[boost] Re: Proposal: random number library

From: Reid Sweatman (borderland_at_[hidden])
Date: 2000-02-21 02:44:04

Obviously this is a pretty OS-specific comment, but a few months back Andy
Glew had some pertinent comments on the use of the x86 Time Stamp Counter
instructions for timing under Windows.  I believe the thread was concerning
the generic timer module.  While the TSC stuff isn't perfect, as Andy
pointed out, it probably is good enough for this purpose.

> -----Original Message-----
> From: Greg Colvin [mailto:gcolvin_at_[hidden]]
> Sent: Sunday, February 20, 2000 7:44 PM
> To: boost_at_[hidden]
> Subject: [boost] Re: Proposal: random number library
>
>
> From: Jacob Hammeken <jacobh_at_[hidden]>
> > 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.
>
> So much the worse for Win32s.  As I recall, Win32s is still close
> enough to DOS that you can just peek into some memory location
> for the system clock.  Maybe this is what GetTickCount() does.
>
> > 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
>
> Although on my NT box Sleep() never comes back in less than 15
> milliseconds.
>
> > 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.
>
> Such frustration is the typical state of a Win32 programmer.
> Too bad we can't just check the source code.
>
> > 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.
>
> Perhaps, but we are getting into pretty machine/OS specific
> programming here.  Even the "portable" versions based on clock()
> can easily fail to produce random numbers.  So the thing to do
> is just test.
>
>
>
>
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