逆ポーランド記法の電卓

main.rb

#usage: ruby main.rb
class Parser
  def initialize
    @stack = []
  end
  def parse(line)
   scanner = Scanner.new(line)
   while (token = scanner.next) do
     add_token(token)
   end
   @stack.pop
  end
  def add_token(token)
    token.append_to(@stack)
  end
end
class Scanner
  def initialize(line)
    @buf = line.chomp.split(//)
    @pos = 0
  end
  def peek
    while is_space do
      @pos += 1
    end
    (@buf.size > @pos)? token : nil
  end
  def next
    v = peek; @pos = @e; v
  end
  private
    def token
      @e = @pos
      if is_op
        @e+=1
        AST::Op.new(@buf[@e-1])
      elsif is_number
        while @buf.size > @pos && is_number(@e) do
          @e+=1
        end
        AST::Num.new(@buf[@pos...@e].join)
      else
        raise "invalid expression: #{@buf.join}"
      end
    end
    def is_space
      @buf[@pos] == ' '
    end
    def is_number(pos=@pos)
      /\d/.match(@buf[pos])
    end
    def is_op
      /[\+\-\*]/.match(@buf[@pos])
    end
end
module AST
  class Exp
    def initialize(l, op, r)
      @l = l; @op = op; @r = r
    end
    def to_s
      [@l, @r, @op].join(" ")
    end
    def append_to(stack)
      stack.push(self)
    end
    def exec
      case @op.to_s
      when "+"; @l.exec + @r.exec
      when "-"; @l.exec - @r.exec
      when "*"; @l.exec * @r.exec
      end
    end
  end
  class Num
    def initialize(v)
      @v = v.to_i
    end
    def to_s; @v.to_s; end
    def exec; @v; end
    def append_to(stack)
      stack.push(self)
    end
  end
  class Op
    def initialize(op)
      @op = op
    end
    def to_s; @op.to_s; end
    def append_to(stack)
      r = stack.pop; l = stack.pop
      stack.push(AST::Exp.new(l, self, r))
    end
  end
end
parser = Parser.new
DATA.each do |line|
  prog = parser.parse(line)
  puts "#{prog.to_s} = #{prog.exec}"
end
__END__
123 168 77 - +
11  7 * 6 +
  15 7*  43 22 -+