Re: [boost] [review][constrained_value] Review of Constrained Value Library begins today

Subject: Re: [boost] [review][constrained_value] Review of Constrained Value Library begins today
From: Gordon Woodhull (gordon_at_[hidden])
Date: 2008-12-10 14:29:35

On Dec 10, 2008, at 7:24 AM, Johan Råde wrote:
> Thorsten Ottosen wrote:
>> John Maddock gave two examples: one where we accapted wrong values  
>> and one where we rejected correct values. I found the latter  
>> somewhat better (because the invariant of bounded_float would be  
>> preserved), and if we are able to get that behavior consistently, I  
>> would be quite happy.
>> So maybe most of our problems will be fixed by using >= and <=  
>> instead of < and >?
>> -Thorsten
> Is the following true: if x and y satisfy  x <= y (or x >= y)
> then they will still do so after any truncation from 80 to 64 bits?

Nope, as John pointed out later in his message, if the test is x<=y  
and x is just slightly greater than y, then x can flip either way.

Epsilon doesn't help here; it just moves the problem around.  Now x<=y  
is going to have the same problem with x very close to y+eps.  The  
only nicer thing about it is that if you are testing *inequality* x<y  
then you can be sure that it will only accept x<y if x really is less  
than y and will remain so after rounding (even though the predicate  
may later fail).  This is the distinction Stjepan was making between  
the invariant and the test.

> Then, as Thorsten suggests, <= and >= will never erroneously accept  
> any value.

Rejecting correct values is indeed better than accepting incorrect  
values.  It seems that you will have to be careful which predicate you  
choose when dealing with floats.

> We could then maybe implement < and > using <=, >= and the  
> std::nextafter function,
> and achieve the same guarantee there.

That's interesting.

I'm really glad to see the discussion of the problems with floating  
point, and I'll make a stab at a few predicates implementing epsilon  
and forced truncation in a couple of weeks when school is out - or if  
anyone else wants to start this, please do.

I believe that this group of people has the ability to get this  
problem right.

Gordon