Improve performance of backtrace filtering

Nikolai Weibull posted the following to the Mocha mailing list :-

Mocha::BacktraceFilter creates a new Regexp object for every line of every backtrace it filters. It also calls File.expand_path for each such line.

The Regexp object creation can easily be removed by creating it once in #initialize. File.expand_path might be more difficult to get rid of, but File.expand_path is slow, at least on Windows. Getting rid of all those calls to it would thus save a lot of time. Rewriting BacktraceFilter to that below makes running my 108 tests with Expectations run in less than a third of the time:

  def initialize(lib_directory = LIB_DIRECTORY)
    @lib_directory = Regexp.new('^' + Regexp.escape(lib_directory))
  end

  def filtered(backtrace)
    backtrace.reject { |location| @lib_directory.match(location) }
  end

I realize that this doesn’t filter backtraces correctly if Mocha’s paths are relative. I was, however, hoping that someone with more knowledge in this area would be able to resolve this. Perhaps setting LIB_DIRECTORY = File.dirname(File.dirname(__FILE__)) + File::SEPARATOR is a good way of doing it? Then both @lib_directory and location will both be relative or absolute, depending on how Mocha was required.