refactor: BloomFilter (#5325)

Author: alxkm

src/main/java/com/thealgorithms/datastructures/bloomfilter/BloomFilter.java

@@ -2,60 +2,108 @@
 
 import java.util.BitSet;
 
+/**
+ * A generic BloomFilter implementation for probabilistic membership checking.
+ *
+ * @param <T> The type of elements to be stored in the Bloom filter.
+ */
 public class BloomFilter<T> {
 
-    private int numberOfHashFunctions;
-    private BitSet bitArray;
-    private Hash<T>[] hashFunctions;
+    private final int numberOfHashFunctions;
+    private final BitSet bitArray;
+    private final Hash<T>[] hashFunctions;
 
-    public BloomFilter(int numberOfHashFunctions, int n) {
+    /**
+     * Constructs a BloomFilter with a specified number of hash functions and bit array size.
+     *
+     * @param numberOfHashFunctions the number of hash functions to use
+     * @param bitArraySize          the size of the bit array
+     */
+    @SuppressWarnings("unchecked")
+    public BloomFilter(int numberOfHashFunctions, int bitArraySize) {
         this.numberOfHashFunctions = numberOfHashFunctions;
-        hashFunctions = new Hash[numberOfHashFunctions];
-        bitArray = new BitSet(n);
-        insertHash();
+        this.bitArray = new BitSet(bitArraySize);
+        this.hashFunctions = new Hash[numberOfHashFunctions];
+        initializeHashFunctions();
     }
 
-    private void insertHash() {
+    /**
+     * Initializes the hash functions with unique indices.
+     */
+    private void initializeHashFunctions() {
         for (int i = 0; i < numberOfHashFunctions; i++) {
-            hashFunctions[i] = new Hash(i);
+            hashFunctions[i] = new Hash<>(i);
         }
     }
 
+    /**
+     * Inserts an element into the Bloom filter.
+     *
+     * @param key the element to insert
+     */
     public void insert(T key) {
         for (Hash<T> hash : hashFunctions) {
-            int position = hash.compute(key) % bitArray.size();
+            int position = Math.abs(hash.compute(key) % bitArray.size());
             bitArray.set(position);
         }
     }
 
+    /**
+     * Checks if an element might be in the Bloom filter.
+     *
+     * @param key the element to check
+     * @return {@code true} if the element might be in the Bloom filter, {@code false} if it is definitely not
+     */
     public boolean contains(T key) {
         for (Hash<T> hash : hashFunctions) {
-            int position = hash.compute(key) % bitArray.size();
+            int position = Math.abs(hash.compute(key) % bitArray.size());
             if (!bitArray.get(position)) {
                 return false;
             }
         }
         return true;
     }
 
-    private class Hash<T> {
+    /**
+     * Inner class representing a hash function used by the Bloom filter.
+     *
+     * @param <T> The type of elements to be hashed.
+     */
+    private static class Hash<T> {
 
-        int index;
+        private final int index;
 
+        /**
+         * Constructs a Hash function with a specified index.
+         *
+         * @param index the index of this hash function
+         */
         Hash(int index) {
             this.index = index;
         }
 
+        /**
+         * Computes the hash of the given key.
+         *
+         * @param key the element to hash
+         * @return the hash value
+         */
         public int compute(T key) {
             return index * asciiString(String.valueOf(key));
         }
 
+        /**
+         * Computes the ASCII value sum of the characters in a string.
+         *
+         * @param word the string to compute
+         * @return the sum of ASCII values of the characters
+         */
         private int asciiString(String word) {
-            int number = 0;
-            for (int i = 0; i < word.length(); i++) {
-                number += word.charAt(i);
+            int sum = 0;
+            for (char c : word.toCharArray()) {
+                sum += c;
             }
-            return number;
+            return sum;
         }
     }
 }

src/test/java/com/thealgorithms/datastructures/bloomfilter/BloomFilterTest.java

@@ -1,12 +1,19 @@
 package com.thealgorithms.datastructures.bloomfilter;
 
 import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
 public class BloomFilterTest {
+    private BloomFilter<String> bloomFilter;
+
+    @BeforeEach
+    void setUp() {
+        bloomFilter = new BloomFilter<>(3, 100);
+    }
 
     @Test
-    public void test1() {
+    public void testIntegerContains() {
         BloomFilter<Integer> bloomFilter = new BloomFilter<>(3, 10);
         bloomFilter.insert(3);
         bloomFilter.insert(17);
@@ -16,12 +23,43 @@ public void test1() {
     }
 
     @Test
-    public void test2() {
-        BloomFilter<String> bloomFilter = new BloomFilter<>(4, 20);
+    public void testStringContains() {
         bloomFilter.insert("omar");
         bloomFilter.insert("mahamid");
 
         Assertions.assertTrue(bloomFilter.contains("omar"));
         Assertions.assertTrue(bloomFilter.contains("mahamid"));
     }
+
+    @Test
+    void testInsertAndContains() {
+        bloomFilter.insert("hello");
+        bloomFilter.insert("world");
+
+        Assertions.assertTrue(bloomFilter.contains("hello"));
+        Assertions.assertTrue(bloomFilter.contains("world"));
+        Assertions.assertFalse(bloomFilter.contains("java"));
+    }
+
+    @Test
+    void testFalsePositive() {
+        bloomFilter.insert("apple");
+        bloomFilter.insert("banana");
+
+        Assertions.assertFalse(bloomFilter.contains("grape"));
+        Assertions.assertFalse(bloomFilter.contains("orange"));
+    }
+
+    @Test
+    void testMultipleInsertions() {
+        for (int i = 0; i < 100; i++) {
+            bloomFilter.insert("key" + i);
+        }
+
+        for (int i = 0; i < 100; i++) {
+            Assertions.assertTrue(bloomFilter.contains("key" + i));
+        }
+
+        Assertions.assertFalse(bloomFilter.contains("key" + 200));
+    }
 }