Add MultiMap.flatten()

pgpainless-core/src/main/java/org/pgpainless/util/MultiMap.java

@@ -100,6 +100,19 @@ public class MultiMap<K, V> {
         return map.entrySet();
     }
 
+    /**
+     * Return all values of the {@link MultiMap} in a single {@link LinkedHashSet}.
+     *
+     * @return set of all values
+     */
+    public Set<V> flatten() {
+        LinkedHashSet<V> flattened = new LinkedHashSet<>();
+        for (Set<V> items : map.values()) {
+            flattened.addAll(items);
+        }
+        return flattened;
+    }
+
     @Override
     public boolean equals(Object o) {
         if (o == null) {

pgpainless-core/src/test/java/org/pgpainless/util/MultiMapTest.java

@@ -14,6 +14,8 @@ import static org.junit.jupiter.api.Assertions.assertTrue;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashSet;
+import java.util.LinkedHashSet;
+import java.util.Set;
 
 import org.junit.jupiter.api.Test;
 
@@ -156,4 +158,23 @@ public class MultiMapTest {
         assertFalse(map.containsKey("foo"));
         assertTrue(map.containsKey("bingo"));
     }
+
+    @Test
+    public void flattenEmptyMap() {
+        MultiMap<String, String> empty = new MultiMap<>();
+        assertEquals(Collections.emptySet(), empty.flatten());
+    }
+
+    @Test
+    public void flattenMap() {
+        MultiMap<String, String> map = new MultiMap<>();
+        map.put("A", "1");
+        map.put("A", "2");
+        map.put("B", "1");
+
+        Set<String> expected = new LinkedHashSet<>();
+        expected.add("1");
+        expected.add("2");
+        assertEquals(expected, map.flatten());
+    }
 }