Add Rail Fence Cipher (#5761)

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

@@ -0,0 +1,62 @@
+package com.thealgorithms.ciphers;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+public class RailFenceTest {
+
+    @Test
+    void testEncryption() {
+        RailFenceCipher cipher = new RailFenceCipher();
+
+        String input = "We are discovered! Flee at once";
+        int rails = 3;
+        String encrypted = cipher.encrypt(input, rails);
+        assertEquals("Wrivdlaneaedsoee!Fe toc  cr e e", encrypted);
+
+        String singleChar = "A";
+        int singleRail = 2;
+        String encryptedSingleChar = cipher.encrypt(singleChar, singleRail);
+        assertEquals("A", encryptedSingleChar);
+
+        String shortString = "Hello";
+        int moreRails = 10;
+        String encryptedShortString = cipher.encrypt(shortString, moreRails);
+        assertEquals("Hello", encryptedShortString);
+
+        String inputSingleRail = "Single line";
+        int singleRailOnly = 1;
+        String encryptedSingleRail = cipher.encrypt(inputSingleRail, singleRailOnly);
+        assertEquals("Single line", encryptedSingleRail);
+    }
+
+    @Test
+    void testDecryption() {
+        RailFenceCipher cipher = new RailFenceCipher();
+
+        // Scenario 1: Basic decryption with multiple rails
+        String encryptedInput = "Wrivdlaneaedsoee!Fe toc  cr e e";
+        int rails = 3;
+        String decrypted = cipher.decrypt(encryptedInput, rails);
+        assertEquals("We are discovered! Flee at once", decrypted);
+
+        // Scenario 2: Single character string decryption
+        String encryptedSingleChar = "A";
+        int singleRail = 2; // More than 1 rail
+        String decryptedSingleChar = cipher.decrypt(encryptedSingleChar, singleRail);
+        assertEquals("A", decryptedSingleChar);
+
+        // Scenario 3: String length less than the number of rails
+        String encryptedShortString = "Hello";
+        int moreRails = 10; // More rails than characters
+        String decryptedShortString = cipher.decrypt(encryptedShortString, moreRails);
+        assertEquals("Hello", decryptedShortString);
+
+        // Scenario 4: Single rail decryption (output should be the same as input)
+        String encryptedSingleRail = "Single line";
+        int singleRailOnly = 1;
+        String decryptedSingleRail = cipher.decrypt(encryptedSingleRail, singleRailOnly);
+        assertEquals("Single line", decryptedSingleRail);
+    }
+}