Hi,
I'm still trying to work out the best way to deal with operations on
heterogenous data in clojure. I have a complex application which has such
requirements. I wrote a simple toy matrix as a means to explore closure
versus map based implementations. Note that in this case the "data structure"
is not pure, rather mutable. Here are 2 implementations I came up with (note
this is my second day with closure, so my not be idiomatic):
The first impl uses closures and provides access via a function (is there a
more efficient way to do this?, avoiding the cond-dispatch?):
(defn matrix-1 [nrow ncol]
(let [data (float-array (* nrow ncol))]
(fn [command & args]
(condp = command
:dim
[nrow ncol]
:get
(let [[i j] args] (aget data (* i j)))
:set
(let [[i j v] args] (aset-float data (* i j) v))))))
The second implementation uses a map:
(defn matrix-2 [nrow ncol]
{ :data (float-array (* nrow ncol)), :nrow nrow, :ncol ncol })
(defn mset! [mat i j v]
(aset-float (get mat :data) (* i j) v))
(defn mget [mat i j]
(aget (get mat :data) (* i j)))
(defn mdim [mat]
[(get mat :nrow) (get mat :ncol)])
Both of these implementations bother me. The first because of the dispatch,
the second because maps clearly are not "near the metal". It would seem
would have to resort to java side classes, unless there is a better way?
BTW, the map implementation is about 3x faster:
(def m1 (matrix-1 10 10))
(def m2 (matrix-2 10 10))
...
(defn benchmark [what times f]
(let [
Tstart
(System/currentTimeMillis)
result
(loop [ntimes times]
(f)
(if (> ntimes 0)
(recur (- ntimes 1))))
Tend
(System/currentTimeMillis)]
(println "evaluating" what "took" (- Tend Tstart) "ms")))
(benchmark "map-based-matrix" 1000000 (fn [] (mset! m2 3 3 0.1234)))
(benchmark "closure-based-matrix" 1000000 (fn [] (m1 :set 3 3 0.1234)))
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