MapStream multimap support

modules/collect/src/main/java/com/opengamma/strata/collect/MapStream.java

@@ -32,6 +32,7 @@
 
 import com.google.common.collect.ImmutableListMultimap;
 import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.Multimap;
 import com.google.common.collect.Streams;
 
 /**
@@ -63,6 +64,20 @@ public static <K, V> MapStream<K, V> of(Map<K, V> map) {
     return new MapStream<>(map.entrySet().stream());
   }
 
+  /**
+   * Returns a stream over all the entries in the multimap.
+   * <p>
+   * This will typically create a stream with duplicate keys.
+   *
+   * @param <K>  the key type
+   * @param <V>  the value type
+   * @param multimap  the multimap to wrap
+   * @return a stream over the entries in the multimap
+   */
+  public static <K, V> MapStream<K, V> of(Multimap<K, V> multimap) {
+    return new MapStream<>(multimap.entries().stream());
+  }
+
   /**
    * Returns a stream of map entries where the values are taken from a collection
    * and the keys are created by applying a function to each value.

modules/collect/src/test/java/com/opengamma/strata/collect/MapStreamTest.java

@@ -28,6 +28,7 @@
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableListMultimap;
 import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.ImmutableMultimap;
 import com.google.common.collect.ListMultimap;
 import com.opengamma.strata.collect.tuple.Pair;
 
@@ -36,6 +37,7 @@ public class MapStreamTest {
 
   private final Map<String, Integer> map = ImmutableMap.of("one", 1, "two", 2, "three", 3, "four", 4);
 
+  //-------------------------------------------------------------------------
   public void keys() {
     List<String> result = MapStream.of(map).keys().collect(toImmutableList());
     assertThat(result).isEqualTo(ImmutableList.of("one", "two", "three", "four"));
@@ -279,6 +281,13 @@ public void forEach() {
     assertThat(mutableMap).isEqualTo(map);
   }
 
+  //-------------------------------------------------------------------------
+  public void ofMultimap() {
+    ImmutableMultimap<String, Integer> input = ImmutableMultimap.of("one", 1, "two", 2, "one", 3);
+    assertThat(MapStream.of(input)).containsExactlyInAnyOrder(entry("one", 1), entry("two", 2), entry("one", 3));
+    assertThat(MapStream.of(input).toMap(Integer::sum)).containsOnly(entry("one", 4), entry("two", 2));
+  }
+
   public void ofCollection() {
     List<String> letters = ImmutableList.of("a", "b", "c");
     Map<String, String> expected = ImmutableMap.of("A", "a", "B", "b", "C", "c");