Re: [Bug-gnubg] Dice Roll Generation

[Joern Thyssen]

On Thu, Aug 14, 2003 at 02:52:00PM -0600, Michael Petch wrote
> 
> I was reviewing the dice generation algorithms on the weekend in more
> detail than I had before, and I was curious about something.
> 
> I noticed that most of the dice generation is based on finding the
> remainder of integer division (dividing a value by 6 and take
> remainder).
> 
> What I find odd about this is that relative lengths have been taken to
> ensure the initial seeds are sufficiently random however using modulus
> to define a range introduces a slight bias for certain remainders.

I've modified the code to use:

        anDice[ 0 ] = 1+(int) (6.0*rand()/(RAND_MAX+1.0));

instead. This is suggested on the rand(3) man page:

       In Numerical Recipes in C: The Art of Scientific Computing
       (William  H.  Press, Brian P. Flannery, Saul A. Teukolsky,
       William T.  Vetterling;  New  York:  Cambridge  University
       Press, 1992 (2nd ed., p. 277)), the following comments are
       made:
              "If you want to generate a random integer between 1
              and 10, you should always do it by using high-order
              bits, as in

                     j=1+(int) (10.0*rand()/(RAND_MAX+1.0));

              and never by anything resembling

                     j=1+(rand() % 10);

              (which uses lower-order bits)."

Jørn

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