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  • First, I appreciate the schadenfreude here as much as the next hacker, but it appears this bug was in the Freescale code for the platform, not the code Microsoft wrote for the Zune. So it effects more than just Zunes...

    Second, the "some type inference systems" is misleading. The one type inference system that Andrew Koenig and MJD discuss is the Hindley-Milner [] type system, which is used in ML and its descendants (OCaml, F#, Haskell, etc.). Koenig's example demonstrated that a recursive algorithm would have never worked properly because it was missing a base case. That's totally different from a while loop in an imperative program. No type inferencing system can find a bug like this in an imperative program.

    Third, this is not the kind of problem simple code coverage would be able to isolate. What happens if $days is 367 and $year is a leap year? Then the inner if branch fires, $year increments, and $days decrements by 1 leap year.

    That's why this code works perfectly, except for one day every 4 years -- the last day of every leap year. You could try 1000 random inputs and demonstrate all branches are exercised, and still miss the case where $days = 366 in a leap year.

    Date math is hard, and full of corner cases like this. This is a problem that by its very nature isn't amenable to automated reasoning, whether you're talking about type inferencing, theorem proving, automated testing or code coverage analysis. Your best bet for all date math related code is to build up a body of known edge cases and try them all, in a brute force manner.

    • Thanks for the extra information.

      I think, ultimately, that part of the problem here could be alleviated by coupling things which must be coupled. For example, having a DayOfYear type with a type coercion system could partially help. Imagine the following type definition (in Perl 6, which doesn't do inferencing, of course, but bear with me):

      subset DayOfYear of Int where { 0 <= $_ <= 366 };

      Obviously, that still leaves edge cases, but I would imagine with inferencing, while you wouldn't get a compile

      • All of this, of course, if from a typing newbie, so if I'm talking complete bollocks, feel free to correct me!

        You're talking complete bollocks. :-)

        The kinds of things a strong static typing system can do is let you create types like DayOfYear, which let you specify a single calendar date, and prevents such nonsense like the 47th of April as being a "date". From there, you can create functions like addDays that take a DayOfYear and an integer number of days and return a new DayOfYear.

        What this code was doing was taking a count of days since the beginning of time, and figuring out the current date by subtractin

      • I think the mindblowingly clever technology you are looking for here is called a truth table. :-)