simple calc (LL parser)

calc.rb

require 'strscan'
require 'test/unit'

class Calc
  def run(str)
    @scanner = StringScanner.new(str)
    expr
  end

  def scan(re)
    @scanner.skip /\s+/
    result = @scanner.scan re
    result.nil? ? false : result
  end

  private
  def fact
    if scan /\(/
      x = expr
      raise unless (scan /\)/)
      x
    else
      (scan /\d+/).to_i
    end
  end

  def expr
    x = term
    while (op = scan /[+-]/)
      m = x.method(op)
      x = m.call(term)
    end
    x
  end
 
  def term
    x = fact
    while (op = scan /[*\/]/)
      m = x.method(op)
      x = m.call(fact)
    end
    x
  end
end
 
include Test::Unit::Assertions
calc = Calc.new
assert_equal(5, calc.run('2 + 3'))
assert_equal(7, calc.run('1 + 2 * 3'))
assert_equal(10, calc.run('5*(3-1)'))
expr = '10/ 2 + (2 - 3)*(4 +5- 6) * ( 101 -1 )'
assert_equal((eval expr), calc.run(expr))