Re: Question for the folks who designed Ada95

[Tucker Taft <stt@averstar.com>]

Fraser Wilson wrote:
> 
> paene lacrimavi postquam Robert Dewar  scripsit:
> 
> >Now, *without* going to the Ada RM, let's see if we can
> >remember exactly what's going on with the etc here. Hands
> >up those who remember how NOT works on non-binary modular
> >types (hint: the above quote is wrong wrt NOT :-)
> 
> not X = Y: X + Y = 0 ?

Close.  X + not(X) + 1 = 0.

> Here's a question: has anyone ever used a modular type which was not
> a power of 2, and lived to tell the tale?  What was the context?

It is convenient for hash functions where you want to use
a prime number as the modulus.  It works even if the 
multiply or add would otherwise overflow.  

It is also convenient for cyclic buffers, though I will admit
that you end up having to put a comment on the "X := X + 1;" line
to avoid confusing the reader, because most readers are not
used to thinking about wrap-around arithmetic.  Though, of course,
this comment is appropriate for both binary as well as non-binary 
moduli, when wrap-around is expected (rather than being just a 
relatively rare "overflow" effect).

> Fraser.

-- 
-Tucker Taft   stt@averstar.com   http://www.averstar.com/~stt/
Technical Director, Distributed IT Solutions  (www.averstar.com/tools)
AverStar (formerly Intermetrics, Inc.)   Burlington, MA  USA