gistfile1.rb

module Search
  # Helper class for performing lexical analysis on a string.
  class Lexer
    attr_accessor :pos, :string

    def initialize(string)
      @string = string.dup
      @pos = 0
    end

    # The current substring to be analyzed.
    def rest
      @string[@pos .. -1]
    end

    # Are we at the end of this string?
    def eos?
      @pos == @string.length
    end

    # Move the scan pointer forward `count` characters, and return
    # the string portion that was consumed.
    def advance(count)
      @pos += count
      @string[@pos - count .. @pos - 1]
    end

    # Accept string and advance the scan pointer, or
    # do nothing if no match.
    def accept(str)
      return unless rest.start_with?(str)
      advance(str.length)
    end

    # Accept regex and advance the scan pointer, or
    # do nothing if no match at begin.
    def match(re)
      return unless rest =~ /\A#{re}/
      advance($~.to_s.length)
    end

    # Accept until `re` matches and advance the scan pointer to the
    # character before the match start, or consume the entire string
    # if no match occurred.
    def match_until(re)
      current = rest

      if current =~ re
        advance($`.length)
      else
        @pos = @string.length
        current
      end
    end
  end
end