Merge branch 'master' into stack_using_two_queues_new_algo

Author: Hardvan

DIRECTORY.md

@@ -42,6 +42,7 @@
             * [AES](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/AES.java)
             * [AESEncryption](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/AESEncryption.java)
             * [AffineCipher](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/AffineCipher.java)
+            * [Autokey](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/Autokey.java)
             * [Blowfish](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/Blowfish.java)
             * [Caesar](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/Caesar.java)
             * [ColumnarTranspositionCipher](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/ColumnarTranspositionCipher.java)
@@ -203,6 +204,7 @@
               * [SameTreesCheck](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/SameTreesCheck.java)
               * [SegmentTree](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/SegmentTree.java)
               * [SplayTree](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/SplayTree.java)
+              * [Treap](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/Treap.java)
               * [TreeRandomNode](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/TreeRandomNode.java)
               * [TrieImp](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/TrieImp.java)
               * [VerticalOrderTraversal](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/VerticalOrderTraversal.java)
@@ -457,6 +459,7 @@
             * [GenerateSubsets](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/Recursion/GenerateSubsets.java)
           * scheduling
             * [FCFSScheduling](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/scheduling/FCFSScheduling.java)
+            * [MLFQScheduler](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/scheduling/MLFQScheduler.java)
             * [PreemptivePriorityScheduling](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/scheduling/PreemptivePriorityScheduling.java)
             * [RRScheduling](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/scheduling/RRScheduling.java)
             * [SJFScheduling](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/scheduling/SJFScheduling.java)
@@ -619,6 +622,7 @@
           * ciphers
             * a5
               * [LFSRTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/a5/LFSRTest.java)
+            * [AutokeyTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/AutokeyTest.java)
             * [BlowfishTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/BlowfishTest.java)
             * [CaesarTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/CaesarTest.java)
             * [DESTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/DESTest.java)
@@ -729,6 +733,7 @@
               * [PreOrderTraversalTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/trees/PreOrderTraversalTest.java)
               * [SameTreesCheckTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/trees/SameTreesCheckTest.java)
               * [SplayTreeTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/trees/SplayTreeTest.java)
+              * [TreapTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/trees/TreapTest.java)
               * [TreeTestUtils](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/trees/TreeTestUtils.java)
               * [TrieImpTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/trees/TrieImpTest.java)
               * [VerticalOrderTraversalTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/trees/VerticalOrderTraversalTest.java)
@@ -912,6 +917,7 @@
             * [GenerateSubsetsTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/Recursion/GenerateSubsetsTest.java)
           * scheduling
             * [FCFSSchedulingTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/scheduling/FCFSSchedulingTest.java)
+            * [MLFQSchedulerTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/scheduling/MLFQSchedulerTest.java)
             * [PreemptivePrioritySchedulingTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/scheduling/PreemptivePrioritySchedulingTest.java)
             * [RRSchedulingTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/scheduling/RRSchedulingTest.java)
             * [SJFSchedulingTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/scheduling/SJFSchedulingTest.java)

src/main/java/com/thealgorithms/bitmanipulation/CountSetBits.java

@@ -48,4 +48,32 @@ public long countSetBits(long num) {
         }
         return cnt;
     }
+
+    /**
+     * This approach takes O(1) running time to count the set bits, but requires a pre-processing.
+     *
+     * So, we divide our 32-bit input into 8-bit chunks, with four chunks. We have 8 bits in each chunk.
+     *
+     * Then the range is from 0-255 (0 to 2^7).
+     * So, we may need to count set bits from 0 to 255 in individual chunks.
+     *
+     * @param num takes a long number
+     * @return the count of set bits in the binary equivalent
+     */
+    public int lookupApproach(int num) {
+        int[] table = new int[256];
+        table[0] = 0;
+
+        for (int i = 1; i < 256; i++) {
+            table[i] = (i & 1) + table[i >> 1]; // i >> 1 equals to i/2
+        }
+
+        int res = 0;
+        for (int i = 0; i < 4; i++) {
+            res += table[num & 0xff];
+            num >>= 8;
+        }
+
+        return res;
+    }
 }

