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From: Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups: comp.lang.c
Subject: Re: Bart, a tcc "issue"
Date: Sun, 10 Nov 2019 03:04:57 +0000
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Richard Damon <Richard@Damon-Family.org> writes:

> On 11/9/19 6:16 PM, Ben Bacarisse wrote:
>> Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
>> <cut>
>>> It's interesting -- three different people posted corrections or
>>> improvements to the uniform interval code, and all of the revisions
>>> have been a little bit off in one way or another.  Care is needed to
>>> deal with edge cases correctly.
>>>
>>>     int
>>>     uniform_less_than( unsigned k ){
>>>         unsigned d, r;
>>>
>>>         if(  k-1 > RAND_MAX  ||  k == 0  )  oops_someone_messed_up();
>>>
>>>         d = (RAND_MAX - (k-1)) / k + 1;
>>>         do  r = rand()/d;  while(  r >= k  );
>>>
>>>         return  r;
>>>     }
>>>
>>>     int
>>>     uniform_on_closed_interval( int a, int b ){
>>>         if(  a > b  )  return  uniform_on_closed_interval( b, a );
>>>
>>>         return  a + uniform_less_than( b + 1u - a );
>>>     }
>>>
>>> A good test for a uniform closed-interval RNG is to check that
>>> it handles the interval 0..RAND_MAX, including both endpoints,
>>> correctly.
>> 
>> Are you considering edge cases in the C standard?  If so, one of those
>> has RAND_MAX == UINT_MAX.
>
> Since rand() returns an int, the only way for RAND_MAX to be UINT_MAX is
> if UINT_MAX == INT_MAX (which generally means a Signed-Magnitude machine
> or some other odd architecture). And yes, if you want to deal with that
> sort of machine a lot of care needs to be taken, as a lot of things we
> often take for granted aren't true, like that an unsigned can count the
> number of values between any two signed numbers!!

Yes, it's a gruesome corner case for some types of code.

-- 
Ben.