module ContextAccessor
  def context
    Thread.current[:context]
  end

  def context=(ctx)
    Thread.current[:context] = ctx
  end

  def execute_in_context
    original_context = self.context
    self.context = self
    yield
    self.context = original_context
  end
end


class Account
  attr_reader :account_id, :balance

  def self.find(id)
    (@@accounts ||= {})[id] ||= new(id)
  end

  def initialize(account_id)
    @account_id = account_id
    @balance = 1000
  end

  def update_log(message, time, amount)
    puts "#{@account_id} :: #{time.strftime('%e %b %Y %H:%M:%S%p')} : #{message}  #{balance}"
  end
end


module MoneySource
  def decrease_balance(amount)
    @balance -= amount
  end
end

module MoneySink
  def increase_balance(amount)
    @balance += amount
  end
end

module TransferMoneySource
  include MoneySource, ContextAccessor

  def transfer_out
    raise "Insufficient funds" if balance < context.amount
    self.decrease_balance context.amount
    self.update_log "-#{context.amount}", Time.now, context.amount
  end
end

module TransferMoneySink
  include MoneySink, ContextAccessor

  def transfer_in
    self.increase_balance context.amount
    self.update_log "+#{context.amount}", Time.now, context.amount
  end
end


class TransferMoneyContext
  attr_reader :source_account, :destination_account, :amount
  include ContextAccessor

  def self.execute(amt, source_account_id, destination_account_id)
    TransferMoneyContext.new(amt, source_account_id, destination_account_id).execute
  end

  def initialize(amt, source_account_id, destination_account_id)
    @source_account = Account.find(source_account_id)
    @source_account.extend TransferMoneySource

    @destination_account = Account.find(destination_account_id)
    @destination_account.extend TransferMoneySink
    @amount = amt
  end

  def execute
    execute_in_context do
      begin
        source_account.transfer_out
        destination_account.transfer_in
      end
    end
  end

end


TransferMoneyContext.execute(500, :savings, :checking)
TransferMoneyContext.execute(100, :savings, :checking)