src/main/java/com/thealgorithms/ciphers/Autokey.java

@@ -0,0 +1,55 @@
+package com.thealgorithms.ciphers;
+
+/**
+ * The Autokey Cipher is an interesting and historically significant encryption method,
+ * as it improves upon the classic Vigenère Cipher by using the plaintext itself to
+ * extend the key. This makes it harder to break using frequency analysis, as it
+ * doesn’t rely solely on a repeated key.
+ * https://en.wikipedia.org/wiki/Autokey_cipher
+ *
+ * @author bennybebo
+ */
+public class Autokey {
+
+    // Encrypts the plaintext using the Autokey cipher
+    public String encrypt(String plaintext, String keyword) {
+        plaintext = plaintext.toUpperCase().replaceAll("[^A-Z]", ""); // Sanitize input
+        keyword = keyword.toUpperCase();
+
+        StringBuilder extendedKey = new StringBuilder(keyword);
+        extendedKey.append(plaintext); // Extend key with plaintext
+
+        StringBuilder ciphertext = new StringBuilder();
+
+        for (int i = 0; i < plaintext.length(); i++) {
+            char plainChar = plaintext.charAt(i);
+            char keyChar = extendedKey.charAt(i);
+
+            int encryptedChar = (plainChar - 'A' + keyChar - 'A') % 26 + 'A';
+            ciphertext.append((char) encryptedChar);
+        }
+
+        return ciphertext.toString();
+    }
+
+    // Decrypts the ciphertext using the Autokey cipher
+    public String decrypt(String ciphertext, String keyword) {
+        ciphertext = ciphertext.toUpperCase().replaceAll("[^A-Z]", ""); // Sanitize input
+        keyword = keyword.toUpperCase();
+
+        StringBuilder plaintext = new StringBuilder();
+        StringBuilder extendedKey = new StringBuilder(keyword);
+
+        for (int i = 0; i < ciphertext.length(); i++) {
+            char cipherChar = ciphertext.charAt(i);
+            char keyChar = extendedKey.charAt(i);
+
+            int decryptedChar = (cipherChar - 'A' - (keyChar - 'A') + 26) % 26 + 'A';
+            plaintext.append((char) decryptedChar);
+
+            extendedKey.append((char) decryptedChar); // Extend key with each decrypted char
+        }
+
+        return plaintext.toString();
+    }
+}

src/main/java/com/thealgorithms/ciphers/a5/CompositeLFSR.java

@@ -5,13 +5,33 @@
 import java.util.Map;
 import java.util.TreeMap;
 
+/**
+ * The CompositeLFSR class represents a composite implementation of
+ * Linear Feedback Shift Registers (LFSRs) for cryptographic purposes.
+ *
+ * <p>
+ * This abstract class manages a collection of LFSR instances and
+ * provides a mechanism for irregular clocking based on the
+ * majority bit among the registers. It implements the BaseLFSR
+ * interface, requiring subclasses to define specific LFSR behaviors.
+ * </p>
+ */
 public abstract class CompositeLFSR implements BaseLFSR {
 
     protected final List<LFSR> registers = new ArrayList<>();
 
     /**
-     * Implements irregular clocking using the clock bit for each register
-     * @return the registers discarded bit xored value
+     * Performs a clocking operation on the composite LFSR.
+     *
+     * <p>
+     * This method determines the majority bit across all registers and
+     * clocks each register based on its clock bit. If a register's
+     * clock bit matches the majority bit, it is clocked (shifted).
+     * The method also computes and returns the XOR of the last bits
+     * of all registers.
+     * </p>
+     *
+     * @return the XOR value of the last bits of all registers.
      */
     @Override
     public boolean clock() {
@@ -26,6 +46,18 @@ public boolean clock() {
         return result;
     }
 
+    /**
+     * Calculates the majority bit among all registers.
+     *
+     * <p>
+     * This private method counts the number of true and false clock bits
+     * across all LFSR registers. It returns true if the count of true
+     * bits is greater than or equal to the count of false bits; otherwise,
+     * it returns false.
+     * </p>
+     *
+     * @return true if the majority clock bits are true; false otherwise.
+     */
     private boolean getMajorityBit() {
         Map<Boolean, Integer> bitCount = new TreeMap<>();
         bitCount.put(Boolean.FALSE, 0);

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

@@ -4,18 +4,41 @@
 import java.util.Map;
 
