Builder pattern in Ruby

gistfile1.rb

# -*- encoding: utf-8 -*-

class Computer
  attr_accessor :display, :motherboard
  attr_reader :drives

  def initialize(display = :crt, motherboard = Motherboard.new, drives = [])
    @display = display
    @motherboard = motherboard
    @drives = drives
  end
end

class CPU;end

class BasicCPU < CPU;end

class TurboCPU < CPU;end

class Motherboard
  attr_accessor :cpu, :memory_size

  def initialize(cpu = BasicCPU.new, memory_size = 1000)
    @cpu = cpu
    @memory_size = memory_size
  end
end

class Drive
  attr_reader :type # :hard_disk or :cd or :dvd
  attr_reader :size # MB
  attr_reader :writable # true if writable

  def initialize(type, size, writable)
    @type = type
    @size = size
    @writable = writable
  end
end

class LaptopDrive
  attr_reader :type # :hard_disk or :cd or :dvd
  attr_reader :size # MB
  attr_reader :writable # true if writable

  def initialize(type, size, writable)
    @type = type
    @size = size
    @writable = writable
  end
end

class DesktopComputer < Computer;end

class LaptopComputer < Computer
  def initialize(motherboard = Motherboard.new, drives = [])
    super(:lcd, motherboard, drives)
  end
end

class ComputerBuilder
  attr_reader :computer

  def turbo(has_turbo_cpu = true)
    @computer.motherboard.cpu = TurboCPU.new
  end

  def memory_size(size_in_mb)
    @computer.motherboard.memory_size = size_in_mb
  end

  def computer
    raise "Not enough memory" if @computer.motherboard.memory_size < 250
    raise "Too many drives" if @computer.drives.size > 4
    hard_disk = @computer.drives.find { |drive| drive.type == :hard_disk }
    raise "No hard disk." unless hard_disk
    @computer
  end

  def method_missing(name, *args)
    words = name.to_s.split('_')
    return super(name, *args) unless words.shift == 'add'
    args_index = 0
    words.each do |w|
      next if w == 'and'
      if w == 'cd'
        add_cd(args[args_index])
      elsif w == 'dvd'
        add_dvd(args[args_index])
      elsif w == 'harddisk'
        add_hard_disk(args[args_index])
      else
        raise "Unknown drive"
      end
      args_index += 1
    end
  end
end

class DesktopBuilder < ComputerBuilder
  def initialize
    @computer = DesktopComputer.new
  end

  def display=(display)
    @computer.display = display
  end
  
  def add_cd(writable = false)
    @computer.drives << Drive.new(:cd, 760, writable)
  end

  def add_dvd(writable = false)
    @computer.drives << Drive.new(:dvd, 4700, writable)
  end

  def add_hard_disk(size_in_mb)
    @computer.drives << Drive.new(:hard_disk, size_in_mb, true)
  end
end

class LaptopBuilder < ComputerBuilder
  def initialize
    @computer = LaptopComputer.new
  end

  def display=(display)
    raise "Laptop display must be lcd" unless display == :lcd
  end

  def add_cd(writer = false)
    @computer.drives << LaptopDrive.new(:cd, 760, writer)
  end

  def add_dvd(writer = false)
    @computer.drives << LaptopDrive.new(:dvd, 4000, writer)
  end

  def add_hard_disk(size_in_mb)
    @computer.drives << LaptopDrive.new(:hard_disk, size_in_mb, true)
  end
end

builder = LaptopBuilder.new
builder.add_cd_and_dvd_and_harddisk(false, false, 100000)
p builder.computer