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RFC 246 (v1) pack/unpack uncontrovercial enhancements

From:
Perl6 RFC Librarian
Date:
September 17, 2000 22:36
Subject:
RFC 246 (v1) pack/unpack uncontrovercial enhancements
Message ID:
20000918053619.17366.qmail@tmtowtdi.perl.org
This and other RFCs are available on the web at
  http://dev.perl.org/rfc/

=head1 TITLE

pack/unpack uncontrovercial enhancements

=head1 VERSION

  Maintainer: Ilya Zakharevich <ilya@math.ohio-state.edu>
  Date: 16 September 2000
  Mailing List: perl6-language-data@perl.org
  Number: 246
  Version: 1
  Status: Developing

=head1 ABSTRACT

This RFC proposes simple enhancements to templates of pack/unpack builtins.
These enhancements do not change the spirit of how pack/unpack is used.
The semantic is enhanced as much as possible under this constaint.

Additional changes to pack/unpack are listed in other proposals.

=head1 DESCRIPTION

The following additional constructs are allowed in pack()/unpack() TEMPLATEs:

=over 8

=item C<,> and C<;>

denotes the same as whitespace.

=item C<[4]>

denotes the same as C<4>, as in C<'i[4] S[4]'>.

=item C<[TEMPLATE]>

denotes the same as C<[$LEN]>; here $LEN is the length taken by the data
described by the template:  C<$LEN = length pack TEMPLATE, @something>
(as far as it does not depend on @something, so C<A*> is prohibited).
This multiplier makes sense for byte-oriented types only.  For example,
the middle part of C<'C[32] C[L8] C[64]'> will extract I<the substring> which
consists of 8 packed longs.  Similarly, C<'C[32] x[L8] C[64]'> will skip
this substring.

=item C<x!LEN>

skip to offset divisible by C<LEN>.  For example, C<'x![L]'> will align
to a long-compatible boundary.

=item C<W>

a BER encoded double.

=item C<w!>

a BER encoded "length", in other words, an integer encoded as BER lengths are.

=item C<(TEMPLATE)>

a group.  May take a repeatition count either as C<(Ns)33> or as C<N/(Ns)>.

=item C<TEMPLATE1//TEMLATE2>

On unpack(): C<TEMPLATE1> extracts a number.  This number specifies the number
of following bytes which the TEMPLATE2 should be applied to.  (The difference
with C<TEMPLATE1/(TEMLATE2)> is that in the latter case the number extracted
by C<TEMPLATE1> is not the number of bytes, but the repetition count.)
During pack(), makes the opposite operation.

For example,

  pack 'n//(A[4] C*)', 'a', 9..14

does the same as

  pack 'n A[4] C*', 10, 'a', 9..14

(this is especially useful with the grouping proposals introduced in a
future RFC).

=item quad-formats

should work on 32-bit architectures too, using doubles instead (with a
possible precision loss).

=back

=head1 MIGRATION ISSUES

None.

=head1 IMPLEMENTATION

Straightforward.

=head1 REFERENCES

RFC142: Enhanced Pack/Unpack




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