/*
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).
*/
/// Applies the ROT13 cipher to the input string.
///
/// ROT13 rotates each ASCII letter by 13 positions:
/// 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.
fn rot13(input: &str) -> String {
input
.chars()
.map(|c| match c {
// Rotate uppercase letters.
'A'..='Z' => {
let offset = c as u8 - b'A';
(b'A' + (offset + 13) % 26) as char
}
// Rotate lowercase letters.
'a'..='z' => {
let offset = c as u8 - b'a';
(b'a' + (offset + 13) % 26) as char
}
// Leave all other characters unchanged.
_ => c,
})
.collect()
}
fn main() {
let input = "Hello, World! 123";
let encoded = rot13(input);
let decoded = rot13(&encoded);
println!("Input : {}", input);
println!("Encoded : {}", encoded);
println!("Decoded : {}", decoded);
}
/*
run:
Input : Hello, World! 123
Encoded : Uryyb, Jbeyq! 123
Decoded : Hello, World! 123
*/
