The Importance of Equality

I’ve been working on a converter from Markdown to NSAttributedString and while running my unit tests against John Gruber’s Markdown test suite, I couldn’t understand why one specific test was failing². Here’s the test Markdown source:

Just a [URL](/url/).

[URL and title](/url/ "title").

[URL and title](/url/  "title preceded by two spaces").

[URL and title](/url/   "title preceded by a tab").

[Empty]().

What was confusing me was that the NSAttributedString that resulted from calling +[NSAttributedString attributedStringByConvertingMarkdown:test withStyles:styles] seemed to repeat the first four links and follow up with the correct fifth link. This didn’t make any sense.

My first thought was that maybe NSAttributedString wasn’t retaining the MCLink instances (which are a subclass of NSURL). And sure enough, after putting some logging into -dealloc, the evidence was clear: the second, third and fourth links were not retained. Only the first and fifth were retained.

I’m not certain why it took me so long to realise that NSAttributedString was probably comparing the new URLs against those already in the string and when it discovered a match, it reused an existing one.

After a quick implementation of -isEqual: and -hash, all was working gloriously.