Rust learning: lessons, notes, and exercise crates

tasks/src/stats.rs

@@ -0,0 +1,26 @@
+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);
+    }
+}