Graham--
I realize this list is about half programming and half feature/help
requests, but I'm hoping you can elaborate on this comment for the
half-programmers on the list... I think you're saying that a "bad"
pseudo-random number generator (PRNG) might be, say, slightly more
likely to pick a Q on the first draw than the expected 1/100, or, more
interestingly perhaps, slightly more likely to pick an exchanged tile
on the next draw, say, and you suggest a shuffling procedure where you
reorder individual elements in a vanishingly small proportion based on
the probability of drawing an endpoint--but the shuffling procedure
you describe is a bit obscure to me, so I might be completely missing
your point...
On 9/12/07, Graham Toal <[EMAIL PROTECTED]> wrote:
> On 9/12/07, jasonkatzbrown <[EMAIL PROTECTED]> wrote:
> > I'm surprised you would ask, even out of jest, but tiles are drawn as
> > pseudo-randomly as they need to be. If you want to further
> > investigate, all tile drawing goes through this one function that
> > draws one letter from the bag:
> >
> > [bag.cpp]
> >
> > Letter Bag::pluck()
> > {
> > return erase(DataManager::self()->randomNumber() % m_tiles.size());
> > }
>
> You're probably aware that there is a *slight* bias in that code
> (consider the degenerate case of a random number generator that
> returned a value in the range 0..7, and you wanted to pick a die face
> in the range 0..5 - then the values 6 and 7 would map to 0 and 1,
> making them twice as likely to turn up. However since the PRNG range
> is probably 32 bits (or at worst 16) then the extra probability of the
> end cases is vanishingly small.
>
> However, to ensure a fair distribution no matter how bad the PRNG, a
> better algorithm is to use the card shuffling trick once at startup,
> and then take the tiles sequentially from the shuffled list. (The
> shuffling alg is to go down the list one by one, swapping the current
> element with the last element if a random function in the range 0..1
> is 0. Then for added safety, create that random function by exoring
> *all* the bits from your PRNG down to a single bit, in case you have a
> bad PRNG with a bias at one end or the other.
>
> regards
>
> Graham
>
>
>
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>
