<?php
/*
Plugin Name: Intersect
Plugin URI: http://www.tester.ca/?name=intersect-plugin
Description: This plugins allows to use categories as intersections. Normally when multiple categories are chosen, they will be ORed (a union), but with this plugin they will be ANDed (an intersection). So if category x and y are chosen, it will not show posts from cat x and cat y, but only those that belong to both categories. (example ?cat=1,2&intersect=1)
Author: Olivier Crête
Version: 1.0
Author URI: http://www.tester.ca/
*/ 


function tester_where_intersect($query)
{
  global $tester_intersect_count;

  if (! empty($_GET['intersect'])) {

    $query = preg_replace("/(category_id = \d+) OR/", 
			   "\\1 AND", $query);

    $tester_intersect_count = 0;

    $query = preg_replace_callback("/(category_id = \d+)/", 
				      "tester_intersect_remplacer", $query);
  }
  return $query;
}


function tester_intersect_remplacer($match)
{
  global $tester_intersect_count;

  return ("p2c".$tester_intersect_count++.".".$match[1]);
}

function tester_join_intersect($query)
{

  global $wpdb;

  if (! empty($_GET['intersect'])) {
    $catcount =  count(preg_split("/,/",$_GET['cat']));

    $query = preg_replace("/( LEFT JOIN $wpdb->post2cat) (ON .+ =) ".$wpdb->post2cat."(\.post_id)/", 
			  "\\1 AS p2c0 \\2 p2c0\\3", $query);

    for ($i=1;$i < $catcount; $i++) {
      $query .= " LEFT JOIN $wpdb->post2cat AS p2c${i} ON ($wpdb->posts.ID = p2c${i}.post_id) ";
	}
  }
  return $query;
}

add_filter('posts_where', 'tester_where_intersect');

add_filter('posts_join', 'tester_join_intersect');


?>