#lang racket  
(provide (all-defined-out))

; shift : char, int -> char
; Shift `ch` by `k` letters (wrapping around at Z).
; Account for upper/lower case.
; Punctuation, however, is not handled correctly (it gets turned into a letter).
;
(define (shift ch k)
  (let* {[offset (char->integer (if (char-upper-case? ch) #\A #\a))]
         [code (- (char->integer ch) offset)]
         [new-code (+ code k)]
         [new-code% (modulo new-code 26)]
         }
    (integer->char (+ new-code% offset))))


(module+  test   ;; a submodule: not run when this file is being require'd elsewhere.
  (require rackunit)
  (check-eq? (shift #\A 1) #\B)
  (check-eq? (shift #\b 2) #\d)
  (check-eq? (shift #\Z 1) #\A)
  (check-eq? (shift #\A -1) #\Z)
  (check-eq? (shift #\A 26) #\A)
  (check-eq? (shift #\A 29) #\D)
  (check-eq? (shift #\a -29) #\x)
  (check-eq? (shift #\B -2601) #\A)
  )