/*
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).
*/
import Foundation
/// Applies the ROT13 cipher to the given string.
///
/// ROT13 rotates each letter by 13 positions in the alphabet:
/// A ↔ N, B ↔ O, ..., M ↔ Z
/// a ↔ n, b ↔ o, ..., m ↔ z
///
/// Non-alphabetic characters are left unchanged.
/// Applying ROT13 twice returns the original text.
///
/// - Parameter input: The text to encode or decode.
/// - Returns: The ROT13-transformed string.
func rot13(_ input: String) -> String {
String(input.map { character in
guard let ascii = character.asciiValue else {
return character
}
switch ascii {
case UInt8(ascii: "A")...UInt8(ascii: "Z"):
// Rotate uppercase letters.
let offset = ascii - Character("A").asciiValue!
return Character(UnicodeScalar(
Character("A").asciiValue! + (offset + 13) % 26
))
case UInt8(ascii: "a")...UInt8(ascii: "z"):
// Rotate lowercase letters.
let offset = ascii - Character("a").asciiValue!
return Character(UnicodeScalar(
Character("a").asciiValue! + (offset + 13) % 26
))
default:
// Leave digits, spaces, punctuation, etc. unchanged.
return character
}
})
}
let input = "Hello, World! 123"
let encoded = rot13(input)
let decoded = rot13(encoded)
print("Input : \(input)")
print("Encoded : \(encoded)")
print("Decoded : \(decoded)")
/*
run:
Input : Hello, World! 123
Encoded : Uryyb, Jbeyq! 123
Decoded : Hello, World! 123
*/
