Run static site with rack

config.ru

APP_ROOT = ::File.expand_path('public', ::File.dirname(__FILE__))
require 'rack'
require 'rack/content_length'
require 'rack/file'

module Rack
  class DirectoryIndex
    def initialize(app, root)
      @app  = app
      @root = root
    end

    def call(env)
      index_path = ::File.join(@root, Rack::Request.new(env).path.split('/'), 'index.html')
      if ::File.exists?(index_path)
        return [200, {"Content-Type" => "text/html"}, [::File.read(index_path)]]
      else
        @app.call(env)
      end
    end
  end

  class NotFound
    def initialize(app, path)
      @app     = app
      @content = ::File.read(::File.expand_path(path))
    end

    def call(env)
      resp = @app.call(env)
      if 404 == resp[0]
        resp = [404, {'Content-Type' => 'text/html; charset=utf-8', 'Content-Length' => @content.size.to_s}, [@content]]
      end
      resp
    end
  end
end

use Rack::ContentLength
use Rack::NotFound, File.join(APP_ROOT, '404.html')
use Rack::DirectoryIndex, APP_ROOT

run Rack::File.new(APP_ROOT)