A counter for Rust

counter.rs

/// A simple counter.

#[warn(non_camel_case_types)]
#[warn(unnecessary_qualification)]
#[warn(non_uppercase_statics)]
#[warn(missing_doc)]

extern crate collections;
use collections::TreeMap;

/// Count the number of occurrences of each value in an iterator
pub fn get_counts<K : Ord, I : Iterator<K>>(mut list : I) -> TreeMap<K, uint> {
	let mut counter : TreeMap<K, uint> = TreeMap::new();
	for key in list {
		// TODO: if key is not in the tree, this looks for its position twice
		//       there used to be an 'insert_or_update_with' method
		
		// Note: can't use following, because of the borrow checker.
		//~ match counter.find(&key) {
			//~ None => {counter.insert(key, 1);},
			//~ Some(mut value) => {(*value) += 1;}
		//~ }
		
		if counter.contains_key(&key) {
			match counter.find_mut(&key) {
				Some(value) => {(*value) += 1;}
				None => {fail!("Impossible!");}
			}
		} else {
			counter.insert(key, 1);
		}
	}
	counter
}