provide an inverse method for multimaps

Author: paulk-asert

src/main/java/org/apache/commons/collections4/MultiMapUtils.java

@@ -20,6 +20,7 @@
 import java.util.Collection;
 import java.util.HashSet;
 import java.util.List;
+import java.util.Map;
 import java.util.Set;
 
 import org.apache.commons.collections4.bag.HashBag;
@@ -170,6 +171,27 @@ public static boolean isEmpty(final MultiValuedMap<?, ?> map) {
         return map == null || map.isEmpty();
     }
 
+    /**
+     * A utility method to invert the mappings from an input MultiValuedMap
+     * and add them to an output MultiValuedMap. Use this method to have complete
+     * control of the output MultiValuedMap or when merging several inverse mappings.
+     * In simple cases, consider just using the {@link MultiValuedMap#inverted()} method.
+     *
+     * @param input take key-to-value mappings from here
+     * @param output add value-to-key mappings here
+     * @param <K> the output MultiValuedMap key type
+     * @param <V> the output MultiValuedMap value type
+     * @param <M> the output MultiValuedMap with key and value types reversed compared with input
+     * @return the updated output MultiValuedMap
+     */
+    public static <K, V, M extends MultiValuedMap<K, V>>
+    M invert(MultiValuedMap<? extends V, ? extends K> input, M output) {
+        for (Map.Entry<? extends V, ? extends K> e : input.entries()) {
+            output.put(e.getValue(), e.getKey());
+        }
+        return output;
+    }
+
     /**
      * Creates a {@link ListValuedMap} with an {@link java.util.ArrayList ArrayList} as
      * collection class to store the values mapped to a key.

src/main/java/org/apache/commons/collections4/MultiValuedMap.java

@@ -140,6 +140,17 @@ public interface MultiValuedMap<K, V> {
      */
     Collection<V> get(K key);
 
+    /**
+     * Returns a new MultiValuedMap with inverted mappings.
+     * The new multimap will have a value-to-key mapping
+     * for each key-to-value mapping in the original.
+     *
+     * @return a new MultiValuedMap with inverted mappings
+     */
+    default MultiValuedMap<V, K> inverted() {
+        throw new UnsupportedOperationException();
+    }
+
     /**
      * Returns {@code true} if this map contains no key-value mappings.
      *

src/main/java/org/apache/commons/collections4/multimap/ArrayListValuedHashMap.java

@@ -25,6 +25,7 @@
 import java.util.HashMap;
 import java.util.Map;
 
+import org.apache.commons.collections4.MultiMapUtils;
 import org.apache.commons.collections4.MultiValuedMap;
 
 /**
@@ -118,6 +119,11 @@ protected ArrayList<V> createCollection() {
         return new ArrayList<>(initialListCapacity);
     }
 
+    @Override
+    public ArrayListValuedHashMap<V, K> inverted() {
+        return MultiMapUtils.invert(this, new ArrayListValuedHashMap<V, K>());
+    }
+
     /**
      * Deserializes an instance from an ObjectInputStream.
      *

src/main/java/org/apache/commons/collections4/multimap/ArrayListValuedLinkedHashMap.java

@@ -25,6 +25,7 @@
 import java.util.LinkedHashMap;
 import java.util.Map;
 
+import org.apache.commons.collections4.MultiMapUtils;
 import org.apache.commons.collections4.MultiValuedMap;
 
 /**
@@ -118,6 +119,11 @@ protected ArrayList<V> createCollection() {
         return new ArrayList<>(initialListCapacity);
     }
 
+    @Override
+    public ArrayListValuedLinkedHashMap<V, K> inverted() {
+        return MultiMapUtils.invert(this, new ArrayListValuedLinkedHashMap<>());
+    }
+
     /**
      * Deserializes an instance from an ObjectInputStream.
      *

src/main/java/org/apache/commons/collections4/multimap/HashSetValuedHashMap.java

@@ -24,6 +24,7 @@
 import java.util.HashSet;
 import java.util.Map;
 
+import org.apache.commons.collections4.MultiMapUtils;
 import org.apache.commons.collections4.MultiValuedMap;
 
 /**
@@ -117,6 +118,11 @@ protected HashSet<V> createCollection() {
         return new HashSet<>(initialSetCapacity);
     }
 
+    @Override
+    public HashSetValuedHashMap<V, K> inverted() {
+        return MultiMapUtils.invert(this, new HashSetValuedHashMap<V, K>());
+    }
+
     /**
      * Deserializes an instance from an ObjectInputStream.
      *

src/main/java/org/apache/commons/collections4/multimap/LinkedHashSetValuedLinkedHashMap.java

@@ -24,6 +24,7 @@
 import java.util.LinkedHashSet;
 import java.util.Map;
 
+import org.apache.commons.collections4.MultiMapUtils;
 import org.apache.commons.collections4.MultiValuedMap;
 
 /**
@@ -117,6 +118,11 @@ protected LinkedHashSet<V> createCollection() {
         return new LinkedHashSet<>(initialSetCapacity);
     }
 
