characters are kept as they are.
pub fn kata_to_hira(kata: &str) -> String {
    // Quick check if conversion is needed
    if !kata.chars().any(is_katakana) {
        return kata.to_string();
    }

    // Preallocate the result with the same capacity
    let mut result = String::with_capacity(kata.len());
    for c in kata.chars() {
        if is_katakana(c) {
            result.push(katakana_char_to_hira(c));
        } else {
            result.push(c);
        }
    }
    result
}

/// Officially kata is the range 30A0-30FF (https://www.unicode.org/charts/PDF/U30A0.pdf),
/// but I am not sure if something like '゠' or 'ヿ' should be converted too
/// (won't matter for deinflection anyway though)
fn is_katakana(c: char) -> bool {
    let cp = c as u32;
    (0x30A1..=0x30F6).contains(&cp)
}

fn katakana_char_to_hira(c: char) -> char {
    char::from_u32(c as u32 - 0x60).unwrap_or(c)
}

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

    #[test]
    fn test_all_correct() {
        let kata = "アイウエオカキクケコサシスセソタチツテトナニヌネノハヒフヘホマミムメモヤユヨラリルレロワヲンガギグゲゴザジズゼゾダヂヅデドバビブベボパピプペポ";
        let hira = "あいうえおかきくけこさしすせそたちつてとなにぬねのはひふへほまみむめもやゆよらりるれろわをんがぎぐげござじずぜぞだぢづでどばびぶべぼぱぴぷぺぽ";
        assert_eq!(kata_to_hira(kata), hira);
    }

    proptest! {
        #[test]
        fn non_katakana_stay_unchanged(s in any::<String>()) {
            let converted = kata_to_hira(&s);

            for (input_char, output_char) in s.chars().zip(converted.chars()) {
                if !is_katakana(input_char) {
                    assert_eq!(
                        input_char, output_char,
                        "Non-katakana character changed: {:?} -> {:?}",
                        input_char, output_char
                    );
                }
            }
        }
    }
}