Stylus plugin to resolve relative paths to images on @import

stylus.js

#!/usr/bin/env node

var S = require('stylus'),
    FS = require('fs'),
    URL = require('url'),
    PATH = require('path');

var path = PATH.resolve('bundle.styl');
S(FS.readFileSync(path, 'utf8'))
    .set('filename', path)
    .define('url', url())
    .render(function(err, css){
        if (err) throw err;
        console.log(css);
    });

/**
 * Url processing function.
 *
 * Translates relative urls to images and so on to the urls
 * relative to the root file.
 *
 * Limitations:
 * - could not translate paths in `@css` litarals
 * - could not translate paths of `@import *.css` literals
 */
function url() {

    function fn(url) {

        // Compile the url
        var compiler = new S.Compiler(url);
        compiler.isURL = true;
        url = url.nodes.map(function(node){
                return compiler.visit(node);
            }).join('');

        // Parse literal
        url = URL.parse(url);

        // Absolute with protocol
        if (url.protocol) return urlLiteral(url.href);

        // TODO: absolute without protocol

        // Relative
        var stack = this.importStack,
            dir = PATH.dirname(this.filename),
            abs = PATH.resolve(dir, PATH.dirname(stack[stack.length - 1]), url.href),
            rel = PATH.relative(dir, abs);

        return urlLiteral(rel);

    };

    fn.raw = true;
    return fn;

}

function urlLiteral(url) {
    return new S.nodes.Literal('url("' + url + '")');
}