
=head1 NAME

Set::Object - set of objects and strings

=head1 SYNOPSIS

  use Set::Object;

  my $set = set();            # or Set::Object->new()

  $set->insert(@thingies);
  $set->remove(@thingies);

  @items = @$set;             # or $set->members;

  $union = $set1 + $set2;
  $intersection = $set1 * $set2;
  $difference = $set1 - $set2;
  $symmetric_difference = $set1 % $set2;

  print "set1 is a proper subset of set2"
      if $set1 < $set2;

  print "set1 is a subset of set2"
      if $set1 <= $set2;

  # common idiom - iterate over any pure Perl structure
  use Set::Object qw(reftype);
  my @stack = $root;
  my $seen = Set::Object->new(@stack);
  while (my $object = pop @stack) {
      if (reftype $object eq "HASH") {
          # do something with hash members

          # add the new nodes to the stack
          push @stack, grep { ref $_ && $seen->insert($_) }
              values %$object;
      }
      elsif (reftype $object eq "ARRAY") {
          # do something with array members

          # add the new nodes to the stack
          push @stack, grep { ref $_ && $seen->insert($_) }
              @$object;

      }
      elsif (reftype $object =~ /SCALAR|REF/) {
          push @stack, $$object
              if ref $$object && $seen->insert($$object);
      }
  }

=head1 DESCRIPTION

This modules implements a set of objects, that is, an unordered
collection of objects without duplication.

The term I<objects> is applied loosely - for the sake of
L<Set::Object>, anything that is a reference is considered an object.

L<Set::Object> 1.09 and later includes support for inserting scalars
(including the empty string, but excluding C<undef>) as well as
objects.  This can be thought of as (and is currently implemented as)
a degenerate hash that only has keys and no values.  Unlike objects
placed into a Set::Object, scalars that are inserted will be flattened
into strings, so will lose any magic (eg, tie) or other special bits
that they went in with; only strings come out.

=head1 CONSTRUCTORS

=head2 Set::Object->new( [I<list>] )

Return a new C<Set::Object> containing the elements passed in I<list>.

=head2 C<set(@members)>

Return a new C<Set::Object> filled with C<@members>.  You have to
explicitly import this method.

B<New in Set::Object 1.22>: this function is now called as a method
to return new sets the various methods that return a new set, such as
C<-E<gt>intersection>, C<-E<gt>union>, etc and their overloaded
counterparts.  The default method always returns C<Set::Object>
objects, preserving previous behaviour and not second guessing the
nature of your derived L<Set::Object> class.

=head2 C<weak_set()>

Return a new C<Set::Object::Weak>, filled with C<@members>.  You have
to explicitly import this method.

=head1 INSTANCE METHODS

=head2 insert( [I<list>] )

Add items to the C<Set::Object>.

Adding the same object several times is not an error, but any
C<Set::Object> will contain at most one occurence of the same object.

Returns the number of elements that were actually added.  As of
Set::Object 1.23, C<undef> will not insert.

=head2 includes( [I<list>] )

=head2 has( [I<list>] )

=head2 contains( [I<list>] )

Return C<true> if B<all> the objects in I<list> are members of the
C<Set::Object>.  I<list> may be empty, in which case C<true> is
always returned.

As of Set::Object 1.23, C<undef> will never appear to be present in
any set (even if the set contains the empty string).  Prior to 1.23,
there would have been a run-time warning.

=head2 member( [I<item>] )

=head2 element( [I<item>] )

Like C<includes>, but takes a single item to check and returns that
item if the value is found, rather than just a true value.

=head2 members

=head2 elements

Return the objects contained in the C<Set::Object> in random (hash)
order.

=head2 size

Return the number of elements in the C<Set::Object>.

=head2 remove( [I<list>] )

=head2 delete( [I<list>] )

Remove objects from a C<Set::Object>.

Removing the same object more than once, or removing an object absent
from the C<Set::Object> is not an error.

Returns the number of elements that were actually removed.

As of Set::Object 1.23, removing C<undef> is safe (but having an
C<undef> in the passed in list does not increase the return value,
because it could never be in the set)

=head2 weaken

Makes all the references in the set "weak" - that is, they do not
increase the reference count of the object they point to, just like
L<Scalar::Util|Scalar::Util>'s C<weaken> function.

