feat: Added Map Reduce Design Pattern (#3184)

* MapReduce design pattern added

* Updated README.md

* added module to parent pom

map-reduce/src/test/java/com/iluwatar/ReducerTest.java

@@ -0,0 +1,74 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar;
+
+import org.junit.jupiter.api.Test;
+import java.util.*;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class ReducerTest {
+
+  @Test
+  void testReduceWithMultipleWords() {
+    Map<String, List<Integer>> input = new HashMap<>();
+    input.put("apple", Arrays.asList(2, 3, 1));
+    input.put("banana", Arrays.asList(1, 1));
+    input.put("cherry", List.of(4));
+
+    List<Map.Entry<String, Integer>> result = Reducer.reduce(input);
+
+    assertEquals(3, result.size());
+    assertEquals("apple", result.get(0).getKey());
+    assertEquals(6, result.get(0).getValue());
+    assertEquals("cherry", result.get(1).getKey());
+    assertEquals(4, result.get(1).getValue());
+    assertEquals("banana", result.get(2).getKey());
+    assertEquals(2, result.get(2).getValue());
+  }
+
+  @Test
+  void testReduceWithEmptyInput() {
+    Map<String, List<Integer>> input = new HashMap<>();
+
+    List<Map.Entry<String, Integer>> result = Reducer.reduce(input);
+
+    assertTrue(result.isEmpty());
+  }
+
+  @Test
+  void testReduceWithTiedCounts() {
+    Map<String, List<Integer>> input = new HashMap<>();
+    input.put("tie1", Arrays.asList(2, 2));
+    input.put("tie2", Arrays.asList(1, 3));
+
+    List<Map.Entry<String, Integer>> result = Reducer.reduce(input);
+
+    assertEquals(2, result.size());
+    assertEquals(4, result.get(0).getValue());
+    assertEquals(4, result.get(1).getValue());
+    // Note: The order of tie1 and tie2 is not guaranteed in case of a tie
+  }
+}