By Michel Colman

Q: What do the words “sudoku” and “kaodoku” mean?

A: “Sudoku” is an abbreviation of “Sūji wa dokushin ni kagiru” which is Japanese for “the digits must occur only once”. A kaodoku has faces (“kao”) instead of digits (“su”).

Q: Where did you get the idea for kaodoku?

A: I was experimenting with different variations of sudoku, like for example LCD sudokus that can also be solved upside down. While these were certainly interesting, solving them still felt comparable to solving a normal sudoku: you basically ended up just writing down all the candidates in each cell and then solving it like any other sudoku. As I was looking for other variations to try, suddenly the idea struck me that I could use a different kind of symbols with two properties, for example smiley faces with different shapes and mouths, and just give one half of the information. I wasn’t expecting much, just another funny variation, but when I tried it out I was surprised how different these puzzles really were, and how many new logic tricks I discovered while solving them. I sent a few to some friends who were immediately enthousiastic as well, and then started sending them to the Belgian Mensa magazine Commensal where they have been included every month for several years now. Mensa Netherlands later started adding them to their magazine too.

Q: Can I include kaodoku puzzles in a magazine?

A: Sure, you can export them right from the Kaodoku app using the standard Share menu, as long as you don’t remove the web address which is automatically added under the puzzle. I suggest you use the pdf format for printed publications, and the png format for online. If you want to remove the web address, or maybe even publish a puzzle book full of kaodokus, contact me at:

Q: I can’t even solve the “very easy” puzzles! Where do I start?

A: They’re not as hard as they look, your mind just has to adjust to working with these new symbols. Here are two examples showing how to start out solving a very easy and a hard kaodoku. The Kaodoku app also has an excellent hint function (💡). It will actually guide you by showing several different ways forward using step by step logic. Whenever you’re stuck, tap the gear-shaped settings icon, then the light bulb. First it will give you a general idea of how many ways forward there are, then you can look at all of them one by one using the “More” and “Other” buttons. A cell with only one possibly symbol, a symbol that has only one place left, a cell that can only have one shape, etcetera. You will be surprised how many things you missed. When the easy methods are exhausted, the hint function will switch to more complicated logic.

Q: How is the level of difficulty determined?

A: The algorithm tries to solve the puzzle in a “human” way using a number of logical methods, the same as those used by the hint function. At every step, it looks at how many ways forward there are, and how easy they would be to spot for a human. All of these together determine the hardness of this particular step. The algorithm fills in the symbol it considers easiest to find, and then goes on to calculate the hardness of the next step. The difficulty of the puzzle is determined by the score of the hardest step, but the algorithm also tries to maximize the overall difficulty of all the steps together. It does this because most people prefer a puzzle that maintains the same level of difficulty as long as possible rather than one with a single bottleneck where you get stuck for a long time after which the puzzle practically solves itself.

Q: How are the puzzles actually generated?

A: As soon as the puzzle parameters screen is displayed, even before you tap “Start”, the app is already working hard in two different background threads to try and find a puzzle that respects the minimum and maximum peak level of difficulty for the chosen settings while maximizing the total difficulty of all the solving steps together. It keeps adding and removing random symbols, making sure there’s still a unique solution and then judging the difficulty. Even after a suitable candidate puzzle is found, the threads continue to look for better ones until about one second after the screen finished fading to black (with some sensible limit to avoid draining your battery). So if you want a puzzle that remains challenging for as long as possible, select the parameters and then give the algorithm a minute or so before you press “Start”.

Q: Which are better, symmetric puzzles or non-symmetric ones?

A: The symmetric puzzles are more visually appealing because the given cells are spread more evenly over the board. However, they do tend to contain more redundant information since the puzzle creation algorithm has less liberty in removing individual givens. This means that, while the peak difficulty is the same, the difficulty in symmetric puzzles usually drops off a bit more quickly after the peak while non-symmetric puzzles remain challenging a lot longer.

Q: Do you have an Android version?

A: No, I don’t have an Android device and don’t know the first thing about programming for Android. If you want to help, let me know and maybe we can work something out.

Q: Why should I upgrade to the full version?

A: The full version offers many different variations with irregularly shaped zones and/or additional gray areas. This is where kaodokus really come into their own, with all these intersecting regions leading to all sorts of fun new ways to deduce even partial information about cells. But even if you’re perfectly happy with the normal puzzles, perhaps consider buying the full version anyway to show your support. Thank you!