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From: tmoran@acm.org
Newsgroups: comp.lang.ada
Subject: Re: calenday (was Re: IBM Acquires Rational Ada
References: <atqd1n$pin$1@slb9.atl.mindspring.net>
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Date: 2002-12-19T08:48:01+00:00
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> deal with the data and you discover your compiler is going to whine and
> snivel about being asked to span things across unusual boundaries.
What's wrong with the routine to align things, followed by an
Unchecked_Conversion to something with an acceptable rep spec, approach?

   type Bit_Width_Lists is array(Integer range <>) of Positive;
   procedure Align (Source : in     Packed_Data;
                    Control: in     Bit_Width_Lists;
                    Target :    out Ada.Streams.Stream_Element_Array);
   -- Copy sequential bit strings, inserting padding as needed to ensure
   -- that each starts on an alignment boundary in Target.

Unless only a few of the resultant fields are accessed, this won't be much
slower than having the compiler do the shifting and masking, and if some
fields are accessed a lot, it will actually be faster.  This doesn't help
with endianness or foreign floating point etc, but those are semantic
level things that a rep spec can't describe anyway.