Re: [PATCH] small glib fix.

[Darin Adler <darin bentspoon com>]

On Wednesday, May 30, 2001, at 03:35  PM, Damian Ivereigh wrote:

> > On Wednesday, May 30, 2001, at 02:21  PM, Anders Carlsson wrote:
> >
> > > Really? I just based it on the g_rand_boolean macro:
> > >
> > > #define g_rand_boolean(rand) (g_rand_int ((rand)) & (1<<15))
> > >
> > > Is that wrong too?
> >
> > On 30 May 2001 13:19:27 -0700, Darin Adler wrote:
> > I think so.
>
> I don't think it makes much difference. If the g_rand_int() function
> where truly random (i.e. each number had an equal chance of occuring,
> regardless of what the last number was) then it would absolutely make no
> difference (all your doing is selecting which bit to use). However I
> would have thought that the least significant bit was more likely to
> have a better spectral spread than the most significant bit.
>
> This is all dragging up college math that is a bit rusty.

My suggested version:

#define g_rand_boolean(rand) ((g_rand_int (rand) >> 15) & 1)

The g_rand_int() function is not "truly random". It's a pseudorandom 
number generator. But this is beside the point, because both the existing 
macro any my proposed changed macro use the 16th significant bit. Whether 
to use the most or least significant bit is not what I'm criticizing.

The issue is whether 0x8000 is a good value for a function that returns a 
"boolean" to return. the original function returns 0 half of the time and 
0x8000 the other half of the time. I prefer to use 0 and 1 for boolean 
values.

    -- Darin

PS: The least significant bits are in pseudorandom number generators are 
often not very random, so decisions should always be influenced primarily 
by the most significant digits. This is mentioned in Chapter 3 of Knuth's 
Art of Computer Programming in his [section 3.6, principle vi] and many 
other places.