Chess, Mini Golf and Solitaire for iPhone

Since releasing Jetshot, I've published three more games on the App Store: Chess Offline, Mini Golf 3D and Solitaire Klondike. All three are classics everyone knows, all three are free, and all three run on iPhone and iPad in English and Czech.

In this article I'll introduce each of them - and share a few stories from under the hood, because each one hides at least one problem that turned out to be far more interesting than it first looked.

Just the Game, Nothing Else

Classic games are everywhere on the App Store, and they are often some of the most cluttered apps there. A game of solitaire interrupted by a video ad, a chess app that asks you to subscribe before it shows you where you went wrong, a mini golf game that wants you to buy coins. I wanted the opposite, so all three games follow the same rules:

No ads - Not even a banner
No in-app purchases - Everything is in the free app
No account and no data collected - Nothing to sign up for
Completely offline - Chess and Solitaire don't contain a single line of networking code

Each App Store description ends with the same kind of sentence: Just chess. Nothing else. That's the whole idea.

Chess Offline: Play and Review

Chess Offline lets you play against the computer at five levels, from Beginner to Grandmaster, or against a friend on one device - the board can turn towards whoever is on the move. Its best feature, though, comes after the game.

When a game ends, the engine plays it back and grades every single move: brilliant, great, best, excellent, good, book, inaccuracy, mistake, miss or blunder. Each player gets an accuracy score, an evaluation graph shows who was winning when, and the move the engine would rather have played is drawn on the board. On many chess platforms, this kind of game review is a premium feature with a daily limit. Here, it has no limit and works in airplane mode.

For newcomers, there's also a nine-chapter guide on how to play, with 27 board diagrams. Fifteen of them are playable - the dots on them are real legal moves calculated by the engine, and tapping one plays it.

An Engine of Its Own

The app doesn't use any third-party chess engine. Its own engine, written in pure Swift, uses the classic techniques of chess programming: alpha-beta search with a transposition table, quiescence search, null-move pruning and static exchange evaluation. That last one is what helps the review tell a brilliant sacrifice from a plain blunder - both give up material on the spot, but only one of them gets something back for it.

The two strongest levels open out of a book of 90 named opening lines, so they never replay the same game. The weaker levels vary their openings by picking a little at random among the moves they searched.

How do you know a chess engine plays by the rules? Chess programmers use a technique called perft (performance test): you count every possible position reachable from a given position in N moves and compare the number with known reference values. From the starting position, there are 20 positions after one move, 400 after two, 8,902 after three and 197,281 after four. If your move generator gets a single rule wrong - castling through check, en passant, a promotion - the number won't match. The test suite runs perft on six well-known positions, including the famous "Kiwipete", which packs castling, en passant, promotions and pins into a single position.

A Chess Game You Can Play by Ear

Every square on the board is an accessibility element. VoiceOver reads its coordinate, what stands on it, whether the piece can move and where a move would land - so a whole game can be played without seeing the screen. The notation is localized too: a knight move written Nf3 in English reads Jf3 in Czech, while the engine and saved games keep using the English notation internally.

A small easter egg for chess fans: the App Store preview video isn't a staged game. It replays the seventeen moves of the famous Opera Game - Paul Morphy against the Duke of Brunswick and Count Isouard, played in a box at the Paris opera in 1858.

Mini Golf 3D: 108 Holes

Mini Golf 3D is a real minigolf game in 3D, built with SwiftUI and RealityKit. It has 108 hand-designed holes across nine worlds - from Green Garden through Frozen Fjord, Neon Nights and Volcano Forge to Orbital Station - each with its own obstacles: sand traps, ice, portals, pinball bumpers, lava, geysers, turntables, cannons, magnets and loops.

You play it like a slingshot: drag back from the ball, and the longer the drag, the harder the shot. A dotted aim line predicts the path, including bank shots off the rails, and turns gold the moment the putt is going in. Each world also has a pair of local critters - a hedgehog, a penguin, a snowman, a crab - and they aren't just scenery. The ball bounces off them.

The holes are modelled on real minigolf. The shapes follow the standard lanes defined by the World Minigolf Sport Federation - the pyramids, the lightning, the labyrinth, the double gate - and the game uses real rules: the stroke limit is par + 3, and water or lava costs a penalty stroke.

Measuring Difficulty with Dijkstra

With 108 holes, the hardest design problem wasn't building them - it was making sure the difficulty grows evenly. The surface area of a hole turned out to be a useless measure: the labyrinth is only two square metres and takes four putts, while an open basin twice the size takes one.

So difficulty is measured as travel: the shortest distance the ball has to roll from the tee to the cup, around every rail and over the ramps and portals. A small offline tool computes it with Dijkstra's algorithm over a grid of 3 cm cells and checks a few rules across all 108 holes. Every hole has between 0.95 and 1.55 metres of travel per par stroke, a world's final hole is always its longest, and the median travel grows from world to world. A second tool walks every hole in 2 cm steps and proves the ball can actually reach both the cup and the hidden bonus star.

