wp-top-level-post-type.php

<?php

add_filter('request', 'hwk_post_type_toplevel_request', 1, 1);
function hwk_post_type_toplevel_request($query){

    $post_type = 'portfolio';

    if(isset($query[$post_type]) && isset($query['post_type']) && $query['post_type'] === 'portfolio')
        return $query;

    $name = false;

    // Found children
    if(isset($query['attachment'])){

        $include_children = true;
        $name = $query['attachment'];
        
    }

    // Default: no children
    elseif(isset($query['name'])){
        
        $include_children = false;
        $name = $query['name'];
        
    }

    if(!$name)
        return $query;

    $get_posts = get_posts(array(
        'name'              => $name,
        'post_type'         => $post_type,
        'posts_per_page'    => 1
    ));

    if(empty($get_posts))
        return $query;

    $post = $get_posts[0];

    if($include_children){
        
        unset($query['attachment']);
        $parent = $post->post_parent;
        
        while($parent){
        
            $parent_post = get_post($parent);
            $name = $parent_post->post_name . '/' . $name;
            $parent = $parent_post->post_parent;
            
        }
        
    }else{

        unset($query['name']);
        
    }

    $query['post_type'] = $post_type;
    $query[$post_type] = $name;

    return $query;

}

add_action('init', 'hwk_post_type_toplevel_add_rewrite_rule');
function hwk_post_type_toplevel_add_rewrite_rule(){

    add_rewrite_rule(
        '(.?.+?)?(:/([0-9]+))?/?$',
        'index.php?portfolio=$matches[1]&post_type=portfolio&page=$matches[2]',
        'bottom'
    );

}

add_filter('post_type_link', 'hwk_post_type_toplevel_permalink', 10, 2);
function hwk_post_type_toplevel_add_rewrite_rule($post_link, $post, $leavename){

    if('portfolio' != $post->post_type !== 'portfolio' || $post->post_status !== 'publish')
        return $post_link;

    $post_link = str_replace('/' . $post->post_type . '/', '/', $post_link);

    return $post_link;

}