#!/usr/local/bin/perl
#!/usr/bin/perl
use strict;
use warnings;

package Set;
sub main::set { __PACKAGE__->new(@_) } # exportの代わり

use overload (
    '@{}' => \&array_ref,
    '""'  => \&string,
    '|'   => \&union,
    '&'   => \&intersection,
    '-'   => \&difference,
    '^'   => \&symmetric_difference,
    '<='  => \&issubset,
    '>='  => \&issuperset,
);

sub new {
    my $class = shift;
    my %self = map { $_ => 1 } @_;
    bless \%self, $class;
}

sub copy { bless { %{ $_[0] } }, __PACKAGE__ }
sub array     { keys %{ $_[0] } }
sub array_ref { [ shift->array ] }
sub string    { 'set(' . join( ", ", sort shift->array ) . ')' }
sub len       { 0 + @{ $_[0] } }

sub union {
    bless { %{ $_[0] }, %{ $_[1] } }, __PACKAGE__;
}

sub intersection {
    my %result;
    $_[0]->{$_} && $result{$_}++ for $_[1]->array;
    bless \%result, __PACKAGE__;
}

sub difference {
    my %result = %{ $_[0] };
    delete @result{ $_[1]->array };
    bless \%result, __PACKAGE__;
}

sub symmetric_difference {
    ( $_[0] | $_[1] ) - ( $_[0] & $_[1] );
}

sub issubset {
    $_[1]->{$_} or return for $_[0]->array;
    return 1;
}

sub issuperset { $_[1]->issubset( $_[0] ) }

package main;

local ( $,, $\ ) = ( ", ", "\n" ); # to prettyprint
my $a = set( 1, 2, 3, 4 );
my $b = set( 3, 4, 5, 6 );
print $a;
print $b;
print $a->len;
print $a | $b;
print $a & $b;
print $a - $b;
print $a ^ $b;
print( ( $a <= $b )      ? "true" : "false" );
print( ( $a & $b ) <= $a ? "true" : "false" );
print( ( $a | $b ) >= $a ? "true" : "false" );