This was introduced with Set::Object 1.16, and uses a brand new type
of magic.  B<Use with caution>.  If you get segfaults when you use
C<weaken>, please reduce your problem to a test script before
submission.

B<New:> as of Set::Object 1.19, you may use the C<weak_set> function
to make weak sets, or C<Set::Object::Weak-E<gt>new>, or import the
C<set> constructor from C<Set::Object::Weak> instead.  See
L<Set::Object::Weak> for more.

B<Note to people sub-classing C<Set::Object>:> this method re-blesses
the invocant to C<Set::Object::Weak>.  Override the method C<weak_pkg>
in your sub-class to control this behaviour.

=head2 is_weak

Returns a true value if this set is a weak set.

=head2 strengthen

Turns a weak set back into a normal one.

B<Note to people sub-classing C<Set::Object>:> this method re-blesses
the invocant to C<Set::Object>.  Override the method C<strong_pkg> in
your sub-class to control this behaviour.

=head2 invert( [I<list>] )

For each item in I<list>, it either removes it or adds it to the set,
so that a change is always made.

Also available as the overloaded operator C</>, in which case it
expects another set (or a single scalar element), and returns a new
set that is the original set with all the second set's items inverted.

=head2 clear

Empty this C<Set::Object>.

=head2 as_string

Return a textual Smalltalk-ish representation of the C<Set::Object>.
Also available as overloaded operator "".

=head2 equal( I<set> )

Returns a true value if I<set> contains exactly the same members as
the invocant.

Also available as overloaded operator C<==> (or C<eq>).

=head2 not_equal( I<set> )

Returns a false value if I<set> contains exactly the same members as
the invocant.

Also available as overloaded operator C<!=> (or C<ne>).

=head2 intersection( [I<list>] )

Return a new C<Set::Object> containing the intersection of the
C<Set::Object>s passed as arguments.

Also available as overloaded operator C<*>.

=head2 union( [I<list>] )

Return a new C<Set::Object> containing the union of the
C<Set::Object>s passed as arguments.

Also available as overloaded operator C<+>.

=head2 difference ( I<set> )

Return a new C<Set::Object> containing the members of the first
(invocant) set with the passed C<Set::Object>s' elements removed.

Also available as overloaded operator C<->.

=head2 unique ( I<set> )

=head2 symmetric_difference ( I<set> )

Return a new C<Set::Object> containing the members of all passed sets
(including the invocant), with common elements removed.  This will be
the opposite (complement) of the I<intersection> of the two sets.

Also available as overloaded operator C<%>.

=head2 subset( I<set> )

Return C<true> if this C<Set::Object> is a subset of I<set>.

Also available as operator C<E<lt>=>.

=head2 proper_subset( I<set> )

Return C<true> if this C<Set::Object> is a proper subset of I<set>
Also available as operator C<E<lt>>.

=head2 superset( I<set> )

Return C<true> if this C<Set::Object> is a superset of I<set>.
Also available as operator C<E<gt>=>.

=head2 proper_superset( I<set> )

Return C<true> if this C<Set::Object> is a proper superset of I<set>
Also available as operator C<E<gt>>.

=head2 is_null( I<set> )

Returns a true value if this set does not contain any members, that
is, if its size is zero.

=head1 Set::Scalar compatibility methods

By and large, L<Set::Object> is not and probably never will be
feature-compatible with L<Set::Scalar>; however the following
functions are provided anyway.

=head2 compare( I<set> )

returns one of:

  "proper intersect"
  "proper subset"
  "proper superset"
  "equal"
  "disjoint"

=head2 is_disjoint( I<set> )

Returns a true value if the two sets have no common items.

=head2 as_string_callback( I<set> )

Allows you to define a custom stringify function.  This is only a
class method.  If you want anything fancier than this, you should
sub-class Set::Object.


=head1 FUNCTIONS

The following functions are defined by the Set::Object XS code for
convenience; they are largely identical to the versions in the
Scalar::Util module, but there are a couple that provide functions not
catered to by that module.

Please use the versions in L<Scalar::Util> in preference to these
functions.

=over

=item B<blessed>

Returns a true value if the passed reference (RV) is blessed.  See
also L<Acme::Holy>.

