[boost] Re: Proposal: random number library

From: Jacob Hammeken (jacobh_at_[hidden])
Date: 2000-02-20 21:26:36

Greg Colvin wrote:
> I've been playing around with this, and made a small modification
> so that you can vary the quality of the result by specifying nbits
> other than 8.  Also made it compile as C (:-)
...
> I didn't mask off the higher bits of the counter
...
> I also made a Win32-specific version that has the virtue of not
> tying up the CPU so much:
...
>          QueryPerformanceCounter(&clicks);
...
Unfortunately, a high-performance counter may not be available, in
which case clicks will be 0. In that case, the return-value of
rand_byte will also be 0. By the way, the function is not available in
Win32s.
I suspect the granularity of GetTickCount (miliseconds) is way too
coarse to be used instead of QueryPerformanceCounter, in particular
because this is also the granularity of Sleep. But if not, perhaps one
could use GetTickCount instead if QueryPerformanceCounter fails?
My documentation states the following about GetTickCount:
>If the function succeeds, the return value is the number of
milliseconds that have elapsed since Windows was started
Unfortunately, it neglects to mention what happens if it does not
succeed... I can not quite imagine why it would not succeed, but it is
very frustrating not to know for sure.

It could perhaps look like this (I have only modified one line):
    unsigned char rand_byte(int nbits)
    {
       unsigned int result=0;
       int i;
       LARGE_INTEGER clicks;
       for (i=0; i<nbits; ++i) {
          Sleep(1);
          if(QueryPerformanceCounter(&clicks)==0)
clicks.LowPart=GetTickCount();
           // rotate low byte left 1 bit and XOR in LSB of counter
          result = (((result << 1) | (result >> 7)) & 0xFF) ^
(clicks.LowPart);
       }
       return result;
    }

Alternatively, one could perhaps use a different algorithm if
QueryPerformanceCounter fails.

Jacob Hammeken