Added into_group_map_by, into_group_map_by_fold

Author: Sagebati

src/group_map.rs

@@ -19,4 +19,42 @@ pub fn into_group_map<I, K, V>(iter: I) -> HashMap<K, Vec<V>>
     }
 
     lookup
-}
+}
+
+pub fn into_group_map_by<I, K, V>(iter: I, f: impl Fn(&V) -> K) -> HashMap<K, Vec<V>>
+    where I: Iterator<Item=V>,
+          K: Hash + Eq,
+{
+    let mut lookup = HashMap::new();
+
+    for val in iter {
+        let key = f(&val);
+        lookup.entry(key).or_insert(Vec::new()).push(val);
+    }
+
+    lookup
+}
+
+pub fn into_group_map_by_fold<I, K, V, Acc, Fold>(iter: I, f_key: impl Fn(&V) -> K,
+                                             init: Acc,
+                                             fold: Fold) -> HashMap<K, Acc>
+    where I: Iterator<Item=V>,
+          K: Hash + Eq,
+          Acc: Clone,
+          Fold : FnMut(Acc,V) -> Acc + Clone
+
+
+{
+    let mut lookup = HashMap::new();
+
+    for val in iter {
+        let key = f_key(&val);
+        lookup.entry(key).or_insert(Vec::new()).push(val);
+    }
+
+    lookup
+        .into_iter()
+        .map(move |(key,value)| (key, value.into_iter().fold(init.clone(), fold.clone())))
+        .collect()
+}
+