=item B<reftype>

A bit like the perl built-in C<ref> function, but returns the I<type>
of reference; ie, if the reference is blessed then it returns what
C<ref> would have if it were not blessed.  Useful for "seeing through"
blessed references.

=item B<refaddr>

Returns the memory address of a scalar.  B<Warning>: this is I<not>
guaranteed to be unique for scalars created in a program; memory might
get re-used!

=item B<is_int>, B<is_string>, B<is_double>

A quick way of checking the three bits on scalars - IOK (is_int), NOK
(is_double) and POK (is_string).  Note that the exact behaviour of
when these bits get set is not defined by the perl API.

This function returns the "p" versions of the macro (SvIOKp, etc); use
with caution.

=item B<is_overloaded>

A quick way to check if an object has overload magic on it.

=item B<ish_int>

This function returns true, if the value it is passed looks like it
I<already is> a representation of an I<integer>.  This is so that you
can decide whether the value passed is a hash key or an array
index.

=item B<is_key>

This function returns true, if the value it is passed looks more like
an I<index> to a collection than a I<value> of a collection.

But wait, you say - Set::Object has no indices, one of the fundamental
properties of a Set is that it is an I<unordered collection>.  Which
means I<no indices>.  Well, if this module were ever to be derived to
be a more general multi-purpose collection, then this (and C<ish_int>)
might be a good function to use to distinguish different types of
indexes from values.

=item B<get_magic>

Pass to a scalar, and get the magick wand (C<mg_obj>) used by the weak
set implementation.  The return will be a list of integers which are
pointers to the actual C<ISET> structure.  Whatever you do don't
change the array :).  This is used only by the test suite, and if you
find it useful for something then you should probably conjure up a
test suite and send it to me, otherwise it could get pulled.

=back

=head1 CLASS METHODS

These class methods are probably only interesting to those
sub-classing C<Set::Object>.

=over

=item strong_pkg

When a set that was already weak is strengthened using
C<-E<gt>strengthen>, it gets re-blessed into this package.

=item weak_pkg

When a set that was NOT already weak is weakened using
C<-E<gt>weaken>, it gets re-blessed into this package.

=item tie_array_pkg

When the object is accessed as an array, tie the array into this
package.

=item tie_hash_pkg

When the object is accessed as a hash, tie the hash into this package.

=back

=head1 SERIALIZATION

It is possible to serialize C<Set::Object> objects via L<Storable> and
duplicate via C<dclone>; such support was added in release 1.04.  As
of C<Set::Object> version 1.15, it is possible to freeze scalar items,
too.

However, the support for freezing scalar items introduced a backwards
incompatibility.  Earlier versions than 1.15 will C<thaw> sets frozen
using Set::Object 1.15 and later as a set with one item - an array
that contains the actual members.

Additionally, version 1.15 had a bug that meant that it would not
detect C<freeze> protocol upgrades, instead reverting to pre-1.15
behaviour.

C<Set::Object> 1.16 and above are capable of dealing correctly with
all serialized forms, as well as correctly aborting if a "newer"
C<freeze> protocol is detected during C<thaw>.

=head1 PERFORMANCE

The following benchmark compares C<Set::Object> with using a hash to
emulate a set-like collection (this is an old benchmark, but still
holds true):

   use Set::Object;

   package Obj;
   sub new { bless { } }

   @els = map { Obj->new() } 1..1000;

   require Benchmark;

   Benchmark::timethese(100, {
      'Control' => sub { },
      'H insert' => sub { my %h = (); @h{@els} = @els; },
      'S insert' => sub { my $s = Set::Object->new(); $s->insert(@els) },
      } );

   %gh = ();
   @gh{@els} = @els;

   $gs = Set::Object->new(@els);
   $el = $els[33];

   Benchmark::timethese(100_000, {
	   'H lookup' => sub { exists $gh{33} },
	   'S lookup' => sub { $gs->includes($el) }
      } );

On my computer the results are:

   Benchmark: timing 100 iterations of Control, H insert, S insert...
      Control:  0 secs ( 0.01 usr  0.00 sys =  0.01 cpu)
               (warning: too few iterations for a reliable count)
     H insert: 68 secs (67.81 usr  0.00 sys = 67.81 cpu)
     S insert:  9 secs ( 8.81 usr  0.00 sys =  8.81 cpu)
   Benchmark: timing 100000 iterations of H lookup, S lookup...
     H lookup:  7 secs ( 7.14 usr  0.00 sys =  7.14 cpu)
     S lookup:  6 secs ( 5.94 usr  0.00 sys =  5.94 cpu)

=head1 THREAD SAFETY

This module has none.

