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From Alex Herbert <alex.d.herb...@gmail.com>
Subject Re: [rng] RNG-101 new MarsagliaTsangWang discrete probability sampler
Date Fri, 10 May 2019 13:52:54 GMT

On 10/05/2019 14:27, Gilles Sadowski wrote:
> Hi Alex.
>
> Le ven. 10 mai 2019 à 13:57, Alex Herbert <alex.d.herbert@gmail.com> a écrit
:
>> Can I get a review of the PR for RNG-101 please.
> Thanks for this work!
>
> I didn't go into the details; however, I see many fields and methods like
>    table1 ... table5
>    fillTable1 ... fillTable5
>    getTable1 ... getTable5
> Wouldn't it be possible to use a 2D table:
>    table[5][];
> so that e.g. only one "fillTable(int tableIndex, /* other args */)" method
> is necessary (where "tableIndex" runs from 0 to 4)?

Yes. The design is based around using 5 tables as per the example code.

The sample() method knows which table it needs so it can directly jump 
to the table in question. I'd have to look at the difference in speed 
when using a 2D table as you are adding another array access but 
reducing the number of possible method calls (although you still need a 
method call). Maybe this will be optimised out by the JVM.

If the speed is not a factor then I'll rewrite it. Otherwise it's 
probably better done for speed as this is the entire point of the 
sampler given it disregards any probability under 2^-31 (i.e. it's not a 
perfectly fair sampler).

Note that 5 tables are needed for 5 hex digits (base 2^6). The paper 
states using 3 tables of base 2^10 then you get a speed increase 
(roughly 1.16x) at the cost of storage (roughly 9x). Changing to 2 
tables of base 2^15 does not make it much faster again.

I'll have a rethink to see if I can make the design work for different 
base sizes.

>
> The diff for "DiscreteSamplersList.java" refers to
>     MarsagliaTsangWangBinomialSampler
> but
>    MarsagliaTsangWangSmallMeanPoissonSampler
> seems to be missing.

Oops, I missed adding that back. I built the PR from code where I was 
testing lots of implementations.

I've just added it back and it is still passing locally. Travis should 
see that too as I pushed the change to the PR.

>
> Regards,
> Gilles
>
>> This is a new sampler based on the source code from the paper:
>>
>> George Marsaglia, Wai Wan Tsang, Jingbo Wang (2004)
>> Fast Generation of Discrete Random Variables.
>> Journal of Statistical Software. Vol. 11, Issue. 3, pp. 1-11.
>>
>> https://www.jstatsoft.org/article/view/v011i03
>>
>> The code has no explicit licence.
>>
>> The paper states:
>>
>> "We have provided C versions of the two methods described here, for
>> inclusion in the “Browse
>> files”section of the journal. ... You may then want to examine the
>> components of the two files, for illumination
>> or for extracting portions that might be usefully applied to your
>> discrete distributions."
>>
>> So I assuming that it can be incorporated with little modification.
>>
>> The Java implementation has been rewritten to allow the storage to be
>> optimised for the required size. The generation of the tables has been
>> adapted appropriately and checks have been added on the input parameters
>> to ensure the sampler does not generate exceptions once constructed (I
>> found out the hard way that the original code was not entirely correct).
>>
>> Thanks.
>>
>> Alex
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