Reverse Polish Notation parser

parser_spec.rb

require 'spec_helper'

class Parser

  def parse(string)
    stack = []
    tokens = string.split(' ')
    tokens.each do |token|
      if token =~ /\d/
        value = token.to_i
        stack.push(value)
      else
        raise "Not enough values" if stack.count < 2
        operand = token.to_sym
        left, right = stack.pop(2)
        stack.push(left.send(token, right))
      end
    end
    raise "Too many values" if stack.count > 1
    stack.first
  end

end

describe Parser do
  let(:parser) { Parser.new }
  it 'handles 2 numbers' do
    parser.parse('4 2 +').should == 4 + 2
    parser.parse('4 2 /').should == 4 / 2
    parser.parse('4 2 *').should == 4 * 2
    parser.parse('4 2 -').should == 4 - 2
  end

  it 'handles complex expressions' do
    parser.parse('5 1 2 + 4 * + 3 -').should == 14
    parser.parse('1 2 + 3 * 4 +').should == 13
  end

end