/**
ROT13 is a simple substitution cipher that replaces a letter with the 13th letter
after it in the alphabet. Because the English alphabet has 26 letters (13 * 2),
applying ROT13 twice restores the original text (it is its own inverse).
*/
public class Rot13Demo {
/**
* Transforms a string using the ROT13 substitution cipher.
*
* @param text The input string to transform.
* @return A new string with the ROT13 cipher applied, or null if input is null.
*/
public static String rot13(String text) {
if (text == null) {
return null; // Guard against null pointer exceptions
}
// Use StringBuilder with pre-allocated capacity for optimal performance
StringBuilder result = new StringBuilder(text.length());
for (int i = 0; i < text.length(); i++) {
char c = text.charAt(i);
if (c >= 'a' && c <= 'z') {
// Shift lowercase letters by 13, wrapping around with modulo 26
result.append((char) ((c - 'a' + 13) % 26 + 'a'));
} else if (c >= 'A' && c <= 'Z') {
// Shift uppercase letters by 13, wrapping around with modulo 26
result.append((char) ((c - 'A' + 13) % 26 + 'A'));
} else {
// Non-alphabetic characters (digits, spaces, punctuation) pass through untouched
result.append(c);
}
}
return result.toString();
}
public static void main(String[] args) {
String message = "Hello, World! 123";
System.out.println("Original: " + message);
// Apply ROT13 encryption
String encrypted = rot13(message);
System.out.println("ROT13: " + encrypted);
// ROT13 is its own inverse; applying it again decrypts the message
String decrypted = rot13(encrypted);
System.out.println("Decrypted: " + decrypted);
}
}
/*
run:
Original: Hello, World! 123
ROT13: Uryyb, Jbeyq! 123
Decrypted: Hello, World! 123
*/
