Hello.
I've tried to translate nice Hilbert-curve-index calculating function
to clojure (http://blog.notdot.net/2009/11/Damn-Cool-Algorithms-
Spatial-indexing-with-Quadtrees-and-Hilbert-Curves).
I've got sth like that:
(def hilbert-map {
:a {[0 0] [0 :d], [0 1] [1 :a], [1 0] [3 :b], [1 1] [2 :a]}
:b {[0 0] [2 :b], [0 1] [1 :b], [1 0] [3 :a], [1 1] [0 :c]}
:c {[0 0] [2 :c], [0 1] [3 :d], [1 0] [1 :c], [1 1] [0 :b]}
:d {[0 0] [0 :a], [0 1] [3 :c], [1 0] [1 :d], [1 1] [2 :d]} } )
(defn point-to-hilbert
"Return index of point (x,y) on Hilbert curve defined on square
(2^order)*(2^order).
x,y should be in 0 .. (2^order)-1
result will be in 0 .. (2^(order*2))-1"
[x y order]
(let [step-inside (fn [x i] (if (== 0 (bit-and x (bit-shift-left 1
i))) 0 1))
step (fn [square i] ((hilbert-map square) [(step-inside x i)
(step-inside y i)]))]
(loop [ square :a, position 0, i (dec order)]
(if (< i (int 0))
position
(let [ v (step square i)]
(recur (v 1), (-> position (bit-shift-left 2) (bit-or (v
0))), (dec i)))))))
(time (dotimes [ _ (int 1000)] (point-to-hilbert (rand-int (int
-1000)) (int 1) (int 16))))
I would like to somehow hide the global hilbert-map into my function,
but I can't see how to do that.
Is this possible? I know that I can just inert literal into my let,
but that degrades performance, when function is called many times.
I would like to have something like static local variable in C++, or
just regular constant local variable in my function.
Besides, if you see a way to improve my code, please, tell how.
Greetings.
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