On Fri, Jan 28, 2011 at 10:48 AM, Robert McIntyre <r...@mit.edu> wrote:
> I tried to convert this java code line for line to clojure to compare
> the speed differences, and boy was I surprised!
>
> public static void ConvertToAWT(byte[] cpuArray){
> // Given an array of bytes representing a c-style bgra image,
> // converts to a java style abgr image
> int len = java.lang.reflect.Array.getLength(cpuArray);
> for (int i = 0; i < len; i+=4){
> byte b = cpuArray[i+0];
> byte g = cpuArray[i+1];
> byte r = cpuArray[i+2];
> byte a = cpuArray[i+3];
> cpuArray[i+0] = a;
> cpuArray[i+1] = b;
> cpuArray[i+2] = g;
> cpuArray[i+3] = r; }}
>
> (defn java-like []
> (loop [i (int 0)] (if (< i buffer-size)
> (let [ + clojure.core/unchecked-add
> b (aget cpuArray i)
> g (aget cpuArray (+ 1 i))
> r (aget cpuArray (+ 2 i))
> a (aget cpuArray (+ 3 i))]
> (aset-byte cpuArray i a)
> (aset-byte cpuArray (+ 1 i) b)
> (aset-byte cpuArray (+ 2 i) g)
> (aset-byte cpuArray (+ 3 i) r)
> (recur (int (+ i 4)))))))
>
> (defn clojure-like []
> (doall (flatten (map (fn [[b g r a]] [a b g r]) (partition 4 4
> cpuArray)))))
>
>
> for a byte-array of size 1920000, the java-like clojure function, a
> line for line translation, takes several minutes, while the java
> method takes around 3 milliseconds.
> the clojure-like one takes 6 seconds.
>
> Why is the clojure function so much more obnoxiously slow than its
> java counterpart?
> Can anyone shed some light on what I'm doing wrong?
Boxing, most likely, and/or reflection. Your cpuArray needs to be
produced with (int-array ...) and hinted with ^ints to get top
performance.
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