Re: First time Ada has let me down

[Tucker Taft <stt@averstar.com>]

You have received a bunch of answers, not all relevant
to Ada 83.  It is true that Ada 83 doesn't have support
for bit-wise "and"/"or" on integers.  However, it has
several other features that make that unnecessary.

Defining a record type with fields of a 4-bit
type (such as "type Bit4 is range 0..15;")
and then putting a record representation clause
on it would seem to be a good starting point.

If you really want to manipulate bits, a packed
array of boolean does have "and" and "or", and
gives you complete access to individual bits.

Finally, you could define a packed array of the 4-bit type
(such as the one defined above), and use that to get
at individual "nibbles".

Unchecked conversion may be necessary somewhere along the
way, unless you can use an appropriate rep-spec'ed record or a
packed array to represent the entire packet.

Although a packed boolean array most directly matches
the (limited ;-) capabilities of C, the other mechanisms (such 
as rep-speced records, or packed array of 4-bit elements)
are probably a better fit for the problem.

Shifty wrote:
> 
> OK, here's the story:
> 
> I had a problem at work where I needed to calculate the offset of a
> "magic number" within a buffer containing an IP datagram.
> 
> The magic number is 3 protocol headers deep within the datagram, and
> all three headers are variable length.  I needed to find the HLEN
> fields of the IP and TCP headers in memory.  Turns out both of these
> are stored in 4 bits.  The data in the other 4 bits of the octet
> belong to another field in the header.

> ... I can't believe Ada (83) doesn't have native
> language support for these!!!  Please tell me that I am hopelessly
> confused and dead wrong!
> 
> Cheers,
> Andrew.

-- 
-Tucker Taft   stt@averstar.com   http://www.averstar.com/~stt/
Technical Director, Commercial Division, AverStar (formerly Intermetrics)
(http://www.averstar.com/services/IT_consulting.html)  Burlington, MA  USA