mykatas/rust/tests/2_counting_duplicates.rs

use std::collections::HashMap;

fn count_duplicates(text: &str) -> u32 {
    text.to_lowercase()
        .chars()
        .fold(HashMap::<char, u32>::new(), |mut hm, c| {
            (*hm.entry(c).or_default()) += 1;
            hm
        })
        .values()
        .filter(|&&count| count > 1)
        .count() as u32
}

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

    #[test]
    fn test_abcde() {
        assert_eq!(count_duplicates("abcde"), 0);
    }

    #[test]
    fn test_abcdea() {
        assert_eq!(count_duplicates("abcdea"), 1);
    }
    #[test]
    fn test_indivisibility() {
        assert_eq!(count_duplicates("indivisibility"), 1);
    }
}