gistfile1.sh

# Example

    > ./rpn.rb '3 + 4 * 2 / (1 - 5) * 2'
    RPN: 3 4 2 * 1 5 - / 2 * +
    Result: -1.0

rpn.rb

#!/usr/bin/env ruby

class RPN
  class Token < String
    OPS_PRIORITY_MAP = [
      '',
      %w(- +),
      %w(* /) 
    ]

    def priority
      OPS_PRIORITY_MAP.index{|ops| ops.include? self} || 0
    end

    def operator?
      OPS_PRIORITY_MAP.flatten.include? self
    end

    def open_bracket?
      self == '('
    end


    def close_bracket?
      self == ')'
    end
  end

  def self.transform(expression)
    result = []
    operators = []

    self.operands(expression).each do |token|
      if token.operator?
        if operators.last && token.priority <= operators.last.priority
          result << operators.pop
        end
        operators << token
      elsif token.close_bracket?
        until operators.last.open_bracket? do
          result << operators.pop
        end
        operators.pop
      elsif token.open_bracket?
        operators << token
      else
        result << token
      end
    end

    (result + operators.reverse).join(' ')
  end

  def self.calc(expression)
    stack = []
    operands(expression).each do |token|
      if token.operator?
        b = stack.pop.to_f
        a = stack.pop.to_f
        stack << a.send(token.to_s, b)
      else
        stack << token
      end
    end

    stack.last
  end

  private

  # Split string to operators
  #
  # @param String expression
  # @return Array
  def self.operands expression
    ops = Token::OPS_PRIORITY_MAP.flatten
    expression.split(/(?:\s+|(?=[0-9]+)(?<![0-9])|(?=[#{ops.join}()]))/).map{|x| Token.new(x)}
  end

end


expression = ARGV.join(' ')
rpn_expression = RPN.transform(expression)
puts "RPN: #{rpn_expression}"
result = RPN.calc(rpn_expression)
puts "Result: #{result}"