=head1 AUTHOR

Original Set::Object module by Jean-Louis Leroy, <jll@skynet.be>

Set::Scalar compatibility, XS debugging, weak references support and
general maintainership courtesy of Sam Vilain, <samv@cpan.org>.
Maximum respect to those who send me test scripts, enhancements, etc
as patches against my git tree, browsable at
L<http://utsl.gen.nz/gitweb/?p=Set-Object>.

=head1 LICENCE

Copyright (c) 1998-1999, Jean-Louis Leroy. All Rights Reserved.
This module is free software. It may be used, redistributed
and/or modified under the terms of the Perl Artistic License

Portions Copyright (c) 2003 - 2005, Sam Vilain.  Same license.

Portions Copyright (c) 2006, 2007, Catalyst IT (NZ) Limited.  Same
license.

=head1 SEE ALSO

perl(1), perltie(1), L<Set::Scalar>, overload.pm

=cut

package Set::Object;

use strict;
use Carp;
use vars qw($VERSION @ISA @EXPORT @EXPORT_OK);

require Exporter;
require DynaLoader;
require AutoLoader;

@ISA = qw(Exporter DynaLoader);
# Items to export into callers namespace by default. Note: do not export
# names by default without a very good reason. Use EXPORT_OK instead.
# Do not simply export all your public functions/methods/constants.

@EXPORT_OK = qw( ish_int is_int is_string is_double blessed reftype
		 refaddr is_overloaded is_object is_key set weak_set );
$VERSION = '1.26';

bootstrap Set::Object $VERSION;

# Preloaded methods go here.

our $cust_disp;

sub as_string
{
    return $cust_disp->(@_) if $cust_disp;
    my $self = shift;
    croak "Tried to use as_string on something other than a Set::Object"
	unless (UNIVERSAL::isa($self, __PACKAGE__));

    ref($self).'(' . (join ' ', sort { $a cmp $b }
		     $self->members) . ')'
}

sub equal
{
   my ($s1, $s2) = @_;
   return undef unless (UNIVERSAL::isa($s2, __PACKAGE__));

   $s1->size() == $s2->size() && $s1->includes($s2->members);
}

sub not_equal
{
   !shift->equal(shift);
}

sub union
{
    $_[0]->set
	    ( map { $_->members() }
	      grep { UNIVERSAL::isa($_, __PACKAGE__) }
	      @_ );
}

sub op_union
{
    my $self = shift;
    my $other;
    if (ref $_[0]) {
	$other = shift;
    } else {
	$other = $self->set(shift);
    }

    croak("Tried to form union between Set::Object & "
	  ."`$other'")
	if ref $other and not UNIVERSAL::isa($other, __PACKAGE__);

    $self->union($other);

}

sub intersection
{
   my $s = shift;
   my $rem = $s->set($s->members);

   while ($s = shift)
   {
       if (!ref $s) {
	   $s = $rem->new($s);
       }

       croak("Tried to form intersection between Set::Object & "
	     .(ref($s)||$s)) unless UNIVERSAL::isa($s, __PACKAGE__);

       $rem->remove(grep { !$s->includes($_) } $rem->members);
   }

   $rem;
}

sub op_intersection
{
    my $s1 = shift;
    my $s2;
    if (ref $_[0]) {
	$s2 = shift;
    } else {
	$s2 = $s1->set(shift);
    }
    my $r = shift;
    if ( $r ) {
	return intersection($s2, $s1);
    } else {
	return intersection($s1, $s2);
    }

