ROTx

ROT13 with a shift

"Hello, World!".rot13() // Uryyb, Jbeyq!

"Uryyb, Jbeyq!".rot13() // Hello, World!

String+ROTx

shifted

"abcdefghijklmnopqrstuvwxyz".shifted(by: 1) // bcdefghijklmnopqrstuvwxyza

"abcdefghijklmnopqrstuvwxyz".shifted(by: 0) // abcdefghijklmnopqrstuvwxyz

"abcdefghijklmnopqrstuvwxyz".shifted(by: -1) // zabcdefghijklmnopqrstuvwxy

rot13

"abcdefghijklmnopqrstuvwxyz".rot13() // nopqrstuvwxyzabcdefghijklm

"nopqrstuvwxyzabcdefghijklm".rot13() // abcdefghijklmnopqrstuvwxyz