Enabled Root-based Category URLs in WordPress (Any bugs? Post comments below.)

root-based-category-urls.php

<?php
/*
 * Plugin Name: Root-based Category URLs
 * Description: Enables root-based Category URLs, i.e. Makes /category/my-category/ URLs route as /my-category/
 * Author: Mike Schinkel
 * Author URI: http://about.me/mikeschinkel
 * Plugin URI: https://gist.github.com/1421235
 * Version: 0.1.1
 */

class Rootbased_Category_URLs {
	function on_load() {
		add_filter( 'request', [ $this, 'request' ] );
		add_filter( 'category_link', [ $this, 'category_link' ] );
	}

	function request( $query_vars ) {
		global $wp;
		if ( preg_match( '#' . self::get_category_base() . '(.*)#', $wp->request, $match ) ) {
			wp_safe_redirect( self::category_link( $wp->request ), 301 );
			exit;
		}
		$term = get_term_by( 'slug', $wp->request, 'category' );
//		$category_hash = array_flip( self::get_category_slugs() );
//		if ( isset( $category_hash[ $wp->request ] ) ) {
//			$query_vars = [ 'category_name' => $wp->request ];
//		}

		if ( $term instanceof WP_Term ) {
			$query_vars = [ 'category_name' => $wp->request ];
		}

		return $query_vars;
	}

	function get_category_base() {
		$category_base = get_option( 'category_base' );
		if ( empty( $category_base ) ) {
			$category_base = 'category';
		}

		return "/{$category_base}/";
	}

	function category_link( $category_link ) {
		return str_replace( self::get_category_base(), '/', $category_link );
	}

	function get_category_slugs() {
		$categories = get_categories();
		foreach ( $categories as $index => $category ) {
			$categories[ $index ] = $category->slug;
		}

		return $categories;
	}
}

$root_cat = new Rootbased_Category_URLs();
$root_cat->on_load();