Merge branch 'master' into leftist_heap_improve

Author: siriak

DIRECTORY.md

@@ -0,0 +1,44 @@
+package com.thealgorithms.bitmanipulation;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * This class provides a method to generate all subsets (power set)
+ * of a given set using bit manipulation.
+ *
+ * @author Hardvan
+ */
+public final class GenerateSubsets {
+    private GenerateSubsets() {
+    }
+
+    /**
+     * Generates all subsets of a given set using bit manipulation.
+     * Steps:
+     * 1. Iterate over all numbers from 0 to 2^n - 1.
+     * 2. For each number, iterate over all bits from 0 to n - 1.
+     * 3. If the i-th bit of the number is set, add the i-th element of the set to the current subset.
+     * 4. Add the current subset to the list of subsets.
+     * 5. Return the list of subsets.
+     *
+     * @param set the input set of integers
+     * @return a list of all subsets represented as lists of integers
+     */
+    public static List<List<Integer>> generateSubsets(int[] set) {
+        int n = set.length;
+        List<List<Integer>> subsets = new ArrayList<>();
+
+        for (int mask = 0; mask < (1 << n); mask++) {
+            List<Integer> subset = new ArrayList<>();
+            for (int i = 0; i < n; i++) {
+                if ((mask & (1 << i)) != 0) {
+                    subset.add(set[i]);
+                }
+            }
+            subsets.add(subset);
+        }
+
+        return subsets;
+    }
+}

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

@@ -0,0 +1,30 @@
+package com.thealgorithms.bitmanipulation;
+
+/**
+ * This class provides a method to find the next higher number
+ * with the same number of set bits as the given number.
+ *
+ * @author Hardvan
+ */
+public final class NextHigherSameBitCount {
+    private NextHigherSameBitCount() {
+    }
+
+    /**
+     * Finds the next higher integer with the same number of set bits.
+     * Steps:
+     * 1. Find {@code c}, the rightmost set bit of {@code n}.
+     * 2. Find {@code r}, the rightmost set bit of {@code n + c}.
+     * 3. Swap the bits of {@code r} and {@code n} to the right of {@code c}.
+     * 4. Shift the bits of {@code r} and {@code n} to the right of {@code c} to the rightmost.
+     * 5. Combine the results of steps 3 and 4.
+     *
+     * @param n the input number
+     * @return the next higher integer with the same set bit count
+     */
+    public static int nextHigherSameBitCount(int n) {
+        int c = n & -n;
+        int r = n + c;
+        return (((r ^ n) >> 2) / c) | r;
+    }
+}

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

@@ -3,23 +3,38 @@
 import java.util.Arrays;
 import java.util.HashMap;
 import java.util.Map;
+
 /**
- * The ADFGVX cipher is a historically significant cipher used by
- * the German Army during World War I. It is a fractionating transposition
- * cipher that combines a Polybius square substitution with a columnar
- * transposition. It's named after the six letters (A, D, F, G, V, X)
- * that it uses in its substitution process.
- * https://en.wikipedia.org/wiki/ADFGVX_cipher
+ * The ADFGVX cipher is a fractionating transposition cipher that was used by
+ * the German Army during World War I. It combines a **Polybius square substitution**
+ * with a **columnar transposition** to enhance encryption strength.
+ * <p>
+ * The name "ADFGVX" refers to the six letters (A, D, F, G, V, X) used as row and
+ * column labels in the Polybius square. This cipher was designed to secure
+ * communication and create complex, hard-to-break ciphertexts.
+ * <p>
+ * Learn more: <a href="https://en.wikipedia.org/wiki/ADFGVX_cipher">ADFGVX Cipher - Wikipedia</a>.
+ * <p>
+ * Example usage:
+ * <pre>
+ * ADFGVXCipher cipher = new ADFGVXCipher();
+ * String encrypted = cipher.encrypt("attack at 1200am", "PRIVACY");
+ * String decrypted = cipher.decrypt(encrypted, "PRIVACY");
+ * </pre>
  *
  * @author bennybebo
  */
 public class ADFGVXCipher {
 
+    // Constants used in the Polybius square
     private static final char[] POLYBIUS_LETTERS = {'A', 'D', 'F', 'G', 'V', 'X'};
     private static final char[][] POLYBIUS_SQUARE = {{'N', 'A', '1', 'C', '3', 'H'}, {'8', 'T', 'B', '2', 'O', 'M'}, {'E', '5', 'W', 'R', 'P', 'D'}, {'4', 'F', '6', 'G', '7', 'I'}, {'9', 'J', '0', 'K', 'L', 'Q'}, {'S', 'U', 'V', 'X', 'Y', 'Z'}};
+
+    // Maps for fast substitution lookups
     private static final Map<String, Character> POLYBIUS_MAP = new HashMap<>();
     private static final Map<Character, String> REVERSE_POLYBIUS_MAP = new HashMap<>();
 
