[ruby] add state pattern

state-pattern/state_pattern.rb

@@ -0,0 +1,194 @@
+# What is the State Design Pattern?
+#
+# - Allows an object to alter its behavior when its internal state changes.
+#   The object will appear to change its class
+# - Context (account): Maintains an instance of a ConcreateState subclass
+#   that defines the current state
+# - State: Defines an interface for encapsulating the behavior associated
+#   with a particular state of the Context
+# - Concrete State: Each subclass implements a behavior associated with
+#   a state of Context
+#
+# @see http://www.newthinktank.com/2012/10/state-design-pattern-tutorial/
+
+require 'forwardable'
+
+class ATMState
+  def insert_card
+    raise NotImplementedError
+  end
+
+  def eject_card
+    raise NotImplementedError
+  end
+
+  def insert_pin(_pin)
+    raise NotImplementedError
+  end
+
+  def request_cash(_amount)
+    raise NotImplementedError
+  end
+end
+
+class ATMMachine
+  extend Forwardable
+
+  def_delegators :@state, :insert_card, :eject_card, :insert_pin, :request_cash
+
+  attr_accessor :state,
+                :cash,
+                :has_card,
+                :no_card,
+                :has_pin,
+                :has_correct_pin,
+                :out_of_cash
+
+  def initialize
+    @has_card = HasCard.new(self)
+    @no_card = NoCard.new(self)
+    @has_pin = HasPin.new(self)
+    @out_of_cash = NoCash.new(self)
+    @state = @no_card
+    @cash = 2000
+    @has_correct_pin = false
+  end
+end
+
+class HasCard < ATMState
+  def initialize(atm)
+    @atm = atm
+  end
+
+  def insert_card
+    puts "you can't enter more than one card"
+  end
+
+  def eject_card
+    puts 'card ejected'
+    atm.state = atm.no_card
+  end
+
+  def insert_pin(pin)
+    if pin == 1234
+      puts 'correct PIN'
+      atm.has_correct_pin = true
+      atm.state = atm.has_pin
+    else
+      puts 'wrong PIN'
+      atm.has_correct_pin = false
+      puts 'card rejected'
+      atm.state = atm.no_card
+    end
+  end
+
+  def request_cash(_amount)
+    puts 'enter PIN first'
+  end
+
+  private
+
+  attr_reader :atm
+end
+
+class NoCard < ATMState
+  def initialize(atm)
+    @atm = atm
+  end
+
+  def insert_card
+    puts 'please enter a PIN'
+    atm.state = atm.has_card
+  end
+
+  def eject_card
+    puts 'enter a card first'
+  end
+
+  def insert_pin(_pin)
+    puts 'enter a card first'
+  end
+
+  def request_cash(_amount)
+    puts 'enter a card first'
+  end
+
+  private
+
+  attr_reader :atm
+end
+
+class HasPin < ATMState
+  def initialize(atm)
+    @atm = atm
+  end
+
+  def insert_card
+    puts "You can't enter more than one card"
+  end
+
+  def eject_card
+    puts 'card ejected'
+    atm.state = atm.no_card
+  end
+
+  def insert_pin(_pin)
+    puts 'already entered PIN'
+  end
+
+  def request_cash(amount)
+    if !atm.has_correct_pin
+      puts 'wrong pin'
+      eject_card
+    elsif amount > atm.cash
+      puts "don't have that cash"
+      eject_card
+    else
+      puts amount.to_s + ' is provided by the machine'
+      atm.cash -= amount
+      eject_card
+      atm.state = atm.out_of_cash if atm.cash.zero?
+    end
+  end
+
+  private
+
+  attr_reader :atm
+end
+
+class NoCash < ATMState
+  def initialize(atm)
+    @atm = atm
+  end
+
+  def insert_card
+    puts 'ATM out of cash'
+  end
+
+  def eject_card
+    puts 'no card inserted'
+  end
+
+  def insert_pin(_pin)
+    puts 'ATM out of cash'
+  end
+
+  def request_cash(_amount)
+    puts 'ATM out of cash'
+  end
+
+  private
+
+  attr_reader :atm
+end
+
+################################################################################
+
+atm = ATMMachine.new
+atm.insert_card
+atm.eject_card
+atm.insert_card
+atm.insert_pin 1234
+atm.request_cash 2000
+atm.insert_card
+atm.insert_pin 1234