Enhance docs, add tests in LinearProbingHashMap (#5977)

Author: Hardvan

src/main/java/com/thealgorithms/datastructures/hashmap/hashing/LinearProbingHashMap.java

@@ -2,24 +2,51 @@
 
 import java.util.ArrayList;
 
-/***
- * This class is an implementation of a hash table using linear probing.
+/**
+ * This class implements a hash table using linear probing to resolve collisions.
+ * Linear probing is a collision resolution method where each slot in the hash table is checked in a sequential manner
+ * until an empty slot is found.
+ *
+ * <p>
+ * The class allows for storing key-value pairs, where both the key and value are generic types.
+ * The key must be of a type that implements the Comparable interface to ensure that the keys can be compared for sorting.
+ * </p>
+ *
+ * <p>
+ * This implementation supports basic operations such as:
+ * <ul>
+ *     <li><b>put(Key key, Value value)</b>: Adds a key-value pair to the hash table. If the key already exists, its value is updated.</li>
+ *     <li><b>get(Key key)</b>: Retrieves the value associated with the given key.</li>
+ *     <li><b>delete(Key key)</b>: Removes the key and its associated value from the hash table.</li>
+ *     <li><b>contains(Key key)</b>: Checks if the hash table contains a given key.</li>
+ *     <li><b>size()</b>: Returns the number of key-value pairs in the hash table.</li>
+ *     <li><b>keys()</b>: Returns an iterable collection of keys stored in the hash table.</li>
+ * </ul>
+ * </p>
+ *
+ * <p>
+ * The internal size of the hash table is automatically resized when the load factor exceeds 0.5 or falls below 0.125,
+ * ensuring efficient space utilization.
+ * </p>
+ *
  * @see <a href="https://en.wikipedia.org/wiki/Linear_probing">Linear Probing Hash Table</a>
  *
- * @param <Key> keys type.
- * @param <Value> values type.
+ * @param <Key> the type of keys maintained by this map
+ * @param <Value> the type of mapped values
  */
 public class LinearProbingHashMap<Key extends Comparable<Key>, Value> extends Map<Key, Value> {
     private int hsize; // size of the hash table
-    private Key[] keys;
-    private Value[] values;
-    private int size; // amount of elements in the hash table
+    private Key[] keys; // array to store keys
+    private Value[] values; // array to store values
+    private int size; // number of elements in the hash table
 
+    // Default constructor initializes the table with a default size of 16
     public LinearProbingHashMap() {
         this(16);
     }
 
     @SuppressWarnings("unchecked")
+    // Constructor to initialize the hash table with a specified size
     public LinearProbingHashMap(int size) {
         this.hsize = size;
         keys = (Key[]) new Comparable[size];
@@ -81,7 +108,7 @@ public boolean delete(Key key) {
 
         i = increment(i);
         while (keys[i] != null) {
-            // delete keys[i] an vals[i] and reinsert
+            // Save the key and value for rehashing
             Key keyToRehash = keys[i];
             Value valToRehash = values[i];
             keys[i] = null;

src/test/java/com/thealgorithms/datastructures/hashmap/hashing/LinearProbingHashMapTest.java

@@ -1,8 +1,91 @@
 package com.thealgorithms.datastructures.hashmap.hashing;
 
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.Test;
+
 class LinearProbingHashMapTest extends MapTest {
+
     @Override
     <Key extends Comparable<Key>, Value> Map<Key, Value> getMap() {
         return new LinearProbingHashMap<>();
     }
+
+    @Test
+    void putNullKey() {
+        Map<Integer, String> map = getMap();
+        assertFalse(map.put(null, "value"), "Putting a null key should return false");
+    }
+
+    @Test
+    void putDuplicateKeys() {
+        Map<Integer, String> map = getMap();
+        map.put(1, "one");
+        map.put(1, "uno");
+        assertEquals("uno", map.get(1), "Value should be updated to 'uno'");
+    }
+
+    @Test
+    void putResizeTest() {
+        Map<Integer, String> map = getMap();
+        for (int i = 0; i < 20; i++) {
+            map.put(i, String.valueOf(i));
+        }
+        assertEquals(20, map.size(), "Map size should be 20 after inserting 20 elements");
+    }
+
+    @Test
+    void deleteNonExistentKey() {
+        Map<Integer, String> map = getMap();
+        assertFalse(map.delete(999), "Deleting a non-existent key should return false");
+    }
+
+    @Test
+    void deleteAndReinsert() {
+        Map<Integer, String> map = getMap();
+        map.put(1, "one");
+        map.delete(1);
+        assertFalse(map.contains(1), "Map should not contain the deleted key");
+        map.put(1, "one again");
+        assertTrue(map.contains(1), "Map should contain the key after reinsertion");
+    }
+
+    @Test
+    void resizeDown() {
+        Map<Integer, String> map = getMap();
+        for (int i = 0; i < 16; i++) {
+            map.put(i, String.valueOf(i));
+        }
+        for (int i = 0; i < 12; i++) {
+            map.delete(i);
+        }
+        assertEquals(4, map.size(), "Map size should be 4 after deleting 12 elements");
+    }
+
+    @Test
+    void keysOrderTest() {
+        Map<Integer, String> map = getMap();
+        for (int i = 10; i > 0; i--) {
+            map.put(i, String.valueOf(i));
+        }
+        int expectedKey = 1;
+        for (Integer key : map.keys()) {
+            assertEquals(expectedKey++, key, "Keys should be in sorted order");
+        }
+    }
+
+    @Test
+    void stressTest() {
+        Map<Integer, String> map = getMap();
+        for (int i = 0; i < 1000; i++) {
+            map.put(i, String.valueOf(i));
+            assertEquals(i + 1, map.size(), "Size should match number of inserted elements");
+        }
+        for (int i = 0; i < 500; i++) {
+            map.delete(i);
+            assertEquals(1000 - (i + 1), map.size(), "Size should decrease correctly");
+        }
+    }
 }