}

sub difference
{
   my ($s1, $s2, $r) = @_;
   if ( ! ref $s2 ) {
       if ( is_int($s2) and !is_string($s2) and $s2 == 0 ) {
	   return __PACKAGE__->new();
       } else {
	   my $set = __PACKAGE__->new($s2);
	   $s2 = $set;
       }
   }
   croak("Tried to find difference between Set::Object & "
	 .(ref($s2)||$s2)) unless UNIVERSAL::isa($s2, __PACKAGE__);

   my $s;
   if ( $r ) {
       $s = $s2->set( grep { !$s1->includes($_) } $s2->members );
   } else {
       $s = $s1->set( grep { !$s2->includes($_) } $s1->members );
   }
   $s;
}

sub op_invert
{
    my $self = shift;
    my $other;
    if (ref $_[0]) {
	$other = shift;
    } else {
	$other = __PACKAGE__->new(shift);
    }

    croak("Tried to form union between Set::Object & "
	  ."`$other'")
	if ref $other and not UNIVERSAL::isa($other, __PACKAGE__);

    my $result = $self->set( $self->members() );
    $result->invert( $other->members() );
    return $result;

}

sub op_symm_diff
{
    my $self = shift;
    my $other;
    if (ref $_[0]) {
	$other = shift;
    } else {
	$other = __PACKAGE__->new(shift);
    }
    return $self->symmetric_difference($other);
}

sub unique {
    my $self = shift;
    $self->symmetric_difference(@_);
}

sub symmetric_difference
{
   my ($s1, $s2) = @_;
   croak("Tried to find symmetric difference between Set::Object & "
	 .(ref($s2)||$s2)) unless UNIVERSAL::isa($s2, __PACKAGE__);

   $s1->difference( $s2 )->union( $s2->difference( $s1 ) );
}

sub proper_subset
{
   my ($s1, $s2) = @_;
   croak("Tried to find proper subset of Set::Object & "
	 .(ref($s2)||$s2)) unless UNIVERSAL::isa($s2, __PACKAGE__);
   $s1->size < $s2->size && $s1->subset( $s2 );
}

sub subset
{
   my ($s1, $s2, $r) = @_;
   croak("Tried to find subset of Set::Object & "
	 .(ref($s2)||$s2)) unless UNIVERSAL::isa($s2, __PACKAGE__);
   $s2->includes($s1->members);
}

sub proper_superset
{
   my ($s1, $s2, $r) = @_;
   croak("Tried to find proper superset of Set::Object & "
	 .(ref($s2)||$s2)) unless UNIVERSAL::isa($s2, __PACKAGE__);
   proper_subset( $s2, $s1 );
}

sub superset
{
   my ($s1, $s2) = @_;
   croak("Tried to find superset of Set::Object & "
	 .(ref($s2)||$s2)) unless UNIVERSAL::isa($s2, __PACKAGE__);
   subset( $s2, $s1 );
}

# following code pasted from Set::Scalar; thanks Jarkko Hietaniemi

use overload
   '""'  =>		\&as_string,
   '+'   =>		\&op_union,
   '*'   =>		\&op_intersection,
   '%'   =>		\&op_symm_diff,
   '/'   =>		\&op_invert,
   '-'   =>		\&difference,
   '=='  =>		\&equal,
   '!='  =>		\&not_equal,
   '<'   =>		\&proper_subset,
   '>'   =>		\&proper_superset,
   '<='  =>		\&subset,
   '>='  =>		\&superset,
   '%{}'  =>		sub { my $self = shift;
			      my %h = {};
			      tie %h, $self->tie_hash_pkg, [], $self;
			      \%h },
   '@{}'  =>		sub { my $self = shift;
			      my @h = {};
			      tie @h, $self->tie_array_pkg, [], $self;
			      \@h },
   'bool'  =>		sub { 1 },
    fallback => 1,
   ;

sub tie_hash_pkg { "Set::Object::TieHash" };
sub tie_array_pkg { "Set::Object::TieArray" };

