#lang rhombus/static

import lib("racket/base.rkt") expose:
  #{string->list} as string_to_list
  #{list->string} as list_to_string
  modulo

fun make_rot13_char(key :: Int):
  fun (char :: Char):
    let char_int = char.to_int()
    fun rot13_by_case(char_int, a_char :: Char) :: Int :
      modulo(char_int - a_char.to_int() + key, 26) + a_char.to_int()
    if char.is_alphabetic():
    | if char.is_lowercase():
      | Char.from_int(rot13_by_case(char_int, Char"a"))
      | Char.from_int(rot13_by_case(char_int, Char"A"))
    | char

fun rot13_string(str :: String, key :: Int) :: String :
  let chars :: PairList = string_to_list(str)
  let rot13_char = make_rot13_char(key)
  let str :: MutableString = list_to_string(chars.map(rot13_char))
  str.snapshot()

rot13_string("Hello, World!", 14) // Output: "Vszzc, Kcfzr!""