# C/C++ 로 배우는 자료구조론 연습문제 6.34

class DropOutStack

  def initialize(size)
    @top = -1

    @memory = []
    @stack_size = size
  end

  def is_full
    return @top >= (@stack_size - 1)
  end

  def is_empty
    return @top == -1
  end

  def push(data)
    if is_full
      shift_down
    end

    @top += 1

    @memory[@top] = data
  end

  def pop
    return nil if is_empty

    data = @memory[@top]

    @top -= 1

    return data
  end

  private
  def shift_down
    return if is_empty
	
	(0..@top).each do |idx|
	  @memory[idx] = @memory[idx+1]
	end
	
	@top -= 1
  end
end

stack = DropOutStack.new(3)
puts stack.push(5)
puts stack.push(6)
puts stack.push(7)
puts stack.push(8)
puts stack.pop
puts stack.pop
puts stack.pop
puts stack.pop

5
6
7
8
8
7
6
nil