Comment by sirclueless

> Norvig didn't do intensive formal testing on his solution.

Ummm, he solved a hundred or so sudoku problems from Project Euler and verified they were correct, and did a performance test on a million random boards. That sounds like as much testing as one would need for correctness.

You might want to add automated tests for regressions, and maybe Norvig did so, but it would add little to the blog post (which is not really about testing at all).

I think the lesson to take away is that Jeffries never sat down and thought about the problem enough to come up with the key piece of insight from Norvig: "Coding up strategies like this is a possible route, but would require hundreds of lines of code (there are dozens of these strategies), and we'd never be sure if we could solve every puzzle." I personally consider this to be a failing of TDD: It encourages you to write code before you understand your problem. Others may take away different things.