Reverse Polish Notation Calculator

# 许多特殊情况都没考虑
# 除数为0 用户输入字母 括号


  class Stack
    def initialize(size)
      @stack = Array.new(size)
      @sp = 0
    end

    def push(value)
      @stack[@sp] = value
      @sp += 1
    end

    def pop
      return nil if @sp == 0
      @sp -= 1
      @stack[@sp]
    end

    def size
      @sp
    end

    def to_string
      "#{@sp} elements: #{@stack[0...@sp]}"
    end
  end

  class Calculator
    PRIORITY = {"+" => 0, "-" => 0, "*" => 1, "/" => 1}

    def initialize(input)
      p PRIORITY['+']
      @operands_stack = Stack.new(input.length)
      @operators_stack = Stack.new(input.length)

      input.split().each do |chr|
        case chr
        when '+', '-', '*', '/'
          @operators_stack.push chr
        else
          @operands_stack.push chr.to_i
        end
      end
    end

    def operate(operator, fo, so)
      case operator
      when '+'
        fo + so
      when '-'
        fo - so
      when '*'
        fo * so
      when '/'
        fo / so
      end
    end

    def result
      until @operators_stack.size == 0
        p @operands_stack.to_string
        operator = @operators_stack.pop
        next_operator = @operators_stack.pop

        if next_operator.nil? || PRIORITY[operator] >= PRIORITY[next_operator]
          return nil if @operands_stack.size <= 1
          so = @operands_stack.pop
          fo = @operands_stack.pop

          @operands_stack.push operate(operator, fo, so)
          @operators_stack.push next_operator
        else
          return nil if @operands_stack.size <= 1
          to = @operands_stack.pop
          so = @operands_stack.pop
          fo = @operands_stack.pop

          @operands_stack.push operate(next_operator, fo, so)
          @operands_stack.push to
          @operators_stack.push operator
        end
      end

      def to_string
        @operands_stack.to_string
      end
    end
  end

  puts 'Give me a string'
  input = gets
  cal = Calculator.new(input)
  cal.result
# p cal.to_string