Point snapping function

snap.clj

(defn offset
  "Calculate distance between point and nearest tile pixel"
  [point pixel-size tile-pixel-count]
  (mod point (* pixel-size tile-pixel-count)))

(defn near
  "Find nearest point"
  [point interval shift]
  (-> point
      (- shift)
      (/ interval)
      (Math/floor)
      (* interval)
      (+ shift)))

(defn point->tile
  "Find the nearest tile"
  [{x :x y :y :as point}
   {:keys [xi yi xo yo] :as tile}]
  {:x (near x xi xo)
   :y (near y yi yo)})

(let [p1 (point->tile {:x -2040584 :y 3164804}
                      {:xi 3000 :xo 2415 :yi -3000 :yo -195})
      p2 (point->tile {:x -2040585 :y 3164805}
                      {:xi 3000 :xo 2415 :yi -3000 :yo -195})
      p3 (point->tile {:x -2040586 :y 3164806}
                      {:xi 3000 :xo 2415 :yi -3000 :yo -195})]
  [p1 p2 p3])

;; [{:x -2040585.0, :y 3164805.0}
;;  {:x -2040585.0, :y 3164805.0}
;;  {:x -2043585.0, :y 3167805.0}]

xi and yi – interval of a tile in projection coordinate units.
xo and yo – offset of tile grid from origin in projection coordinate units.