auto merge of rust-lang#8320 : mihneadb/rust/freq_count, r=cmr

Author: bors

src/libextra/stats.rs

@@ -10,6 +10,7 @@
 
 use sort;
 use std::cmp;
+use std::hashmap;
 use std::io;
 use std::num;
 
@@ -352,6 +353,16 @@ pub fn write_boxplot(w: @io::Writer, s: &Summary, width_hint: uint) {
     w.write_str(histr);
 }
 
+/// Returns a HashMap with the number of occurences of every element in the
+/// sequence that the iterator exposes.
+pub fn freq_count<T: Iterator<U>, U: Eq+Hash>(mut iter: T) -> hashmap::HashMap<U, uint> {
+    let mut map = hashmap::HashMap::new::<U, uint>();
+    for elem in iter {
+        map.insert_or_update_with(elem, 1, |_, count| *count += 1);
+    }
+    map
+}
+
 // Test vectors generated from R, using the script src/etc/stat-test-vectors.r.
 
 #[cfg(test)]