Simplest implementation of rack compatible web server

server.rb

require 'socket'
require 'http_tools'
require 'sinatra/base'
require 'sinatra/cookies'

class Server
  DEFAULT_HOST = 'localhost'
  DEFAULT_PORT = 3000

  def self.run(app, **options)
    new(app, options).listen
  end

  attr_reader :app, :server

  def initialize(app, **options)
    @app = app
    @server = TCPServer.new(
      options[:host] || DEFAULT_HOST,
      options[:port] || DEFAULT_PORT)
  end

  def listen
    loop do
      Thread.new(server.accept) do |client|
        handle_connection(client)
      end
    end
  end

  private

  def handle_connection(client)
    env = build_env(client)
    status, headers, body = app.call(env)
    client << HTTPTools::Builder.response(status, headers)
    body.each { |row| client << row }
  ensure
    client.close
  end

  def build_env(client)
    parser = HTTPTools::Parser.new
    parser << client.gets until parser.finished?
    parser.env
  end
end

class Application < Sinatra::Application
  helpers Sinatra::Cookies

  get '/' do
    "Hello, #{cookies[:name] || 'guest'}!"
  end

  get '/set' do
    cookies[:name] = params[:name]
    redirect to('/')
  end
end

Server.run(Application.new)