Added utilities and a hashmap inverter.

src/main/java/ca/recrown/islandsurvivalcraft/Utilities.java

@@ -0,0 +1,21 @@
+package ca.recrown.islandsurvivalcraft;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.Map.Entry;
+
+public class Utilities {
+    public static <K, V> HashMap<V, ArrayList<K>> invertHashMap(HashMap<K, V> hashMap) {
+        HashMap<V, ArrayList<K>> res = new HashMap<>();
+        for (Entry<K, V> entry : hashMap.entrySet()) {
+            if (!res.containsKey(entry.getValue())) {
+                ArrayList<K> l = new ArrayList<K>();
+                l.add(entry.getKey());
+                res.put(entry.getValue(), l);
+            } else {
+                res.get(entry.getValue()).add(entry.getKey());
+            }
+        }
+        return res;
+    }
+}

src/test/java/ca/recrown/islandsurvivalcraft/UtilitiesTest.java

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+package ca.recrown.islandsurvivalcraft;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.HashMap;
+
+import junit.framework.Test;
+import junit.framework.TestCase;
+import junit.framework.TestSuite;
+
+/**
+ * Unit test for simple App.
+ */
+public class UtilitiesTest 
+    extends TestCase
+{
+    /**
+     * Create the test case
+     *
+     * @param testName name of the test case
+     */
+    public UtilitiesTest( String testName )
+    {
+        super( testName );
+    }
+
+    /**
+     * @return the suite of tests being tested
+     */
+    public static Test suite()
+    {
+        return new TestSuite( UtilitiesTest.class );
+    }
+
+    /**
+     * Basic hashmap inversion test.
+     */
+    public void testInvertHashMap()
+    {
+        HashMap<String, Integer> hashMap = new HashMap<>();
+        hashMap.put("a", 1);
+        hashMap.put("b", 1);
+        hashMap.put("c", 2);
+        hashMap.put("d", 2);
+
+        HashMap<Integer, ArrayList<String>> res = Utilities.invertHashMap(hashMap);
+        ArrayList<String> first = res.get(1);
+        ArrayList<String> expectedFirst = new ArrayList<>();
+        expectedFirst.add("a");
+        expectedFirst.add("b");
+
+        ArrayList<String> second = res.get(2);
+        ArrayList<String> expectedSecond = new ArrayList<>();
+        expectedSecond.add("c");
+        expectedSecond.add("d");
+        assertEquals(first, expectedFirst);
+        assertEquals(second, expectedSecond);
+    }
+}