I'm 72. As for what I have shipped over the half-century of my career, you can read all about that in part two of my book We, Programmers. Suffice it to say I've shipped a LOT of code.
Actually, I presented that figure as an example of just how difficult that problem was to understand, and how (on a bike ride) I was finally able to visualize what was going on. The ascii image was presented a bit tongue in cheek.
However, if you study that image, you might come to the same insight that I (on my bike ride) came to; and that the english comments never helped me with.
In 2006 Ron Jeffries wrote four blogs about solving Sudoku in Ruby with TDD. His blogging effort ended before he completed the solution. I think he got interested in something else and left the whole thing hanging.
The anti-TDD lobby, at the time, hailed the two documents as proof that TDD didn't work. Ha Ha, Nya Nya Boo Boo.
I was aware of this silliness, but never bothered to study it. I had better things to do. Until March of 2020. Then I thought I'd use Sudoku as a case study for Episode 62 in http://cleancoders.com.
I had not read either of the previous documents and decided to maintain that ignorance while writing the solver in Clojure using TDD. It turned out to be a rather trivial problem to solve. You can see my solution in http://github.com/unclebob/sudoku
I don't know why Ron stopped blogging his Sudoku solver in 2006; but he picked it up again in 2024 and has written much more about it.
The trick I used to solve Sudoku with TDD was to consider the degenerate cases. Sudoku is usually a 3x3x3x3 grid. Let's call this a rank-3 problem. I started with a rank 1 problem which is trivial to solve. Then I moved on to a rank 2 problem which was relatively simple to solve; but was also very close to a general solution. After that I could solve rank N problems.
The TDD strategy of starting with the most degenerate case (rank 1) and then gradually adding complexity may not have been well known in 2006. TDD was pretty new back then. If you explore Ron's first four blogs you can see that he briefly considered rank 2 but opted to go straight into the rank 3 case. The sheer number of variables for each test (81) may have played a role in his loss of interest. In my case (rank 2) I had far fewer variables to deal with.