card-at.clj

(def ranks [2 3 4 5 6 7 8 9 0 "J" "Q" "K" "A"])
(def suits ["C" "D" "H" "S"])

(defn card-at
  "Retrieves the deck of card at the nth position"
  [n]
  (if (< n 52)
    (str
      (ranks (mod n 13))
      (suits
        (int (/ n 13))))
    (throw
      (ex-info "Deck of cards should not be more than 51."
        {:type :validation-failed}))))

;; Unit Tests for the cardAt method.
(def test-cases [
  [0  "2C"]
  [1  "3C"]
  [34 "0H"]
  [35 "JH"]
  [51 "AS"]
])

(defn test-card-at
  "Test the card-at method with the given input array"
  [case]
  (=
    (card-at (first case))
    (last case)))

;; Iterate over the test cases and see if they pass.
(print
  "Does every test case pass?"
  (if
    (every?
      true?
      (map test-card-at test-cases))
    "Yes!" "No."))

;; Invoke card-at directly in the REPL to see if the result is expected.
(card-at 0)
(card-at 1)
(card-at 34)
(card-at 35)
(card-at 51)

;; Ideally, we should test for exceptions too in the case that
;; the card exceeds 51, but that would
;; clutter the REPL due to the way repl.it works:
;;
;; (is (thrown-with-msg?
;;   clojure.lang.ExceptionInfo
;;   #"Deck of cards should not be more than"
;;   (card-at 52)))