Add Junit tests for AffineCipher.java, improve documentation

DIRECTORY.md

@@ -618,6 +618,7 @@
           * ciphers
             * a5
               * [LFSRTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/a5/LFSRTest.java)
+            * [AffineCipherTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/AffineCipherTest.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)

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

@@ -1,5 +1,23 @@
 package com.thealgorithms.ciphers;
 
+/**
+ * The AffineCipher class implements the Affine cipher, a type of monoalphabetic substitution cipher.
+ * It encrypts and decrypts messages using a linear transformation defined by the formula:
+ *
+ *     E(x) = (a * x + b) mod m
+ *     D(y) = a^-1 * (y - b) mod m
+ *
+ * where:
+ * - E(x) is the encrypted character,
+ * - D(y) is the decrypted character,
+ * - a is the multiplicative key (must be coprime to m),
+ * - b is the additive key,
+ * - x is the index of the plaintext character,
+ * - y is the index of the ciphertext character,
+ * - m is the size of the alphabet (26 for the English alphabet).
+ *
+ * The class provides methods for encrypting and decrypting messages, as well as a main method to demonstrate its usage.
+ */
 final class AffineCipher {
     private AffineCipher() {
     }
@@ -8,45 +26,56 @@ private AffineCipher() {
     static int a = 17;
     static int b = 20;
 
+    /**
+     * Encrypts a message using the Affine cipher.
+     *
+     * @param msg the plaintext message as a character array
+     * @return the encrypted ciphertext
+     */
     static String encryptMessage(char[] msg) {
-        /// Cipher Text initially empty
+        // Cipher Text initially empty
         String cipher = "";
         for (int i = 0; i < msg.length; i++) {
             // Avoid space to be encrypted
-            /* applying encryption formula ( a x + b ) mod m
+            /* applying encryption formula ( a * x + b ) mod m
             {here x is msg[i] and m is 26} and added 'A' to
-            bring it in range of ascii alphabet[ 65-90 | A-Z ] */
+            bring it in the range of ASCII alphabet [65-90 | A-Z] */
             if (msg[i] != ' ') {
-                cipher = cipher + (char) ((((a * (msg[i] - 'A')) + b) % 26) + 'A');
+                cipher += (char) ((((a * (msg[i] - 'A')) + b) % 26) + 'A');
             } else { // else simply append space character
                 cipher += msg[i];
             }
         }
         return cipher;
     }
 
+    /**
+     * Decrypts a ciphertext using the Affine cipher.
+     *
+     * @param cipher the ciphertext to decrypt
+     * @return the decrypted plaintext message
+     */
     static String decryptCipher(String cipher) {
         String msg = "";
         int aInv = 0;
-        int flag = 0;
+        int flag;
 
-        // Find a^-1 (the multiplicative inverse of a
-        // in the group of integers modulo m.)
+        // Find a^-1 (the multiplicative inverse of a in the group of integers modulo m.)
         for (int i = 0; i < 26; i++) {
             flag = (a * i) % 26;
 
-            // Check if (a*i)%26 == 1,
+            // Check if (a * i) % 26 == 1,
             // then i will be the multiplicative inverse of a
             if (flag == 1) {
                 aInv = i;
             }
         }
         for (int i = 0; i < cipher.length(); i++) {
-            /*Applying decryption formula a^-1 ( x - b ) mod m
+            /* Applying decryption formula a^-1 * (x - b) mod m
             {here x is cipher[i] and m is 26} and added 'A'
-            to bring it in range of ASCII alphabet[ 65-90 | A-Z ] */
+            to bring it in the range of ASCII alphabet [65-90 | A-Z] */
             if (cipher.charAt(i) != ' ') {
-                msg = msg + (char) (((aInv * ((cipher.charAt(i) + 'A' - b)) % 26)) + 'A');
+                msg += (char) (((aInv * ((cipher.charAt(i) - 'A') - b + 26)) % 26) + 'A');
             } else { // else simply append space character
                 msg += cipher.charAt(i);
             }

src/test/java/com/thealgorithms/ciphers/AffineCipherTest.java

@@ -0,0 +1,39 @@
+package com.thealgorithms.ciphers;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+public class AffineCipherTest {
+
+    @Test
+    public void testEncryptMessage() {
+        String plaintext = "AFFINE CIPHER";
+        char[] msg = plaintext.toCharArray();
+        String expectedCiphertext = "UBBAHK CAPJKX"; // Expected ciphertext after encryption
+
+        String actualCiphertext = AffineCipher.encryptMessage(msg);
+        assertEquals(expectedCiphertext, actualCiphertext, "The encryption result should match the expected ciphertext.");
+    }
+
+    @Test
+    public void testEncryptDecrypt() {
+        String plaintext = "HELLO WORLD";
+        char[] msg = plaintext.toCharArray();
+
+        String ciphertext = AffineCipher.encryptMessage(msg);
+        String decryptedText = AffineCipher.decryptCipher(ciphertext);
+
+        assertEquals(plaintext, decryptedText, "Decrypted text should match the original plaintext.");
+    }
+
+    @Test
+    public void testSpacesHandledInEncryption() {
+        String plaintext = "HELLO WORLD";
+        char[] msg = plaintext.toCharArray();
+        String expectedCiphertext = "JKZZY EYXZT";
+
+        String actualCiphertext = AffineCipher.encryptMessage(msg);
+        assertEquals(expectedCiphertext, actualCiphertext, "The encryption should handle spaces correctly.");
+    }
+}