rpn.rb

class RPN
  def initialize(expression)
    @tree = Tree.new(expression)
  end
 
  def calculate
    @tree.calculate
  end
 
  class Tree
    def initialize(expression)
      @root = Node.new(expression)
      @root.partition
    end
 
    def calculate
      @root.calculate
    end
 
    class Node
      attr_reader :token, :left, :right
 
      def initialize(token, left = nil, right = nil)
        @token = token
        @left  = self.class.new(left)  if left
        @right = self.class.new(right) if right
      end
 
      def partition
        if expression?
          regexp =
            case @token
            when /[+\-]/; /[+\-]/
            when /[*\/]/; /[*\/]/
            end
 
          l, t, r = @token.partition(regexp)
          @token = t
          @left  = self.class.new(l)
          @right = self.class.new(r)
 
          @left.partition
          @right.partition
        end
 
        self
      end
 
      def calculate
        l = @left.operator?  ? @left.calculate  : @left.token.to_f
        r = @right.operator? ? @right.calculate : @right.token.to_f
        l.send(@token.intern, r)
      end
 
      def expression?
        !(operand? || operator?)
      end
 
      def operand?
        Float(@token)
      rescue
        false
      else
        true
      end
 
      def operator?
        !!(@token =~ /\A[+\-*\/]\z/)
      end
    end
  end
end