CORP.rb

class PurchasePower
  def self.base
    500
  end

  def process_request(request)
    raise NotImplementedError
  end  
 
  private

  def next
    if successor.nil?
      successor.process_request(request)
    else
      yield block if block_given?
    end
  end

  attr_accessor :successor
end

class Manager < PurchasePower
  ALLOWABLE = 10 * base

  def process_request(request)
    if request.amount < ALLOWABLE
      puts("Manager will approve $#{request.amount}")
      return
    end
    next    
  end
end

class Director < PurchasePower
  ALLOWABLE = 20 * base

  def process_request(request)
    if request.amount < ALLOWABLE
      puts("Director will approve $#{request.amount}")
      return
    end
    next
  end
end

class VicePresident < PurchasePower
  ALLOWABLE = 40 * base

  def process_request(request)
    if request.amount < ALLOWABLE
      puts("VicePresident will approve $#{request.amount}")
      return
    end
  next
  end
end

class President < PurchasePower
  ALLOWABLE = 60 * base

  def process_request(request)
    if request.amount < ALLOWABLE
      puts("President will approve $#{request.amount}")
      return
    end
    next{ puts "Your request for $#{request.amount} needs a board meeting!" }     
  end
end

class PurchaseRequest
  def initialize(number, amount, purpose)
    @number = number
    @amount = amount
    @purpose = purpose
  end

  attr_reader :amount
end

class Approvers
  def initialize(*approvers)
    @approvers = approvers.map(&:new)
  end

  def <<(approver)
    @approvers << approver
  end

  def -(approver)
    @approvers.delete approver
  end

  attr_reader :approvers
end

class CheckAuthority
  def initialize(approvers)
    @approvers         = approvers
    @starting_approver = build_chains.first
  end

  def checking
    loop do
      $stdout.flush
      puts "Enter the amount to check who should approve your expenditure."
      puts ">>"
      @starting_approver.process_request PurchaseRequest.new(0, gets.to_i, 'General')
    end
  end

  private

  def build_chains
    @approvers.each_cons(2).map { |_prev, _next|  _prev.send :successor=, _next }
  end
end

approvers = Approvers.new(PurchasePower.descendants) #Warning this is rails dependent
CheckAuthority.new(approvers).checking

output

Arup-iMac:arup_ruby shreyas$ ruby a.rb
Enter the amount to check who should approve your expenditure.
>>
1200
Director will approve $1200
Enter the amount to check who should approve your expenditure.
>>
2000
Director will approve $2000
Enter the amount to check who should approve your expenditure.
>>
^[[A^[[B
Director will approve $0
Enter the amount to check who should approve your expenditure.
>>
3000
Director will approve $3000
Enter the amount to check who should approve your expenditure.
>>
40000
Your request for $40000 needs a board meeting!
Enter the amount to check who should approve your expenditure.
>>
4000
Director will approve $4000
Enter the amount to check who should approve your expenditure.
>>
30000
Your request for $30000 needs a board meeting!
Enter the amount to check who should approve your expenditure.
>>
10000
VicePresident will approve $10000
Enter the amount to check who should approve your expenditure.
>>