This attempts to copy chomium's algorithm for making sense of things typed into the url bar. You can download the chromium source to play along, but note that it is currently 2.1 GB.
The ported code is very similar to how it is written in the original C++. It is a great example of the imperative style of programming by state mutation. I'm not gonna lie, it's pretty gross. But hey, it passes the tests.
Some examples from the tests:
URLGrey.new("google.com").fixed
#=> "http://google.com/"
URLGrey.new("www.google.com#foo").fixed
#=> "http://www.google.com/#foo"
URLGrey.new("\u6C34.com").fixed
#=> "http://xn--1rw.com/"
URLGrey.new("http://foo.com/s?q=\uC5C5").fixed
#=> "http://foo.com/s?q=%EC%97%85"
URLGrey.new("http;/www.google.com/").fixed
#=> "http://www.google.com/"
URLGrey.new(" foo.com/asdf bar").fixed
#=> "http://foo.com/asdf%20%20bar"
URLGrey.new("[::]:80/path").fixed
#=> "http://[::]/path"