plates.rb

#!/usr/bin/env ruby
require "bundler/inline"

gemfile do
  source "https://rubygems.org"
  gem "rack"
  gem "foxy", git: "git://github.com/weapp/foxyrb.git"
end

class App
  def grade(plate)
    url = "http://www.dgt.es/es/seguridad-vial/distintivo-ambiental/index.shtml"
    qs = { accion: 1, matriculahd: nil, submit: "Comprobar" }
    img = Foxy::Client.new
                      .eraw(path: url, params: qs.merge(matricula: plate))
                      .foxy
    img &&= img.find(id: "resultadoBusqueda")
    img &&= img.find(tagname: "img")
    img &&= img.attr("src")
    img && img[-5] if img != "/resources/cnr/imgm/icn-pdf.png"
  end

  def call(env)
    if env["REQUEST_METHOD"] == "GET"
      data = grade(env["PATH_INFO"][1..-1])
      if data
        [200, {}, [{ data: data }.to_json]]
      else
        [404, {}, [{ error: "Not found" }.to_json]]
      end
    else
      [405, {}, ["Method Not Allowed"]]
    end
  end
end

Rack::Server.start(app: App.new, Port: 3000)