crappy quad tree

(defn quad-tree
  ([c l] {:mid c :len l})
  ([c l v] (assoc (quad-tree c l) :value v)))

(defn next-mid
  [qt quad]
  (let [[x y] (:mid qt)
        d (/ (:len qt) 2)]
    (case quad
      :I   [(+ x d) (+ y d)]
      :II  [(- x d) (+ y d)]
      :III [(- x d) (- y d)]
      :IV  [(+ x d) (- y d)]
      :else (println "Invalid quadrant"))))

(defn next-quad [[x y] [px py]]
  (cond
   (and (> px x) (> py y)) :I
   (and (< px x) (> py y)) :II
   (and (< px x) (< py y)) :III
   (and (> px x) (< py y)) :IV))

(declare add-points)

(defn add-point
  [qt point]
  (let [[x y] (:mid qt)
        [px py] point
        quad (next-quad [x y] point)
        val (:value qt)
        loc (quad qt)]
    (cond
     val (-> qt (dissoc :value) (add-points [val point]))
     loc (assoc qt quad (add-point loc point))
     :else (assoc qt quad (quad-tree (next-mid qt quad) (/ (:len qt) 2) point)))))

(defn add-points
  [qt pts] (reduce add-point qt pts))