ReversePolishNotation

gistfile1.rb

module ReversePolishNotation
  class << self
    def calc(formula)
      chain = prepare formula
      stack = []
      while chain.any? do
        item = chain.shift
        if item.kind_of? Float
          stack.push item
        else
          b = stack.pop
          a = stack.pop
          res = a.send item, b
          stack.push res
        end

        puts stack.inspect
      end
      stack.pop
    end

    private
    def prepare(formula)
      chain = parse formula
      result = []
      stack = []

      while chain.any? do
        c = chain.shift

        case c
          when Float
            result << c
          when '+', '-', '*', '/'
            op1 = c
            while (op2 = stack.last) && (op2 != '(') && (precedence(op1) <= precedence(op2))
              stack.pop
              result << op2
            end
            stack << op1
          when '('
            stack << c
          when ')'
            while (h = stack.pop) != '('
              raise 'Error' unless h
              result << h
            end
        end
      end
      result << stack.reverse
      result.flatten
    end

    def parse(string)
      raw = string.gsub /([\+\-\*\/\(\)])/, ' \1 '
      array = raw.split ' '
      chain = array.delete_if{ |i| i.empty? }
      chain.map { |i| i[/\d+/] ? i.to_f : i }
    end

    def precedence(op)
      case op
      when '+', '-'
        1
      when '*', '/'
        2
      end
    end
  end
end

puts ReversePolishNotation.calc '(8+2*5)/(1+3*2-4)'