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Offline Roquen

JGO Kernel


Medals: 518



« Reply #150 - Posted 2013-03-27 16:27:25 »

Sigh:  Know your math kids.  I was peeking around at skip lists to see if I could find a "search finger" version to test out in a given situation I have.  No luck, but I glanced at quite of few skip list implementations and what struck me is:  people don't know their math.  A classic skip list needs to generate some "levels" with probabilities of : 50%, 25% 12.5%, etc.  The 'fastest' way this seems to be commonly done (in java-speak) is:

(int)(Math.log(1.-rng.nextDouble())/Math.log(0.5))

and quite of few do it iteratively. Doh!!  This can be cheaply be computed as follows:

Integer.numberOfLeadingZeros(rng.nextInt())

Why? Top bit has a 50% chance of being zero, and likewise for all following bits.  So having two zeros in a row is: 0.52 and likewise for n-zeros is 0.5n.  End of story.  (NOTE: leading zero counts are intrinsic and directly map to a hardware instruction in HotSpot)
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