use Sort::Packed 'sort_packed';

sub pwc_p {
    my ( $from, $to, $target ) = @_;
    
    use integer;
    my @witnesses = ([ 4, 7 ], [ 2 .. 9 ]);
    
    my ( %pairs, $j );
    for ( my $i = ( $from + 2 ) / 3 * 3; $i <= $to; $i += 3 ) {
        sort_packed C => my $i_key = $i;
        my $k = $i % 9 ? 0 : 1;
        
        for ( @{ $witnesses[ $k ]}) {
            last if length( $j = $i * $_ ) != length( $i );
            sort_packed C => my $j_key = $j;
            $pairs{ $i }{ $j } = $_
                unless $i_key ne $j_key
        }
        for ( @{ $witnesses[ $k ]}) {
            last if length( $j = $i / $_ ) != length( $i );
            next if $i % $_;
            sort_packed C => my $j_key = $j;
            $pairs{ $i }{ $j } = -$_
                unless exists $pairs{ $j } && 
                       exists $pairs{ $j }{ $i }
                    || $i_key ne $j_key
        }
    }
    return scalar grep { %$_ >= $target } values %pairs
}