{ package Set::Object::TieArray;
  sub TIEARRAY {
      my $p = shift;
      my $tie = bless [ @_ ], $p;
      require Scalar::Util;
      Scalar::Util::weaken($tie->[0]);
      Scalar::Util::weaken($tie->[1]);
      return $tie;
  }
  sub promote {
      my $self = shift;
      @{$self->[0]} = sort $self->[1]->members;
      return $self->[0];
  }
  sub commit {
      my $self = shift;
      $self->[1]->clear;
      $self->[1]->insert(@{$self->[0]});
  }
  sub FETCH {
      my $self = shift;
      my $index = shift;
      $self->promote->[$index];
  }
  sub STORE {
      my $self = shift;
      my $index = shift;
      $self->promote->[$index] = shift;
      $self->commit;
  }
  sub FETCHSIZE {
      my $self = shift;
      return $self->[1]->size;
  }
  sub STORESIZE {
      my $self = shift;
      my $count = shift;
      $#{$self->promote}=$count-1;
      $self->commit;
  }
  sub EXTEND {
  }
  sub EXISTS {
      my $self = shift;
      my $index = shift;
      if ( $index+1 > $self->[1]->size) {
	  return undef;
      } else {
	  return 1;
      }
  }
  sub DELETE {
      my $self = shift;
      delete $self->promote->[(shift)];
      $self->commit;
  }
  sub PUSH {
      my $self = shift;
      $self->[1]->insert(@_);
  }
  sub POP {
      my $self = shift;
      my $rv = pop @{$self->promote};
      $self->commit;
      return $rv;
  }
  sub CLEAR {
      my $self = shift;
      $self->[1]->clear;
  }
  sub SHIFT {
      my $self = shift;
      my $rv = shift @{$self->promote};
      $self->commit;
      return $rv;
  }
  sub UNSHIFT {
      my $self = shift;
      $self->[1]->insert(@_);
  }
  sub SPLICE {
      my $self = shift;
      my @rv;
      # perl5--
      if ( @_ == 1 ) {
	  splice @{$self->promote}, $_[0];
      }
      elsif ( @_ == 2 ) {
	  splice @{$self->promote}, $_[0], $_[1];
      }
      else {
	  splice @{$self->promote}, $_[0], $_[1], @_;
      }
      $self->commit;
      @rv;
  }
}

{ package Set::Object::TieHash;
  sub TIEHASH {
      my $p = shift;
      my $tie = bless [ @_ ], $p;
      require Scalar::Util;
      Scalar::Util::weaken($tie->[0]);
      Scalar::Util::weaken($tie->[1]);
      return $tie;
  }
  sub FETCH {
      my $self = shift;
      return $self->[1]->includes(shift);
  }
  sub STORE {
      my $self = shift;
      my $item = shift;
      if ( shift ) {
	  $self->[1]->insert($item);
      } else {
	  $self->[1]->remove($item);
      }
  }
  sub DELETE {
      my $self = shift;
      my $item = shift;
      $self->[1]->remove($item);
  }
  sub CLEAR {
      my $self = shift;
      $self->[1]->clear;
  }
  sub EXISTS {
      my $self = shift;
      $self->[1]->includes(shift);
  }
  sub FIRSTKEY {
      my $self = shift;
      @{$self->[0]} = $self->[1]->members;
      $self->NEXTKEY;
  }
  sub NEXTKEY {
      my $self = shift;
      if ( @{$self->[0]} ) {
	  return (shift @{$self->[0]});
      } else {
	  return ();
      }
  }
  sub SCALAR {
      my $self = shift;
      $self->[1]->size;
  }
}

# Autoload methods go after =cut, and are processed by the autosplit program.
# This function is used to differentiate between an integer and a
# string for use by the hash container types


# This function is not from Scalar::Util; it is a DWIMy function to
# decide whether the passed thingy could reasonably be considered
# to be an array index, and if so returns the index
sub ish_int {
    my $i;
    local $@;
    eval { $i = _ish_int($_[0]) };

    if ($@) {
	if ($@ =~ /overload/i) {
	    if (my $sub = UNIVERSAL::can($_[0], "(0+")) {
		return ish_int(&$sub($_[0]));
	    } else {
		return undef;
	    }
	} elsif ($@ =~ /tie/i) {
	    my $x = $_[0];
	    return ish_int($x);
	}
    } else {
	return $i;
    }
}