+    @Override
+    public LinkedHashSetValuedLinkedHashMap<V, K> inverted() {
+        return MultiMapUtils.invert(this, new LinkedHashSetValuedLinkedHashMap<V, K>());
+    }
+
     /**
      * Deserializes an instance from an ObjectInputStream.
      *

src/test/java/org/apache/commons/collections4/MultiMapUtilsTest.java

@@ -29,6 +29,8 @@
 import java.util.Set;
 
 import org.apache.commons.collections4.multimap.ArrayListValuedHashMap;
+import org.apache.commons.collections4.multimap.HashSetValuedHashMap;
+import org.apache.commons.collections4.multimap.LinkedHashSetValuedLinkedHashMap;
 import org.junit.jupiter.api.Test;
 
 /**
@@ -116,6 +118,30 @@ void testGetValuesAsSet() {
         assertEquals(new HashSet<>(Arrays.asList(values)), set);
     }
 
+    @Test
+    void testInvert() {
+        final HashSetValuedHashMap<String, String> usages = new HashSetValuedHashMap<>();
+
+        final LinkedHashSetValuedLinkedHashMap<String, String> deps = new LinkedHashSetValuedLinkedHashMap<>();
+        deps.put("commons-configuration2", "commons-logging");
+        deps.put("commons-configuration2", "commons-lang3");
+        deps.put("commons-configuration2", "commons-text");
+        deps.put("commons-beanutils", "commons-collections");
+        deps.put("commons-beanutils", "commons-logging");
+        MultiMapUtils.invert(deps, usages);
+        final Set<String> loggingUsagesCompile = usages.get("commons-logging");
+        assertEquals("[commons-configuration2, commons-beanutils]", loggingUsagesCompile.toString());
+        final Set<String> codecUsagesCompile = usages.get("commons-codec");
+        assertEquals("[]", codecUsagesCompile.toString());
+
+        final LinkedHashSetValuedLinkedHashMap<String, String> optionalDeps = new LinkedHashSetValuedLinkedHashMap<>();
+        optionalDeps.put("commons-configuration2", "commons-codec");
+        optionalDeps.put("commons-collections", "commons-codec");
+        MultiMapUtils.invert(optionalDeps, usages);
+        final Set<String> codecUsagesAll = usages.get("commons-codec");
+        assertEquals("[commons-collections, commons-configuration2]", codecUsagesAll.toString());
+    }
+
     @Test
     void testIsEmptyWithEmptyMap() {
         assertTrue(MultiMapUtils.isEmpty(new ArrayListValuedHashMap<>()));

src/test/java/org/apache/commons/collections4/multimap/ArrayListValuedHashMapTest.java

@@ -93,6 +93,19 @@ void testEqualsHashCodeContract() {
         assertNotSame(map1.hashCode(), map2.hashCode());
     }
 
+    @Test
+    void testInverted() {
+        final ArrayListValuedHashMap<String, String> shopping = new ArrayListValuedHashMap<>(4);
+        shopping.put("Alice", "Bread");
+        shopping.put("Alice", "Milk");
+        shopping.put("Alice", "Milk");
+        shopping.put("Bob", "Pizza");
+        shopping.put("Bob", "Bread");
+        shopping.put("Bob", "Bread");
+        assertEquals("{Pizza=[Bob], Bread=[Bob, Bob, Alice], Milk=[Alice, Alice]}",
+                shopping.inverted().toString());
+    }
+
     @Test
     @SuppressWarnings("unchecked")
     void testListValuedMapAdd() {

src/test/java/org/apache/commons/collections4/multimap/ArrayListValuedLinkedHashMapTest.java

@@ -99,6 +99,19 @@ void testEqualsHashCodeContract() {
         assertNotSame(map1.hashCode(), map2.hashCode());
     }
 
+    @Test
+    void testInverted() {
+        final ArrayListValuedLinkedHashMap<String, String> shopping = new ArrayListValuedLinkedHashMap<>(4);
+        shopping.put("Alice", "Bread");
+        shopping.put("Alice", "Milk");
+        shopping.put("Alice", "Milk");
+        shopping.put("Bob", "Pizza");
+        shopping.put("Bob", "Bread");
+        shopping.put("Bob", "Bread");
+        assertEquals("{Bread=[Alice, Bob, Bob], Milk=[Alice, Alice], Pizza=[Bob]}",
+                shopping.inverted().toString());
+    }
+
     @Test
     @SuppressWarnings("unchecked")
     void testListValuedMapAdd() {

src/test/java/org/apache/commons/collections4/multimap/HashSetValuedHashMapTest.java

@@ -110,6 +110,19 @@ void testHashSetValueHashMap_1() {
         assertEquals("{}", map3.toString());
     }
 
+    @Test
+    void testInverted() {
+        final HashSetValuedHashMap<String, String> dependencies = new HashSetValuedHashMap<>();
+        dependencies.put("commons-configuration2", "commons-logging");
+        dependencies.put("commons-configuration2", "commons-lang3");
+        dependencies.put("commons-configuration2", "commons-text");
+        dependencies.put("commons-beanutils", "commons-collections");
+        dependencies.put("commons-beanutils", "commons-logging");
+        final Set<String> loggingUsages = dependencies.inverted().get("commons-logging");
+        assertEquals("[commons-beanutils, commons-configuration2]",
+                loggingUsages.toString());
+    }
+
     @Test
     @SuppressWarnings("unchecked")
     void testSetValuedMapAdd() {