 /**
- * Java program for LFU Cache (https://en.wikipedia.org/wiki/Least_frequently_used)
+ * 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.
+ *
+ * @param <K> The type of keys maintained by this cache.
+ * @param <V> The type of mapped values.
+ *
+ * <p>
+ * Reference: <a href="https://en.wikipedia.org/wiki/Least_frequently_used">LFU Cache - Wikipedia</a>
+ * </p>
+ *
  * @author Akshay Dubey (https://github.com/itsAkshayDubey)
  */
 public class LFUCache<K, V> {
 
+    /**
+     * The {@code Node} class represents an element in the LFU cache.
+     * Each node contains a key, a value, and a frequency count.
+     * It also has pointers to the previous and next nodes in the doubly linked list.
+     */
     private class Node {
         private final K key;
         private V value;
         private int frequency;
         private Node previous;
         private Node next;
 
+        /**
+         * Constructs a new {@code Node} with the specified key, value, and frequency.
+         *
+         * @param key The key associated with this node.
+         * @param value The value stored in this node.
+         * @param frequency The frequency of usage of this node.
+         */
         Node(K key, V value, int frequency) {
             this.key = key;
             this.value = value;
@@ -29,10 +52,19 @@ private class Node {
     private final int capacity;
     private static final int DEFAULT_CAPACITY = 100;
 
+    /**
+     * Constructs an LFU cache with the default capacity.
+     */
     public LFUCache() {
         this(DEFAULT_CAPACITY);
     }
 
+    /**
+     * Constructs an LFU cache with the specified capacity.
+     *
+     * @param capacity The maximum number of items that the cache can hold.
+     * @throws IllegalArgumentException if the specified capacity is less than or equal to zero.
+     */
     public LFUCache(int capacity) {
         if (capacity <= 0) {
             throw new IllegalArgumentException("Capacity must be greater than zero.");
@@ -42,10 +74,12 @@ public LFUCache(int capacity) {
     }
 
     /**
-     * Retrieves the value for the given key from the cache. Increases the frequency of the node.
+     * 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
+     * in the linked list based on its updated frequency.
      *
-     * @param key The key to look up.
-     * @return The value associated with the key, or null if the key is not present.
+     * @param key The key whose associated value is to be returned.
+     * @return The value associated with the key, or {@code null} if the key is not present in the cache.
      */
     public V get(K key) {
         Node node = cache.get(key);
@@ -59,10 +93,12 @@ public V get(K key) {
     }
 
     /**
-     * Adds or updates a key-value pair in the cache. If the cache is full, the least frequently used item is evicted.
+     * Inserts or updates a key-value pair in the cache.
+     * If the key already exists, the value is updated and its frequency is incremented.
+     * If the cache is full, the least frequently used item is removed before inserting the new item.
      *
-     * @param key   The key to insert or update.
-     * @param value The value to insert or update.
+     * @param key The key associated with the value to be inserted or updated.
+     * @param value The value to be inserted or updated.
      */
     public void put(K key, V value) {
         if (cache.containsKey(key)) {
@@ -73,7 +109,7 @@ public void put(K key, V value) {
             addNodeWithUpdatedFrequency(node);
         } else {
             if (cache.size() >= capacity) {
-                cache.remove(this.head.key);
+                cache.remove(this.head.key); // Evict least frequently used item
                 removeNode(head);
             }
             Node node = new Node(key, value, 1);
@@ -84,8 +120,9 @@ public void put(K key, V value) {
 
     /**
      * Adds a node to the linked list in the correct position based on its frequency.
+     * The linked list is ordered by frequency, with the least frequently used node at the head.
      *
-     * @param node The node to add.
+     * @param node The node to be inserted into the list.
      */
     private void addNodeWithUpdatedFrequency(Node node) {
         if (tail != null && head != null) {
@@ -122,9 +159,10 @@ private void addNodeWithUpdatedFrequency(Node node) {
     }
 
     /**
-     * Removes a node from the linked list.
+     * Removes a node from the doubly linked list.
+     * This method ensures that the pointers of neighboring nodes are properly updated.
      *
-     * @param node The node to remove.
+     * @param node The node to be removed from the list.
      */
     private void removeNode(Node node) {
         if (node.previous != null) {