FilterIterator位掩码（或位域）

Question

I'm struggling with bitmasks (or is it bitfields?). I'm not sure how to do it anymore.

I want to create a DirectoryFilterIterator that accepts flags of what to filter. I thought I'd use these bits for that:

const DIR_NO_DOT = 1;
const DOT        = 2;
const DIR        = 3;
const FILE       = 4;

Because a DOT is also considered a DIR I'd like to be able to distinguish between those two also. If I'm correct I thought something like that should be possible like this:

DirectoryFilterIterator::DIR & ~DirectoryFilterIterator::DOT

In other words, this should filter out DIR unless it's a DOT . But I'm totally stuck on how to get the filtering to work (in the accept method):

class DirectoryFilterIterator
    extends FilterIterator
{
    const DIR_NO_DOT = 1;

    const DOT        = 2;

    const DIR        = 3;

    const FILE       = 4;

    protected $_filter;

    public function __construct( DirectoryIterator $iterator, $filter = self::DIR )
    {
        parent::__construct( $iterator );
        $this->_filter = $filter;
    }

    public function accept()
    {
        $item = $this->getInnerIterator()->current();
        return (
            !( ( $this->_filter & self::DOT ) == self::DOT && $item->isDot() ) &&
            !( ( $this->_filter & self::DIR ) == self::DIR && $item->isDir() ) &&
            !( ( $this->_filter & self::FILE ) == self::FILE && $item->isFile() )
        );
    }
}

...especially because of all the negating I have going on, I'm kind of lost. How can I get this to work properly?

Answer 1

When using bitmasks you need to set their values to powers of two. If you set DIR to 3, it means it's synonymous to both DIR_NO_DOT and DOT combined.

Try setting their values to something like this:

const DOT  = 1;
const DIR  = 2;
const FILE = 4;

Now you can check if an item is a DIR but not a DOT with the expression DIR & !DOT .