Cassidy's Interview Question - 24/5

editor.clj

(def S (atom ""))
(def undo-stack (atom []))
(def redo-stack (atom []))

;; Helpers

(defn reset-stacks!
  "Empties both stacks"
  []
  (reset! undo-stack [])
  (reset! redo-stack []))

(defn reset-app! []
  (reset-stacks!)
  (reset! S ""))

(defn update-stack
  "Updates the stack with the latest state"
  [st s]
  (swap! st conj s))

(defn reset-stack
  "Removes the top of st1 stack and moves it to the
  st2 stack. Updates S with the current top of st1"
  [st1 st2]
  (if (empty? @st1)
    (throw (Exception. "st1 is empty!"))
    (let [top (last @st1)
          new-st1 (vec (butlast @st1))
          new-S top]
      (update-stack st2 top)
      (reset! st1 new-st1)
      (reset! S new-S))))

;; Editor functions

(defn append
  "Appends the text x to S"
  [x]
  (swap! S (partial apply str) x)
  (update-stack undo-stack @S))

(defn delete
  "Deletes last n characters of S"
  [n]
  (when (zero? n)
    reset! S S)
  (when (neg? n)
    (throw (Exception. "Can't remove negative characters!")))
  (let [current-S @S
        new-S (subs current-S 0 (- (.length current-S) n))]
    (reset! S new-S)
    (update-stack undo-stack @S)))

(defn undo
  "Reverts S to previous saved state"
  []
  (reset-stack undo-stack redo-stack))

(defn redo
  "Reverts undo"
  []
  (reset-stack redo-stack undo-stack))

;; Playground

(reset-app!)

(append "this")
(append "isme")

(delete 3)
(delete 1)

(append "ismeeee!")

@S

(undo)
@undo-stack
@redo-stack
@S

(redo)
@undo-stack
@redo-stack
@S