Add $.groupBy(iterator, function, binaryOperator) method.

Author: javadev

src/main/java/com/github/underscore/$.java

@@ -805,6 +805,32 @@ public <K, E> Map<K, List<E>> groupBy(final Function<E, K> func) {
         return groupBy((Iterable<E>) iterable, func);
     }
 
+    public static <K, E> Map<K, Optional<E>> groupBy(final Iterable<E> iterable, final Function<E, K> func,
+        final BinaryOperator<E> binaryOperator) {
+        final Map<K, List<E>> retVal = newLinkedHashMap();
+        for (E e : iterable) {
+            final K key = func.apply(e);
+            List<E> val;
+            if (retVal.containsKey(key)) {
+                val = retVal.get(key);
+            } else {
+                val = newArrayList();
+            }
+            val.add(e);
+            retVal.put(key, val);
+        }
+        final Map<K, Optional<E>> retVal2 = newLinkedHashMap();
+        for (Map.Entry<K, List<E>> entry : retVal.entrySet()) {
+            retVal2.put(entry.getKey(), reduce(entry.getValue(), binaryOperator));
+        }
+        return retVal2;
+    }
+
+    @SuppressWarnings("unchecked")
+    public <K, E> Map<K, Optional<E>> groupBy(final Function<E, K> func, final BinaryOperator<E> binaryOperator) {
+        return groupBy((Iterable<E>) iterable, func, binaryOperator);
+    }
+
     @SuppressWarnings("unchecked")
     public static <K, E> Map<K, List<E>> indexBy(final Iterable<E> iterable, final String property) {
         return groupBy(iterable, new Function<E, K>() {

src/test/java/com/github/underscore/CollectionsTest.java

@@ -1485,6 +1485,42 @@ public Double apply(Double num) {
         assertEquals("{1.0=[1.3], 2.0=[2.1, 2.4]}", resultChain.toString());
     }
 
+/*
+_.groupBy([1.3, 2.1, 2.4], function(num){ return Math.floor(num); });
+=> {1: [1.3], 2: [2.1, 2.4]}
+*/
+    @Test
+    @SuppressWarnings("unchecked")
+    public void groupByWithSumming() {
+        final Map<Double, Optional<Double>> result =
+        $.groupBy(asList(1.3, 2.1, 2.4),
+            new Function<Double, Double>() {
+            public Double apply(Double num) {
+                return Math.floor(num);
+            }
+            },
+            new BinaryOperator<Double>() {
+            public Double apply(Double a, Double b) {
+                return a + b;
+            }
+            }
+        );
+        assertEquals("{1.0=Optional.of(1.3), 2.0=Optional.of(4.5)}", result.toString());
+        final Map<Double, Optional<Double>> resultObj =
+        new $(asList(1.3, 2.1, 2.4)).groupBy(
+            new Function<Double, Double>() {
+            public Double apply(Double num) {
+                return Math.floor(num);
+            }
+            },
+            new BinaryOperator<Double>() {
+            public Double apply(Double a, Double b) {
+                return a + b;
+            }
+            });
+        assertEquals("{1.0=Optional.of(1.3), 2.0=Optional.of(4.5)}", resultObj.toString());
+    }
+
 /*
 var stooges = [{name: 'moe', age: 40}, {name: 'larry', age: 50}, {name: 'curly', age: 60}];
 _.indexBy(stooges, 'age');