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    defined $ValueSet[$b] or $ValueSet[$b] = _extract_value($Array[$b], $Rule);
    
    # If the function returns zero or undef, the values are equivalent
    my $rc = &{ $Rule->{compare_func} }
	or next RULE;
    
    # Else return the comparison results, reversing them first if necessary
    return $rc * $Rule->{order_sign};
  }
  # If the items are equivalent for all of the rules, don't change their order
  # warn "Comparing $a and $b: '$ValueSet[$a]' " . ('=') . " '$ValueSet[$b]'\n";
  return $a <=> $b;
}

sub _sorted_packed {
  my @packed;
  if ( @Rules == 1 ) {
    @packed = map
    &{ $Rules[0]->{extract_func} }( $Array[$_], @{ $Rules[0]->{extract_args} } )
      . "\0" . $_, 
    ( 0 .. $#Array );
  } else {
    @packed = map { 
      my $item = $Array[$_]; 
      join( "\0", 
	map(&{ $_->{extract_func} }( $item, @{ $_->{extract_args} } ), @Rules),
	$_ 
      ) 
    } ( 0 .. $#Array );
  }
  
  # warn "Packed: " . join(', ', map "'$_'", @packed ) . "\n";
  
  return @Array[ map substr($_, 1 + rindex $_, "\0"), sort @packed ];
}

########################################################################

=head1 STATUS AND SUPPORT

This release of Data::Sorting is intended for public review and feedback. 

  Name            DSLIP  Description
  --------------  -----  ---------------------------------------------
  Data::
  ::Sorting       bdpfp  Multi-key sort using function results

Further information and support for this module is available at www.evoscript.org.

Please report bugs or other problems to E<lt>bugs@evoscript.comE<gt>.

=head1 BUGS AND TO DO

The following issues have been noted for future improvements:

Convert more types of comparisons to packed-default sorts for speed.

Further investigate the current status of the Sort::Records module.

Add a comparator function for an alpha-numeric-spans sorting model
like Sort::Naturally.

Interface to Sort::PolySort for alternate comparator styles, like
"name" and "usdate".

For non-scalar values, compare referents along the lines of
Ref::cmpref().

Provide better handling for nested sorts; perhaps throw an exception
from the inner instance to the outer, catch and set $PreCalculate,
then go back into the loop?

Replace dynamic scoping with object instances for thread safety.
May not be necessary given changes in threading models.

=head1 CREDITS AND COPYRIGHT

=head2 Developed By

  M. Simon Cavalletto, simonm@cavalletto.org
  Evolution Softworks, www.evoscript.org

=head2 Copyright

Copyright 2003 Matthew Cavalletto. 

Portions copyright 1996, 1997, 1998, 1999 Evolution Online Systems, Inc. 

=head2 License

You may use, modify, and distribute this software under the same terms as Perl.

=cut

########################################################################

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