example.clj

(defn diffuse [grid diff-ratio dt]
  (let [a (float (* dt diff-ratio grid-size grid-size))
	a4-1 (float (+ 1 (* 4 a)))
	grid #^floats (deref grid)
	diffused-grid #^floats (make-grid)
	grid-size (int grid-size)
	line-length (int (+ grid-size 2))]
    (dotimes [n 20]
      (dotimes [y grid-size]
	(let [line-offset (* (inc y) line-length)]
	  (loop [x (int 1)
		 c (int (+ x line-offset))]
	    (aset diffused-grid c
		  (/ (+ (aget grid c)
			(*  a
			    (+ (+ (aget diffused-grid
					(unchecked-dec c))
				  (aget diffused-grid
					(unchecked-inc c)))
			       (+ (aget diffused-grid
					(unchecked-subtract c line-length))
				  (aget diffused-grid
					(unchecked-add c line-length))))))
		     a4-1))
	    (when (< x grid-size)
	      (recur (unchecked-inc x)
		     (unchecked-inc c)))))))
    diffused-grid))