use std::{collections::HashMap, hash::Hash};

pub fn tally<T, K, F>(items: &[T], key: F) -> HashMap<K, usize>
where
    K: Eq + Hash,
    F: Fn(&T) -> K,
{
    let mut counts = HashMap::new();
    for item in items {
        *counts.entry(key(item)).or_insert(0) += 1;
    }
    counts
}

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

    #[test]
    fn owned_key() {
        let words = ["apple", "avocado", "banana"];
        let counts = tally(&words, |w| w.chars().next().unwrap().to_string());
        assert_eq!(counts.get("a"), Some(&2), "keys: {:?}", counts);
        assert_eq!(counts.len(), 2, "keys: {:?}", counts);
    }
}