Alux,
Welcome to Clojrue! Thanks for posting this example.
At first glance I would combine power-of-two? and what-power-of-2 into
one fn. Also, I would change the order you wrote the method calls in
to favor the .method style
(defn power-of-2
"Returns log_2(n) iff n is an exact pwoer of 2. nil otherwise"
(if (= (.bitCount (big-int n)) 1)
(dec (.bitLength (big-int n))))
Also, letfn is for mutually recursive fns (you know, the kind that
swear at each other :). I would move big-int, step & power-of-2 into
their own defns. That should clean up your definition of conway-PM.
Sean
PS - Do you use github or lisppaste? The general practice is to put
code in a pastebin, and simply include a link in the mailing list.
On Feb 13, 11:48 am, alux <alu...@googlemail.com> wrote:
> Hi,
>
> I just finished my first (very small) Clojure program. I come from
> Java, so it will be everything but idiomatic Clojure, so I post it
> here and you may give as much criticism as you want. I'm here to
> learn.
>
> It implements JHConway prime machine. The program does not want to be
> a quick prime generator (the algorithm is interesting but not quick at
> all), but it shall be/become a good implementation of this algorithm.
>
> http://www.jstor.org/pss/2690263(The first page is enough to
> implement the algorithm, even if the explanation will come on the next
> pages only.)
>
> Started like:
>
> (conway-PM)
>
> It came up to 563 after about 6 hours on my 1.2 GHz Centrino ;-)
>
> Thats the program, feel free to tell me stuff (or to ignore me, would
> be a lessen too :)
>
> (defn conway-PM []
> (let [
> factors '(17/91 78/85 19/51 23/38 29/33 77/29 95/23 77/19
> 1/17 11/13
> 13/11 15/14 15/2 55/1)
> big-int-class (. 999999999999999999999999999999999 getClass)]
> (letfn [
> (big-int [n];"makes a bigint from int, keeps n if
> bigint already"
> (let [current-class (. n getClass)]
> (if (= current-class big-int-class) n
> (. java.math.BigInteger
> valueOf n))))
> (power-of-two? [n];"true iff the integer n is an
> power of 2."
> (= (. (big-int n) bitCount) 1))
> (what-power-of-2 [n];"gets n from 2^n"
> (- (. (big-int n) bitLength) 1))
> (step ;"Multiplies with the first factor to give an
> integer."
> [n, unused-factors]
> (let [factor (first unused-factors)
> rest-factors (rest unused-factors)
> dummy (* n factor)]
> (if (integer? dummy)
> dummy
> (step n rest-factors))))]
> (loop [seed 2]
> (let [next (step seed factors)]
> (if (power-of-two? next)
> (println (what-power-of-2
> next)))
> (recur next))))))
>
> Regards, alux
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