[Kde-games-devel] Reproducing random numbers

[Albert Astals Cid]

A Monday, 13 of April de 2009, Ian Wadham va escriure:
> While working on the record/replay feature for KGoldrunner I have
> struck a couple of problems with random numbers.  I would
> appreciate any help or reassurance you can offer re the solutions
> I have adopted.
>
> Enemies in KGoldrunner depend on random numbers to decide
> where to reappear after being killed and when to pick up or drop
> gold.  Up till now the game has been using C library rand() (man
> 3 rand) with a seed based on the time() function (man 2 time), so
> that the levels play differently from one time to another.
>
> That's all very well, but how do you get a replayed level to run exactly
> the same as when it was recorded?  Random number generators
> appear to have no way to "read out" a seed.  The solution adopted
> was to generate a random integer when "live" play of a level is about to
> start, then re-inject that number as the seed and also save it in the
> "recording".  When playing back the recording, read in and use the
> same seed.  This seems a bit dodgy, but it works.
>
> The next problem I found was that something was "swallowing" a
> number from rand() occasionally and it did not seem to be code in
> KGoldrunner (logged every use of rand()).  Maybe some KDE or
> Qt library classes use rand()?  The effect of the swallowing was
> to completely mess up the course of play.
>
> So I switched to using a single KRandomSequence object and
> the problem seems to be solved.  Reading the code for KRS, the state
> appears to be completely self-contained and private, except that
> the code can call KRandom (and rand()?) if you do not supply a seed.
>
> So am I right in thinking that use of KRandomSequence is
> completely reproducible if you supply a seed before starting a
> sequence of numbers?

Reading the code and the documentation is what i understand too 
(KRandomSequence with seed gives a "known" random sequence), if you find this 
to be false i would qualify it as a bug.

Albert

>
> Cheers, Ian w.
>
>
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