It’s just data

URI Equivalence

In researching how Atom and the FeedValidator should handle URI equivalence, I took a look at how language environments with built in URI classes implement equality methods.

testuri.java produces:

 http://example.com/          http://example.com           false
 HTTP://example.com/          http://example.com/          true
 http://example.com/          http://example.com:/         true
 http://example.com/          http://example.com:80/       false
 http://example.com/          http://Example.com/          true
 http://example.com/~smith/   http://example.com/%7Esmith/ false
 http://example.com/~smith/   http://example.com/%7esmith/ false
 http://example.com/%7Esmith/ http://example.com/%7esmith/ true
 http://example.com/%C3%87    http://example.com/C%CC%A7   false

testuri.cs produces:

 http://example.com/          http://example.com           True
 HTTP://example.com/          http://example.com/          True
 http://example.com/          http://example.com:/         True
 http://example.com/          http://example.com:80/       True
 http://example.com/          http://Example.com/          True
 http://example.com/~smith/   http://example.com/%7Esmith/ True
 http://example.com/~smith/   http://example.com/%7esmith/ True
 http://example.com/%7Esmith/ http://example.com/%7esmith/ True
 http://example.com/%C3%87    http://example.com/C%CC%A7   True

Update: testuri.pl produces:

 http://example.com/          http://example.com           1
 HTTP://example.com/          http://example.com/          1
 http://example.com/          http://example.com:/         1
 http://example.com/          http://example.com:80/       1
 http://example.com/          http://Example.com/          1
 http://example.com/~smith/   http://example.com/%7Esmith/ 1
 http://example.com/~smith/   http://example.com/%7esmith/ 1
 http://example.com/%7Esmith/ http://example.com/%7esmith/ 1
 http://example.com/%C3%87    http://example.com/C%CC%A7   0

Java is totally borked.  The first seven examples are straight from section 3.2.3 of RFC 2616; they should all return true.

Test 8 was recently discussed on atom-syntax, and should also return true, although this is not explicitly clear from reading RFC 2396bis.

Posted by Mark at

Gack.  Just hypothetically, if someone wanted to write carefully-done URI comparator, I suppose the cleanest thing would be to subclass URI... maybe not.  Since it doesn't really have any exposed fields that seem useful, you might just as well do a URIEquivalenceChecker class with a single static method taking two URIs and some way of expressing how hard you want to try...

Posted by Tim Bray at

There are a finite number of URI schemes.  It should be possible to write a URI compare module/class that takes the quirks of each scheme into account.

Now that I've pretty much run out of useful features to implement for the Universal Feed Parser, maybe I'll work on this next.

Posted by Mark at

The "irc:" scheme does not appear in the assigned list. It amazed me, I've always used it in mozilla. It can be found in the wild.

Posted by Santiago Gala at

Perhaps I should have qualified: "registered" URI schemes.  The irc:// scheme has had several drafts over the years, but never made it to final RFC status.

Posted by Mark at

A few more can be found here which still does not include feed: nor tag: nor urn:uuid, all of which can be found out in the wild.

Makes me wonder if the registration process is broken to the point where the concept of registered schemes is increasingly becoming less and less relevant.

Posted by Sam Ruby at

Sam Ruby: URI Equivalence

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In Java  equals  is very closely linked to hashcode.

If you encode before comparing in .equals, then you must encode before calculating your hashcode() for storage and retrieval in HashMap/HashTable/HashSet etc. That leaves you with a pretty slow hashcode() function, espeicially if you have to re-size the hashmap. So URI may “suck” for performance reasons. If you don’t encode in hashcode too then  [link] goes into your hashset and [link] won’t get it out.

Note that this definitely means you never want URL to be a key in your hashmap! :) wanna take bets on what hashcode() does to create a hash value (I haven’t checked yet).

Posted by anonymous at

Related discussion

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