bbp_exclude_forum_ids( $type = 'string' )

function bbp_exclude_forum_ids( $type = 'string' ) {

	// Setup arrays
	$private = $hidden = $meta_query = $forum_ids = array();

	// Default return value
	switch ( $type ) {
		case 'string' :
			$retval = '';
			break;

		case 'array'  :
			$retval = array();
			break;

		case 'meta_query' :
			$retval = array( array() ) ;
			break;
	}

	// Exclude for everyone but keymasters
	if ( ! bbp_is_user_keymaster() ) {

		// Private forums
		if ( !current_user_can( 'read_private_forums' ) )
			$private = bbp_get_private_forum_ids();

		// Hidden forums
		if ( !current_user_can( 'read_hidden_forums' ) )
			$hidden  = bbp_get_hidden_forum_ids();

		// Merge private and hidden forums together
		$forum_ids = (array) array_filter( wp_parse_id_list( array_merge( $private, $hidden ) ) );

		// There are forums that need to be excluded
		if ( !empty( $forum_ids ) ) {

			switch ( $type ) {

				// Separate forum ID's into a comma separated string
				case 'string' :
					$retval = implode( ',', $forum_ids );
					break;

				// Use forum_ids array
				case 'array'  :
					$retval = $forum_ids;
					break;

				// Build a meta_query
				case 'meta_query' :
					$retval = array(
						'key'     => '_bbp_forum_id',
						'value'   => implode( ',', $forum_ids ),
						'type'    => 'numeric',
						'compare' => ( 1 < count( $forum_ids ) ) ? 'NOT IN' : '!='
					);
					break;
			}
		}
	}

	// Filter and return the results
	return apply_filters( 'bbp_exclude_forum_ids', $retval, $forum_ids, $type );
}