+    // Static block to initialize the lookup tables from the Polybius square
     static {
         for (int i = 0; i < POLYBIUS_SQUARE.length; i++) {
             for (int j = 0; j < POLYBIUS_SQUARE[i].length; j++) {
@@ -30,26 +45,41 @@ public class ADFGVXCipher {
         }
     }
 
-    // Encrypts the plaintext using the ADFGVX cipher
+    /**
+     * Encrypts a given plaintext using the ADFGVX cipher with the provided keyword.
+     * Steps:
+     * 1. Substitute each letter in the plaintext with a pair of ADFGVX letters.
+     * 2. Perform a columnar transposition on the fractionated text using the keyword.
+     *
+     * @param plaintext The message to be encrypted (can contain letters and digits).
+     * @param key       The keyword for columnar transposition.
+     * @return The encrypted message as ciphertext.
+     */
     public String encrypt(String plaintext, String key) {
-        plaintext = plaintext.toUpperCase().replaceAll("[^A-Z0-9]", "");
+        plaintext = plaintext.toUpperCase().replaceAll("[^A-Z0-9]", ""); // Sanitize input
         StringBuilder fractionatedText = new StringBuilder();
 
-        // Step 1: Polybius square substitution
         for (char c : plaintext.toCharArray()) {
             fractionatedText.append(REVERSE_POLYBIUS_MAP.get(c));
         }
 
-        // Step 2: Columnar transposition
         return columnarTransposition(fractionatedText.toString(), key);
     }
 
-    // Decrypts the ciphertext using the ADFGVX cipher
+    /**
+     * Decrypts a given ciphertext using the ADFGVX cipher with the provided keyword.
+     * Steps:
+     * 1. Reverse the columnar transposition performed during encryption.
+     * 2. Substitute each pair of ADFGVX letters with the corresponding plaintext letter.
+     * The resulting text is the decrypted message.
+     *
+     * @param ciphertext The encrypted message.
+     * @param key        The keyword used during encryption.
+     * @return The decrypted plaintext message.
+     */
     public String decrypt(String ciphertext, String key) {
-        // Step 1: Reverse the columnar transposition
         String fractionatedText = reverseColumnarTransposition(ciphertext, key);
 
-        // Step 2: Polybius square substitution
         StringBuilder plaintext = new StringBuilder();
         for (int i = 0; i < fractionatedText.length(); i += 2) {
             String pair = fractionatedText.substring(i, i + 2);
@@ -59,14 +89,21 @@ public String decrypt(String ciphertext, String key) {
         return plaintext.toString();
     }
 
+    /**
+     * Helper method: Performs columnar transposition during encryption
+     *
+     * @param text The fractionated text to be transposed
+     * @param key  The keyword for columnar transposition
+     * @return The transposed text
+     */
     private String columnarTransposition(String text, String key) {
         int numRows = (int) Math.ceil((double) text.length() / key.length());
         char[][] table = new char[numRows][key.length()];
-        for (char[] row : table) {
-            Arrays.fill(row, '_'); // Fill with underscores to handle empty cells
+        for (char[] row : table) { // Fill empty cells with underscores
+            Arrays.fill(row, '_');
         }
 
-        // Fill the table row by row
+        // Populate the table row by row
         for (int i = 0; i < text.length(); i++) {
             table[i / key.length()][i % key.length()] = text.charAt(i);
         }
@@ -88,6 +125,13 @@ private String columnarTransposition(String text, String key) {
         return ciphertext.toString();
     }
 
+    /**
+     * Helper method: Reverses the columnar transposition during decryption
+     *
+     * @param ciphertext The transposed text to be reversed
+     * @param key        The keyword used during encryption
+     * @return The reversed text
+     */
     private String reverseColumnarTransposition(String ciphertext, String key) {
         int numRows = (int) Math.ceil((double) ciphertext.length() / key.length());
         char[][] table = new char[numRows][key.length()];
@@ -96,19 +140,19 @@ private String reverseColumnarTransposition(String ciphertext, String key) {
         Arrays.sort(sortedKey);
 
         int index = 0;
-        // Fill the table column by column according to the sorted key order
+        // Populate the table column by column according to the sorted key
         for (char keyChar : sortedKey) {
             int column = key.indexOf(keyChar);
             for (int row = 0; row < numRows; row++) {
                 if (index < ciphertext.length()) {
                     table[row][column] = ciphertext.charAt(index++);
                 } else {
-                    table[row][column] = '_'; // Fill empty cells with an underscore
+                    table[row][column] = '_';
                 }
             }
         }
 
-        // Read the table row by row to get the fractionated text
+        // Read the table row by row to reconstruct the fractionated text
         StringBuilder fractionatedText = new StringBuilder();
         for (char[] row : table) {
             for (char cell : row) {

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

@@ -4,63 +4,116 @@
 import java.security.SecureRandom;
 
