Reverse Polish Notation, first try

polish.rb

#
# Requirement: delimiter ',' after each operands
# Example:
#
# ~> ruby polish '8,2,5,*+1,3,2,*+4,-/'
# result: 6
#

class ReversePolishTypeError < StandardError; end

class ReversePolish
  def initialize
    @result_stack = []
    @buffer = ""
  end

  def calculate(str)
    str.each_char do |char|
      if operator?(char)
        b, a = @result_stack.pop, @result_stack.pop
        @result_stack.push(operate(char, a, b))
      elsif delimiter?(char)
        @result_stack.push @buffer.to_i
        @buffer.clear
      elsif operand_path?(char)
        @buffer += char
      else
        raise ReversePolishTypeError
      end
    end

    @result_stack.pop
  end

  def delimiter?(char)
    [","].include?(char)
  end

  def operator?(char)
    ["+", "-", "*", "/"].include?(char)
  end

  def operand_path?(char)
    !!char.match(/[0-9]/)
  end

  def operate(operator, a, b)
    case operator
      when "+"; return a + b
      when "-"; return a - b
      when "/"; return a / b
      when "*"; return a * b
    end
  end

  class << self
    def calculate(str)
      obj = new
      result = obj.calculate(str)
      puts "result: #{result}"
    rescue ReversePolishTypeError => e
      puts "operands must be an integer"
    end
  end

end

str = ARGV.first
ReversePolish.calculate(str)