AoC_2022_day_9.clj

(require ['clojure.string :as 'str])

(def head (atom [0 0]))

(def tail (atom [0 0]))

(def visited-tail-positions (atom #{[0 0]}))

(defn get-x [atom]
  (get @atom 0))

(defn get-y [atom]
  (get @atom 1))

(defn tail-touching-head? []
  (let [head-x (get-x head)
        head-y (get-y head)
        tail-x (get-x tail)
        tail-y (get-y tail)]
  (and (>= 1 (abs (- head-x tail-x)))
       (>= 1 (abs (- head-y tail-y))))))

(defn move-head-right [units]
  (dotimes [_ units]
    (swap! head update-in [0] inc)
    (if (not (tail-touching-head?))
      (let [head-x (get-x head)
            head-y (get-y head)]
        (reset! tail [(dec head-x) head-y])
        (swap! visited-tail-positions conj @tail)))))

(defn move-head-left [units]
  (dotimes [_ units]
    (swap! head update-in [0] dec)
    (if (not (tail-touching-head?))
      (let [head-x (get-x head)
            head-y (get-y head)]
        (reset! tail [(inc head-x) head-y])
        (swap! visited-tail-positions conj @tail)))))

(defn move-head-up [units]
  (dotimes [_ units]
    (swap! head update-in [1] inc)
    (if (not (tail-touching-head?))
      (let [head-x (get-x head)
            head-y (get-y head)]
        (reset! tail [head-x (dec head-y)])
        (swap! visited-tail-positions conj @tail)))))

(defn move-head-down [units]
  (dotimes [_ units]
    (swap! head update-in [1] dec)
    (if (not (tail-touching-head?))
      (let [head-x (get-x head)
            head-y (get-y head)]
        (reset! tail [head-x (inc head-y)])
        (swap! visited-tail-positions conj @tail)))))

(defn move-head [direction units]
  (case direction
    "R" (move-head-right units)
    "L" (move-head-left units)
    "U" (move-head-up units)
    "D" (move-head-down units)
    :else nil))

(defn parse-line [line]
  (let [[direction number] (str/split line #" ")]
    [direction (Integer/parseInt number)]))

(defn parse-input [filename]
  (->> filename
       slurp
       str/split-lines
       (map parse-line)))

(defn solution-part-1 [filename]
  (let [all-moves (parse-input filename)]
    (doseq [move all-moves]
      (apply move-head move))
    (count @visited-tail-positions)))