My close personal friend and colleague Encsé posted a fun little programming exercise and invited us to send him solutions in our favorite Turing-complete tool, especially if it happens to be mod_rewrite or C++ templates. Unfortunately, I don’t think there is a single sane person on the planet who would consider C++ their favorite anything, and I like Encsé better than to let him needlessly associate with any additional crazy people. Obviously, this meant that I had to delve into the murky depths of template madness myself.

Here is a copy of the problem, in case Encsé decides to delete his post once he finds out it has been tainted with C++ — which would be an entirely sensible (and, indeed, healthy) reaction:

Find a number consisting of 9 digits in which each of the digits from 1 to 9 appears only once. This number must also satisfy these divisibility requirements:

1. The number should be divisible by 9.
2. If the rightmost digit is removed, the remaining number should be divisible by 8.
3. If the rightmost digit of the new number is removed, the remaining number should be divisible by 7.
4. And so on, until there’s only one digit (which will necessarily be divisible by 1).

OK, so I implemented a solution using C++ template metaprogramming. It was actually easier than I thought, and the experience was surprisingly interesting. Which is not to say I’d like to do it again. I imagine I would feel this exact same way after performing an autopsy.