Implement groupBy on List

MartinSeeler · MartinSeeler · commit c4b5eb7317f1 · 2014-12-04T22:25:41.000+01:00
diff --git a/core/src/main/java/fj/data/List.java b/core/src/main/java/fj/data/List.java
@@ -1237,6 +1237,50 @@ public final A mode(final Ord<A> o) {
     return sort(o).group(o.equal()).maximum(intOrd.comap(List.<A>length_())).head();
   }
 
+  public final <K> HashMap<K, List<A>> groupBy(final F<A, K> keyFunction) {
+    return groupBy(keyFunction, Function.identity());
+  }
+
+  public final <K, V> HashMap<K, List<V>> groupBy(
+      final F<A, K> keyFunction,
+      final F<A, V> valueFunction) {
+    return this.groupBy(keyFunction, valueFunction, List.<V>nil(),
+        new F2<V, List<V>, List<V>>() {
+          @Override
+          public List<V> f(final V head, final List<V> tail) {
+            return List.cons(head, tail);
+          }
+        });
+  }
+
+  public final <K, V, R> HashMap<K, R> groupBy(
+      final F<A, K> keyFunction,
+      final F<A, V> valueFunction, final R groupingIdentity, final F2<V, R, R> groupingAcc) {
+    return this.foldLeft(
+        new F<HashMap<K, R>, F<A, HashMap<K, R>>>() {
+          @Override
+          public F<A, HashMap<K, R>> f(final HashMap<K, R> map) {
+            return new F<A, HashMap<K, R>>() {
+              @Override
+              public HashMap<K, R> f(final A element) {
+                final K key = keyFunction.f(element);
+                final V value = valueFunction.f(element);
+                map.set(key, map.get(key)
+                    .map(new F<R, R>() {
+                      @Override
+                      public R f(final R existing) {
+                        return groupingAcc.f(value, existing);
+                      }
+                    })
+                    .orSome(groupingAcc.f(value, groupingIdentity)));
+                return map;
+              }
+            };
+          }
+        }, HashMap.<K, R>hashMap()
+    );
+  }
+
   /**
    * Returns whether or not all elements in the list are equal according to the given equality test.
    *
