This is a "progressive webapp", you can play it in the browser, or install it like a "proper" app on your mobile (Chrome/Android trhee-button-menu, "Install app"; other borwsers/platforms, please let me know!). It will keep your game stored even during reboots.
Seconded this. Very small too, at about 10 MiB, or 250 kiB per game, which is tough to beat! The Android port is probably the best but a Linux and Windows version also exist. Get it on F-Droid
There is also Gauguin, a similarly small app but with only 1 puzzle type, this one. There are more options though, including more operation combos, negative and non-consecutive numbers, and even rectangular grids! The controls are a little worse if you ask me and so is the performance but not by a lot. I have spent huge amounts of time playing the game in both apps and I slightly prefer this one thanks to more variation.
This is very cool!
Coming from a beginner, it might make the first few games easier if you added an extra setting that would start the puzzle with one or two numbers already filled in (like a typical sudoku), as that gives a natural starting point.
Obviously, it may not be needed if you mainly develop this for yourself :)
I believe in the Puzzle I attempted, the solution was not unique: at the end, two numbers could be placed interchangeably (in my case, I could switch around all "1"s and "4"s and end up with just as valid a solution - that is, of course, unless I missed something :)
I would kill to just have a simple app to play dominoes on my phone with no ads and no BS. I almost found one. It didn't have ads between games, but it did have a stupid micro transaction scheme or online play or something. Grumble. Grumble.
This might just drive me to start developing apps. I can probably do the basic stuff but I don't know where to begin programming the cpu opponent...