# returns true if the value looks like a key, not an object or a
# collection
sub is_key {
    if (my $class = tied $_[0]) {
	if ($class =~ m/^Tangram::/) { # hack for Tangram RefOnDemands
	    return undef;
	} else {
	    my $x = $_[0];
	    return is_key($x);
	}
    } elsif (is_overloaded($_[0])) {
	# this is a bit of a hack - intrude into the overload internal
	# space
	if (my $sub = UNIVERSAL::can($_[0], "(0+")) {
	    return is_key(&$sub($_[0]));
	} elsif ($sub = UNIVERSAL::can($_[0], '(""')) {
	    return is_key(&$sub($_[0]));
	} elsif ($sub = UNIVERAL::can($_[0], '(nomethod')) {
	    return is_key(&$sub($_[0]));
	} else {
	    return undef;
	}
    } elsif (is_int($_[0]) || is_string($_[0]) || is_double($_[0])) {
	return 1;
    } else {
	return undef;
    }
}

# interface so that Storable may still work
sub STORABLE_freeze {
    my $obj = shift;
    my $am_cloning = shift;
    return ("v3-" . ($obj->is_weak ? "w" : "s"), [ $obj->members ]);
}

#use Devel::Peek qw(Dump);

sub STORABLE_thaw {
    #print Dump $_ foreach (@_);

    $DB::single = 1;
    if ( $_[2] ) {
	if ( $_[2] eq "v2" ) {
	    @_ = (@_[0,1], "", @{ $_[3] });
	}
	elsif ( $_[2] =~ m/^v3-(w|s)/ ) {
	    @_ = (@_[0,1], "", @{ $_[3] });
	    if ( $1 eq "w" ) {
		my $self = shift;
		$self->_STORABLE_thaw(@_);
		$self->weaken();
		return;
	    }
	} else {
	    croak("Unrecognised Set::Object Storable version $_[2]");
	}
    }

    goto &_STORABLE_thaw;
    #print "Got here\n";
}

sub delete {
    my $self = shift;
    return $self->remove(@_);
}

our $AUTOLOAD;
sub AUTOLOAD {
    croak "No such method $AUTOLOAD";
}

sub invert {
    my $self = shift;
    while ( @_ ) {
	my $sv = shift;
	defined $sv or next;
	if ( $self->includes($sv) ) {
	    $self->remove($sv);
	} else {
	    $self->insert($sv);
	}
    }
}

sub compare {
    my $self = shift;
    my $other = shift;

    return "apples, oranges" unless UNIVERSAL::isa($other, __PACKAGE__);

    my $only_self = $self - $other;
    my $only_other = $other - $self;
    my $intersect = $self * $other;

    if ( $intersect->size ) {
	if ( $only_self->size ) {
	    if ( $only_other->size ) {
		return "proper intersect";
	    } else {
		return "proper subset";
	    }
	} else {
	    if ( $only_other->size ) {
		return "proper superset";
	    } else {
		return "equal";
	    }
	}
    } else {
	if ($self->size || $other->size) {
	    return "disjoint";
	} else {
	    # both sets are empty
	    return "equal";
	}
    }
}

sub is_disjoint {
    my $self = shift;
    my $other = shift;

    return "apples, oranges" unless UNIVERSAL::isa($other, __PACKAGE__);
    return !($self*$other)->size;
}

#use Data::Dumper;
sub as_string_callback {
    shift;
    if ( @_ ) {
	$cust_disp = shift;
	if ( $cust_disp &&
	     $cust_disp == \&as_string ) {
	    undef($cust_disp);
	}
    } else {
	\&as_string;
    }
}

sub elements {
    my $self = shift;
    return $self->members(@_);
}

sub has { (shift)->includes(@_) }
sub contains { (shift)->includes(@_) }
sub element { (shift)->member(@_) }
sub member {
    my $self = shift;
    my $item = shift;
    return ( $self->includes($item) ?
	     $item : undef );
}

sub set {
    local $@;
    if (eval { $_[0]->isa(__PACKAGE__) }) {
    	shift;
    }
    __PACKAGE__->new(@_);
}
sub weak_set {
    my $self = __PACKAGE__->new();
    $self->weaken;
    $self->insert(@_);
    return $self;
}

require Set::Object::Weak;
sub weaken {
    my $self = shift;
    $self->_weaken;
    bless $self, $self->weak_pkg;
}

sub strengthen {
    my $self = shift;
    $self->_strengthen;
    bless $self, $self->strong_pkg;
}

sub weak_pkg {
    "Set::Object::Weak";
}
sub strong_pkg {
    "Set::Object";
}
1;

__END__