 /**
- * @author Nguyen Duy Tiep on 23-Oct-17.
+ * RSA is an asymmetric cryptographic algorithm used for secure data encryption and decryption.
+ * It relies on a pair of keys: a public key (used for encryption) and a private key
+ * (used for decryption). The algorithm is based on the difficulty of factoring large prime numbers.
+ *
+ * This implementation includes key generation, encryption, and decryption methods that can handle both
+ * text-based messages and BigInteger inputs. For more details on RSA:
+ * <a href="https://en.wikipedia.org/wiki/RSA_(cryptosystem)">RSA Cryptosystem - Wikipedia</a>.
+ *
+ * Example Usage:
+ * <pre>
+ * RSA rsa = new RSA(1024);
+ * String encryptedMessage = rsa.encrypt("Hello RSA!");
+ * String decryptedMessage = rsa.decrypt(encryptedMessage);
+ * System.out.println(decryptedMessage);  // Output: Hello RSA!
+ * </pre>
+ *
+ * Note: The key size directly affects the security and performance of the RSA algorithm.
+ * Larger keys are more secure but slower to compute.
+ *
+ * @author Nguyen Duy Tiep
+ * @version 23-Oct-17
  */
 public class RSA {
 
     private BigInteger modulus;
     private BigInteger privateKey;
     private BigInteger publicKey;
 
+    /**
+     * Constructor that generates RSA keys with the specified number of bits.
+     *
+     * @param bits The bit length of the keys to be generated. Common sizes include 512, 1024, 2048, etc.
+     */
     public RSA(int bits) {
         generateKeys(bits);
     }
 
     /**
-     * @return encrypted message
+     * Encrypts a text message using the RSA public key.
+     *
+     * @param message The plaintext message to be encrypted.
+     * @throws IllegalArgumentException If the message is empty.
+     * @return The encrypted message represented as a String.
      */
     public synchronized String encrypt(String message) {
+        if (message.isEmpty()) {
+            throw new IllegalArgumentException("Message is empty");
+        }
         return (new BigInteger(message.getBytes())).modPow(publicKey, modulus).toString();
     }
 
     /**
-     * @return encrypted message as big integer
+     * Encrypts a BigInteger message using the RSA public key.
+     *
+     * @param message The plaintext message as a BigInteger.
+     * @return The encrypted message as a BigInteger.
      */
     public synchronized BigInteger encrypt(BigInteger message) {
         return message.modPow(publicKey, modulus);
     }
 
     /**
-     * @return plain message
+     * Decrypts an encrypted message (as String) using the RSA private key.
+     *
+     * @param encryptedMessage The encrypted message to be decrypted, represented as a String.
+     * @throws IllegalArgumentException If the message is empty.
+     * @return The decrypted plaintext message as a String.
      */
     public synchronized String decrypt(String encryptedMessage) {
+        if (encryptedMessage.isEmpty()) {
+            throw new IllegalArgumentException("Message is empty");
+        }
         return new String((new BigInteger(encryptedMessage)).modPow(privateKey, modulus).toByteArray());
     }
 
     /**
-     * @return plain message as big integer
+     * Decrypts an encrypted BigInteger message using the RSA private key.
+     *
+     * @param encryptedMessage The encrypted message as a BigInteger.
+     * @return The decrypted plaintext message as a BigInteger.
      */
     public synchronized BigInteger decrypt(BigInteger encryptedMessage) {
         return encryptedMessage.modPow(privateKey, modulus);
     }
 
     /**
-     * Generate a new public and private key set.
+     * Generates a new RSA key pair (public and private keys) with the specified bit length.
+     * Steps:
+     * 1. Generate two large prime numbers p and q.
+     * 2. Compute the modulus n = p * q.
+     * 3. Compute Euler's totient function: φ(n) = (p-1) * (q-1).
+     * 4. Choose a public key e (starting from 3) that is coprime with φ(n).
+     * 5. Compute the private key d as the modular inverse of e mod φ(n).
+     * The public key is (e, n) and the private key is (d, n).
+     *
+     * @param bits The bit length of the keys to be generated.
      */
     public final synchronized void generateKeys(int bits) {
-        SecureRandom r = new SecureRandom();
-        BigInteger p = new BigInteger(bits / 2, 100, r);
-        BigInteger q = new BigInteger(bits / 2, 100, r);
+        SecureRandom random = new SecureRandom();
+        BigInteger p = new BigInteger(bits / 2, 100, random);
+        BigInteger q = new BigInteger(bits / 2, 100, random);
         modulus = p.multiply(q);
 
-        BigInteger m = (p.subtract(BigInteger.ONE)).multiply(q.subtract(BigInteger.ONE));
+        BigInteger phi = (p.subtract(BigInteger.ONE)).multiply(q.subtract(BigInteger.ONE));
 
         publicKey = BigInteger.valueOf(3L);
-
-        while (m.gcd(publicKey).intValue() > 1) {
+        while (phi.gcd(publicKey).intValue() > 1) {
             publicKey = publicKey.add(BigInteger.TWO);
         }
 
-        privateKey = publicKey.modInverse(m);
+        privateKey = publicKey.modInverse(phi);
     }
 }