bbPress (Topics and Repiles) Numeric Permalinks

hacks.php

<?php

if ( ! did_action( 'bbp_after_setup_actions' ) ) {
     add_action( 'bbp_after_setup_actions', 'n5n_after_bbPress_setup' );
} else {
     n5n_after_bbPress_setup();
}

/**
 * After bbPress Setup.
 *
 * @author Nashwan Doaqan
 */
function n5n_after_bbPress_setup() {
     
     if ( function_exists( 'is_bbpress' ) ) {
          add_action( 'registered_post_type', 'n5n_bbp_add_post_types_rewrite', 1, 2 );
          add_filter( 'rewrite_rules_array', 'n5n_bbp_filter_post_types_rewrite_rules', 1 );
          add_filter( 'post_type_link', 'n5n_bbp_filter_post_type_link', 1, 2 );
     }
     
}

/**
 * Add the numeric permalink structure to bbPress topics and replies.
 *
 * @author Nashwan Doaqan
 */
function n5n_bbp_add_post_types_rewrite( $post_type , $args ) {

     switch( $post_type ) {

         case bbp_get_reply_post_type():
         case bbp_get_topic_post_type():

         add_rewrite_tag( "%{$post_type}_id%", '([0-9]+)', "post_type={$post_type}&p=" );
         add_permastruct( $post_type, "{$args->rewrite['slug']}/%{$post_type}_id%", $args->rewrite );

         break;

     }

}

/**
 * Change the bbPress topics and replies rewrite rules.
 *
 * @author Nashwan Doaqan
 */
function n5n_bbp_filter_post_types_rewrite_rules( $rules ) {

    global $wp_rewrite;

    $edit_rule = '/([^/]+)/edit/?$';
    $edit_id   = bbp_get_edit_rewrite_id();
    $rules[ bbp_get_topic_slug() . $edit_rule ] = 'index.php?post_type=' . bbp_get_topic_post_type()  . '&p=' . $wp_rewrite->preg_index( 1 ) . '&' . $edit_id . '=1';
    $rules[ bbp_get_reply_slug() . $edit_rule ] = 'index.php?post_type=' . bbp_get_reply_post_type()  . '&p=' . $wp_rewrite->preg_index( 1 ) . '&' . $edit_id . '=1';

    return $rules;

}

/**
 * Change bbPress post types links.
 *
 * @author Nashwan Doaqan
 */
function n5n_bbp_filter_post_type_link( $post_link , $_post ) {

    global $wp_rewrite;

    if ( empty( $_post ) )
         return $post_link;

     switch( $_post->post_type ) {

         case bbp_get_reply_post_type():
         case bbp_get_topic_post_type():

            $post_link = $wp_rewrite->get_extra_permastruct( $_post->post_type );
            $post_link = str_replace( "%{$_post->post_type}_id%", $_post->ID, $post_link );
            $post_link = home_url( user_trailingslashit( $post_link ) );

            break;

     }

     return $post_link;

}

Hello , can u tell me how to use this file ? thanks

@janfeng Create your own hacks plugin, or use Code Snippets plugin. and put the codes above in it.. and don't forget to update the Permalinks settings to fire the Rewrite Rules actions