Arithmetic expression parser *without* StringScanner in Ruby, inspired by parser combinators.

parser-oo.rb

class Parser

  def initialize string
    @string = string

    # To improve performance, increments position instead of modifying @string.
    @position = 0
  end

  def self.eval string
    self.new(string).expr
  end

  def left_string
    @string[@position..-1]
  end

  # satify の略
  def sat &block
    c = @string[@position]
    if block.call(c)
      @position += 1
      c
    end
  end

  def digit
    sat {|c| c && '0' <= c && c <= '9' }
  end

  def space
    sat {|c| c =~ /^\s$/ }
  end

  def nat
    many1_s { digit }&.to_i
  end

  def spaces
    many { space }
  end

  def sym str
    if @string.index(str, @position) == @position
      @position += str.length
      str
    end
  end

  def token &block
    spaces && (result = block.call) && spaces
    result
  end

  def symbol str
    token { sym str }
  end

  def natural
    token { nat }
  end

  def many1_s &block
    results = nil
    loop do
      result = block.call
      if result
        results ||= ''
        results << result
      else
        break
      end
    end
    results
  end

  def many &block
    results = []
    loop do
      result = block.call
      if result
        results << result
      else
        break
      end
    end
    results
  end

  def expr
    if t = term
      if symbol("+") && (e = expr)
        t + e
      else
        t
      end
    end
  end

  def term
    if f = factor
      if symbol("*") && (t = term)
        f * t
      else
        f
      end
    end
  end

  def factor
    result =
      symbol("(") &&
        (e = expr) &&
        symbol(")")
    if result
      e
    else
      natural
    end
  end

end

if __FILE__ == $PROGRAM_NAME
  puts "Expected: 9876543210"
  p Parser.eval("9876543210")

  puts "\nExpected: 9"
  p Parser.eval("1+ 2* 4")    #=> 9

  puts "\nExpected: 12"
  p Parser.eval("(1 +2) *4 ") #=> 12

  puts "\nExpected: 9"
  p Parser.eval("(1 +(2 *4))") #=> 9

  puts "\nExpected: 6"
  p Parser.eval("1 + 2 + 3")  #=> 6

  puts "\nExpected: 24"
  p Parser.eval("2*3*4")      #=> 24

  puts "\nExpected: 42"
  p Parser.eval("2*((3+4) * 3)") #=> 42
end