gistfile1.rs

extern crate collections;
use collections::HashMap;
use std::hash::Hash;

fn main() {
    let mut map: CountedSet<&'static str> = CountedSet::new();
    let strings = ["a", "b", "a", "b", "b", "c"];
    let answer = [("b", 3u), ("a", 2), ("c", 1)];
    for k in strings.iter() {
         map.add(*k);
    }
    assert!(answer == map.most_common(3).as_slice())
}

}

struct CountedSet<T> {
    data: HashMap<T, uint>
}

impl<T: Hash + TotalEq> CountedSet<T> {
    fn add<'a>(&'a mut self, k: T) -> &'a mut uint {
        self.data.insert_or_update_with(k, 1u, |_, v| *v += 1)
    }

    fn most_common<'a>(&'a self, n: uint) -> Vec<(&'a T, &'a uint)> {
        let mut v = self.data.iter().collect::<Vec<(&T, &uint)>>();
        v.sort_by(|&(_, a), &(_, b)| b.cmp(a));
        v
    }
    fn new() -> CountedSet<T> {
        CountedSet{data: HashMap::new()}
    }
}