"Related category" functions from a widget built for S. Collins & Son

related-category.php

<?php

class SomeClass {
    
  /**
   * Get X random related categories
   *
   * @param object $category  The current category
   * @param int    $threshold The amount of common tags needed to deem a category "related"
   * @param int    $amount    Amount of categories to return
   *
   * @return array The related categories
   */
  public function get_random_related_categories($category, $threshold, $amount)
  {
    $related_categories = $this->get_related_categories($category, $threshold);
    $related_count = count($related_categories);
  
    if ($related_count == 0) {
      return array();
    } elseif ($related_count < $threshold) {
      $keys = array_rand($related_categories, $related_count);
    } else {
      $keys = array_rand($related_categories, $threshold);
    }
  
    if (is_array($keys)) {
      return array_intersect_key($related_categories, array_flip($keys));
    } else {
      return array($related_categories[$keys]);
    }
  }
  
  /**
   * Get related categories
   *
   * Retreive all categories which share at least 3 product tags
   * with the current category
   *
   * @param object $category  The current category
   * @param int    $threshold The amount of common tags needed to deem a category "related"
   *
   * @return array $relatedCategores The related categories
   */
  public function get_related_categories($category, $threshold)
  {
    $relatedCategories = array();
  
    $excluded = get_ancestors($category->term_id, 'product_cat');
    array_push($excluded, $category->term_id);
  
    $allCategories = get_terms('product_cat', array(
      'exclude' => $excluded
    ));
  
    $tags = $this->get_category_tags($category->term_id);
  
    foreach ($allCategories as $category) {
      $thisTags = $this->get_category_tags($category->term_id);
  
      if ($this->tags_in_common(array($tags, $thisTags)) >= $threshold) {
        $relatedCategories[] = $category;
      }
    }
  
    return $relatedCategories;
  }
  
  /**
   * Tags in common
   *
   * Compare two arrays of tags and return the amount of elements
   * which they both share (common)
   *
   * @param array $tag_sets An array containing the two arrays of tags
   * 
   * @return int Amount of common elements between the two arrays
   */
  function tags_in_common($tag_sets)
  {
    if (count($tag_sets) < 2) return;
    $commonTags = array();
  
    foreach ($tag_sets[0] as $tag) {
      if (in_array($tag, $tag_sets[1])) $commonTags[] = $tag;
    }
  
    return count($commonTags);
  }
  
  /**
   * Get category tags
   *
   * Query the WP database for all distinct tags used in a specified category
   *
   * @param int $category_id
   *
   * @return array $tags
   */
  function get_category_tags($category_id)
  {
    global $wpdb;
    $tags = $wpdb->get_results($wpdb->prepare(
      "SELECT DISTINCT
        terms2.term_id as tag_id,
        terms2.name as tag_name
      FROM 
        {$wpdb->posts} as posts
        LEFT JOIN {$wpdb->term_relationships} as term_relationships
          ON term_relationships.object_id = posts.ID
        LEFT JOIN {$wpdb->term_taxonomy} as term_taxonomy
          ON term_relationships.term_taxonomy_id = term_taxonomy.term_taxonomy_id
        LEFT JOIN {$wpdb->terms} as terms
          ON terms.term_id = term_taxonomy.term_id,
        {$wpdb->posts} as posts2
        LEFT JOIN {$wpdb->term_relationships} as term_relationships2
          ON term_relationships2.object_id = posts2.ID
        LEFT JOIN {$wpdb->term_taxonomy} as term_taxonomy2
          ON term_relationships2.term_taxonomy_id = term_taxonomy2.term_taxonomy_id
        LEFT JOIN {$wpdb->terms} as terms2
          ON terms2.term_id = term_taxonomy2.term_id
      WHERE (
        term_taxonomy.taxonomy = 'product_cat' AND
        terms.term_id = '%d' AND
        term_taxonomy2.taxonomy = 'product_tag' AND
        posts.post_status = 'publish' AND
        posts2.post_status = 'publish' AND
        posts.ID = posts2.ID
      )",
      $category_id
    ));
  
    return $tags;
  }
  
}
?>