Agreed with all the comments on this so far. I would also say that dotimes
is slower than loop for stuff like this so I would also make that change.
(defn inplace-xor [^bytes a ^bytes b ^bytes out]
(let [len (alength a)]
(loop [i 0]
(when (< i len)
(aset-byte out i (bit-xor (aget a i) (aget b i)))
(recur (inc i))))))
In this scenario loop/recur will use a primitive long for i. And then I
would look at the bytecode to verify no boxing is occurring.
On Thursday, March 13, 2014 3:36:14 AM UTC-5, Michael Gardner wrote:
>
> Might be slow because of the polymorphic nature of nth. If you replace nth
> with aget (and turn on *warn-on-reflection*, which is a good idea when
> performance-tuning), you'll get reflection warnings because Clojure doesn't
> know what Java method to use since it doesn't know what type of objects a
> and b are. Once you silence those with type hints like in Walter's example,
> you get roughly a 2x speedup (in my tests).
>
> BTW, I'd write it like this:
>
> (defn inplace-xor [^bytes a ^bytes b ^bytes out]
> (dotimes [i (alength a)]
> (aset-byte out i
> (bit-xor (aget a i) (aget b i)))))
>
>
> On Mar 13, 2014, at 00:26 , Ignacio Corderi
> <[email protected]<javascript:>>
> wrote:
>
> > Hey guys, here is a huge performance problem I'm trying to figure out:
> >
> > ;; Say you have 2 data arrays sitting out there of 1 MB
> >
> > (def one-mb (byte-array (* 1024 1024)))
> > (def another-mb (byte-array (* 1024 1024)))
> >
> > ;; and another one that should have the byte-by-byte XOR of the previous
> two
> >
> > (def out-mb (byte-array (* 1024 1024)))
> >
> > ;; question is... how do you code this guy, so that it doesn't take
> forever
> >
> > (defn inplace-xor [a b out]
> > (def ln (count a))
> > (loop [x 0]
> > (if (< x ln)
> > (do
> > (aset-byte out x (bit-xor (nth a x) (nth b x)))
> > (recur (+ x 1))
> > ))))
> >
> > ;; checking the time
> >
> > (time (inplace-xor one-mb another-mb out-mb))
> >
> > ;; takes about ~400ms which is.... well... A LOT
> >
> > ;; I'm happy to receive a solution that involves calling some java
> library...
> >
> > Thanks in advance!
> > -Ignacio
> >
> >
> >
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