Enhance docs, add more tests in LFUCache (#5949)

Author: Hardvan

src/main/java/com/thealgorithms/datastructures/caches/LFUCache.java

@@ -6,16 +6,21 @@
 /**
  * The {@code LFUCache} class implements a Least Frequently Used (LFU) cache.
  * An LFU cache evicts the least frequently used item when the cache reaches its capacity.
- * It keeps track of how many times each item is used and maintains a doubly linked list
- * for efficient addition and removal of items based on their frequency of use.
+ * It maintains a mapping of keys to nodes, where each node contains the key, its associated value,
+ * and a frequency count that tracks how many times the item has been accessed. A doubly linked list
+ * is used to efficiently manage the ordering of items based on their usage frequency.
  *
- * @param <K> The type of keys maintained by this cache.
- * @param <V> The type of mapped values.
+ * <p>This implementation is designed to provide O(1) time complexity for both the {@code get} and
+ * {@code put} operations, which is achieved through the use of a hashmap for quick access and a
+ * doubly linked list for maintaining the order of item frequencies.</p>
  *
  * <p>
  * Reference: <a href="https://en.wikipedia.org/wiki/Least_frequently_used">LFU Cache - Wikipedia</a>
  * </p>
  *
+ * @param <K> The type of keys maintained by this cache.
+ * @param <V> The type of mapped values.
+ *
  * @author Akshay Dubey (https://github.com/itsAkshayDubey)
  */
 public class LFUCache<K, V> {
@@ -75,7 +80,7 @@ public LFUCache(int capacity) {
 
     /**
      * Retrieves the value associated with the given key from the cache.
-     * If the key exists, the node's frequency is increased and the node is repositioned
+     * If the key exists, the node's frequency is incremented, and the node is repositioned
      * in the linked list based on its updated frequency.
      *
      * @param key The key whose associated value is to be returned.

src/test/java/com/thealgorithms/datastructures/caches/LFUCacheTest.java

@@ -1,6 +1,8 @@
 package com.thealgorithms.datastructures.caches;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import org.junit.jupiter.api.Test;
 
@@ -22,15 +24,15 @@ void testLFUCacheWithIntegerValueShouldPass() {
         lfuCache.put(6, 60);
 
         // will return null as value with key 2 is now evicted
-        assertEquals(null, lfuCache.get(2));
+        assertNull(lfuCache.get(2));
 
         // should return 60
         assertEquals(60, lfuCache.get(6));
 
         // this operation will remove value with key as 3
         lfuCache.put(7, 70);
 
-        assertEquals(null, lfuCache.get(2));
+        assertNull(lfuCache.get(2));
         assertEquals(70, lfuCache.get(7));
     }
 
@@ -41,7 +43,7 @@ void testLFUCacheWithStringValueShouldPass() {
         lfuCache.put(2, "Beta");
         lfuCache.put(3, "Gamma");
         lfuCache.put(4, "Delta");
-        lfuCache.put(5, "Eplison");
+        lfuCache.put(5, "Epsilon");
 
         // get method call will increase frequency of key 1 by 1
         assertEquals("Alpha", lfuCache.get(1));
@@ -50,33 +52,87 @@ void testLFUCacheWithStringValueShouldPass() {
         lfuCache.put(6, "Digamma");
 
         // will return null as value with key 2 is now evicted
-        assertEquals(null, lfuCache.get(2));
+        assertNull(lfuCache.get(2));
 
         // should return string Digamma
         assertEquals("Digamma", lfuCache.get(6));
 
         // this operation will remove value with key as 3
         lfuCache.put(7, "Zeta");
 
-        assertEquals(null, lfuCache.get(2));
+        assertNull(lfuCache.get(2));
         assertEquals("Zeta", lfuCache.get(7));
     }
 
-    /**
-     * test addNodeWithUpdatedFrequency method
-     * @author yuluo
-     */
     @Test
-    void testAddNodeWithUpdatedFrequency() {
+    void testUpdateValueShouldPreserveFrequency() {
         LFUCache<Integer, String> lfuCache = new LFUCache<>(3);
-        lfuCache.put(1, "beijing");
-        lfuCache.put(2, "shanghai");
-        lfuCache.put(3, "gansu");
+        lfuCache.put(1, "A");
+        lfuCache.put(2, "B");
+        lfuCache.put(3, "C");
 
-        assertEquals("beijing", lfuCache.get(1));
+        assertEquals("A", lfuCache.get(1)); // Accessing key 1
+        lfuCache.put(4, "D"); // This should evict key 2
 
-        lfuCache.put(1, "shanxi");
+        assertNull(lfuCache.get(2)); // Key 2 should be evicted
+        assertEquals("C", lfuCache.get(3)); // Key 3 should still exist
+        assertEquals("A", lfuCache.get(1)); // Key 1 should still exist
 
-        assertEquals("shanxi", lfuCache.get(1));
+        lfuCache.put(1, "Updated A"); // Update the value of key 1
+        assertEquals("Updated A", lfuCache.get(1)); // Check if the update was successful
+    }
+
+    @Test
+    void testEvictionPolicyWhenFull() {
+        LFUCache<Integer, String> lfuCache = new LFUCache<>(2);
+        lfuCache.put(1, "One");
+        lfuCache.put(2, "Two");
+
+        assertEquals("One", lfuCache.get(1)); // Access key 1
+        lfuCache.put(3, "Three"); // This should evict key 2 (least frequently used)
+
+        assertNull(lfuCache.get(2)); // Key 2 should be evicted
+        assertEquals("One", lfuCache.get(1)); // Key 1 should still exist
+        assertEquals("Three", lfuCache.get(3)); // Check if key 3 exists
+    }
+
+    @Test
+    void testGetFromEmptyCacheShouldReturnNull() {
+        LFUCache<Integer, String> lfuCache = new LFUCache<>(3);
+        assertNull(lfuCache.get(1)); // Should return null as the cache is empty
+    }
+
+    @Test
+    void testPutNullValueShouldStoreNull() {
+        LFUCache<Integer, String> lfuCache = new LFUCache<>(3);
+        lfuCache.put(1, null); // Store a null value
+
+        assertNull(lfuCache.get(1)); // Should return null
+    }
+
+    @Test
+    void testInvalidCacheCapacityShouldThrowException() {
+        assertThrows(IllegalArgumentException.class, () -> new LFUCache<>(0));
+        assertThrows(IllegalArgumentException.class, () -> new LFUCache<>(-1));
+    }
+
+    @Test
+    void testMultipleAccessPatterns() {
+        LFUCache<Integer, String> lfuCache = new LFUCache<>(5);
+        lfuCache.put(1, "A");
+        lfuCache.put(2, "B");
+        lfuCache.put(3, "C");
+        lfuCache.put(4, "D");
+
+        assertEquals("A", lfuCache.get(1)); // Access 1
+        lfuCache.put(5, "E"); // Should not evict anything yet
+        lfuCache.put(6, "F"); // Evict B
+
+        assertNull(lfuCache.get(2)); // B should be evicted
+        assertEquals("C", lfuCache.get(3)); // C should still exist
+        assertEquals("D", lfuCache.get(4)); // D should still exist
+        assertEquals("A", lfuCache.get(1)); // A should still exist
+        assertEquals("E", lfuCache.get(5)); // E should exist
+        assertEquals("F", lfuCache.get(6)); // F should exist
     }
 }