Hi, Try this:
(defn stationary-convolve [n fs gs]
(let [window-size (count fs)
padding (repeat window-size 0)
padded-gs (concat padding gs padding)
new-center (+ n window-size)
window-of-gs (subvec (vec padded-gs)
(- (inc new-center) window-size)
(inc new-center))
reverse-window-fn (fn [i] (nth window-of-gs
(mod (- -1 i) window-size)))
reverse-window (map reverse-window-fn (range 0 window-size))]
(reduce + (map * fs reverse-window))))
(defn convolve [fs gs]
(map #(stationary-convolve % fs gs)
(range 0 (+ (count fs) (count gs)))))
(let [xs (for [i (range 1000000)] (rand 100))
ys (for [i (range 1000000)] (rand 100))]
(time (dotimes [_ 3] (convolve xs ys))))
"Elapsed time: 2540.967482 msecs"
Seems to run pretty fast and seems to be the kind of convolution you
want.
Thanks for the interesting problem!
Carson
On Jul 16, 12:57 pm, Isaac Hodes <iho...@mac.com> wrote:
> I posted this on StackOverflow yesterday, but to no avail: I don't
> think many people looked at it, or least I didn't get much feedback.
>
> I am trying to create a lazy/functional/efficient/Clojuresque function
> to carry out convolution on two lists/vectors of (ideally BigDecimals
> but that may be inefficient) doubles.
>
> It is turning out to be very difficult. I have four or five versions
> in my buffer right now, and none of them are acceptable. I've even
> tried a number of versions using into-array etc, e.g. mutables.
>
> Instead of posting a lot of links to pastie, I'm just copying the SO
> link here, where my code and the algorithms can be found.
>
> Right now, it seems as though this is something Clojure cannot do.
> David Nolen mentioned that the appropriate Java-interop functionality
> may come in Clojure 1.3, but aside from eschewing the functional style
> by using arrays, it seems as though there should be a better answer.
> (As an aside, where can I find out about 1.3?)
>
> I have a Python implementation to show how nicely it can be expressed
> in an imperitive language (though I suppose I dabbled in the
> functional style initializing my return array).
>
> Is it possible to do this, and do it well?
>
> Thank you so much, in advance.
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