Re: S-expression I/O in Ada

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org> writes:

> On 08/09/2010 11:32 AM, Natacha Kerensikova wrote:
>>
>> In C for such things I usually use a struct bitfield. I guess the Ada
>> equivalent would be a record of Booleans, with a pragma or something
>> to have them packed together. Would there be anything wrong with that?
>> It seems to me more readable than the above IFs, though from what I
>> understood from the rest of the thread it would make CASE on multiple
>> bit difficult, and it won't allow slicing (which I'm not sure is a bad
>> thing, readability-wise). Is there anything else?
>
> Typically, I would address it using:
>
> type Mode_ID is (Alarm, ...);
>
> type Mode_Set is array (Mode_ID) of Boolean;
> for Mode_Set'Component_Size use 1;
>
> Mode : Mode_Set;
>
> if Mode (Alarm) then ...
>
> Of course, I'd usually leave off the 'Component_Size definition; memory
> is usually cheap and plentiful these days. Only if I had to match an
> existing bit-packed representation, or Mode_ID was very large [array
> (Integer) of Boolean, anyone?] would I worry about the representation.

Well, if you're doing "and"s and "or"s (i.e. intersections and unions),
then the packed representation is likely more efficient.  Memory is
cheap in terms of money, but in terms of efficiency, using more
memory can make your program run slower (e.g. worse cache behavior).

I'd use "pragma Pack" instead of the rep clause in this case.
It's guaranteed to do the same thing, but the pragma has more
of a flavor of "for efficiency reasons".  I'd use the Component_Size
clause if I were interfacing to some existing representation,
i.e., "for correctness reasons".

- Bob