<?php
/***
    * Plugin Name: bbPress AutoTagger
    * Plugin URI:  http://example.org/
    * Description: --- description here --
    * Version:     1.0
    * Author:      Shmoo
    * Author URI:  http://example.org/
    * License:     Same as WordPress I guess.
***/

add_action( 'bbp_new_topic',  'shmoo_auto_save_tags', 10, 1 );
add_action( 'bbp_edit_topic', 'shmoo_auto_save_tags', 10, 1 );


function shmoo_auto_save_tags( $topic_id ) {

    // Get all bbPress topic tags inside an array + strip them to plain text
        $bbp_topic_tags = wp_tag_cloud( array( 'format' => 'array', 'taxonomy' => 'topic-tag' ) );

                function tag_stripper( &$items, $format ) {

                        $items = strip_tags( $items );

                }
                array_walk( $bbp_topic_tags, 'tag_stripper' );


// Topic Content


        // Get the topic content
        $bbp_topic_content = bbp_get_topic_content( $topic_id );


// Match Topics Tags against Topic Content


        // Check and echo matches
        $matches = array();

        $match_found = preg_match_all( "/\b(" . implode($bbp_topic_tags,"|") . ")\b/i", $bbp_topic_content, $matches );

        if ( $match_found ) {

            $words = array_unique( $matches[0] );

                    $words = array_unique( array_merge( $words, wp_get_post_terms( $topic_id, bbp_get_topic_tag_tax_id(), array( 'fields' => 'names' ) ) ) );
                        if ( !empty( $words ) )
                            wp_set_post_terms( $topic_id, $words, bbp_get_topic_tag_tax_id(), true );

        }


}