The Loop That Physics Couldn't Handle

Most of the game relies on RealityKit's physics engine, but not the loops. A 3 cm ball at putting speed would either catch on the seam between two track segments and fly off into the sky, or glue itself to the track. So inside a loop, the game takes the ball out of the physics engine and moves it around the circle itself, using the energy budget of a real rolling sphere.

There's a lovely bit of physics in that: a rolling ball slows down on a slope more gently than a sliding one, because part of its energy is stored in the spin. For a solid sphere, only 5/7 of gravity brakes it along the track. Hit the loop too softly and the ball climbs part way and rolls back; just a bit short, and it loses contact near the top and drops inside the circle - exactly like a real one would.

The Same Hole for Everyone, Every Day

Mini Golf also has a daily challenge: one hole a day, the same for every player. Just like the daily run in my brick-breaker Breakup, there's no server behind it. The hole is derived from the date alone - the day is hashed with FNV-1a and fed into a SplitMix64 random generator - so every device picks the same hole independently.

The tricky part was a seemingly simple rule: never the same hole two days in a row. If today's draw matches yesterday's, it's redrawn. But yesterday might have been redrawn as well, so comparing against yesterday's first draw isn't enough - you need to know what yesterday actually produced, which depends on the day before that. The game unwinds this chain four days back.

Solitaire Klondike: Offline

Solitaire Klondike is the classic solitaire, done properly. It has draw one or draw three, and the scoring you remember from the Windows desktop, worked out in full: +10 for a card on a foundation, +5 for turning one over, −2 every ten seconds and a time bonus for finishing quickly. If you prefer, there's Vegas scoring, or no scoring at all.

Its goal is that you're never stuck and never guessing. Undo reaches back through the whole game, hints cycle through every move worth making, and when a deal genuinely runs out of legal moves, the board tells you so. Once the win is certain, a magic wand plays the rest of the game for you.

A Whole Board in 65 Bytes

Undo survives even closing the app, so a deal reopened at a dead end can still be walked back to where it went wrong. That means the full undo history is saved after every single move. Encoding it as JSON would take about 200 bytes per step, rewritten from scratch each time, so the game uses its own compact binary format instead. Each card fits into one byte - suit and rank in seven bits, and the eighth bit says whether it's face up - and a whole board takes just 65 bytes.

400 Games Played on Every Push

The game logic is tested by a fuzz test that plays 400 deals and checks the board after every single move: 52 distinct cards, valid foundations, valid runs, no face-up card ever buried. It also makes sure the hints and the dead-end detection always agree, so the game can never announce that a deal is dead while a hint is still waiting to be given.

Yet one bug slipped through all of that: tapping anywhere on the empty table dealt a card, as if you'd tapped the deck. The logic was fine - the problem was the size of the deck's tap area. In SwiftUI, a view placed with .position claims all the space it's offered, so a tap area applied after it quietly covered the whole board. Unit tests ask the model to make a move; they never actually tap. Today, UI tests launch the app, tap the bare felt and expect nothing to happen.

Apps That Take Their Own Screenshots

One thing all three games share is how their App Store screenshots and preview videos were made. None of them is a mock-up. Each app has special debug modes that let a script launch it in a given state and let it play by itself, so every score, clock and accuracy figure in the pictures was really earned in a game.

For Solitaire, this led to a fun search. I needed a screenshot of the dead-end banner - a deal where the game's own move advisor plays itself into a position with no legal move left, with a score still on the board. Finding one meant playing through hundreds of thousands of deals. Only two in two hundred thousand fit.

Built Natively, Built Small

All three games are written in Swift and SwiftUI, with no third-party dependencies - only Apple's own frameworks. Solitaire and Chess synthesize their sounds at runtime, Solitaire draws its cards in code, and Mini Golf generates its 3D graphics and textures procedurally. As a result, the apps are tiny by today's standards:

Solitaire Klondike - about 2.4 MB
Chess Offline - about 5.6 MB
Mini Golf 3D - about 28 MB, most of which is its soundtrack of 32 music tracks

All three are also built with accessibility in mind - VoiceOver, Dynamic Type and Reduce Motion are supported throughout. And all three are open source on GitHub under the MIT license: chess, minigolf and solitaire.

Give Them a Try

If you have an iPhone or iPad, I'd be really happy if you tried them out. A rating or a short review on the App Store helps a lot - for a small independent developer, it's the main way other players can find the games.

Final Thoughts

It's easy to think that classic games like chess or solitaire are "solved" - that there's nothing left to build. But doing them properly is full of interesting problems: a chess engine you can trust, a difficulty curve you can measure, an undo history that fits in a few bytes. And sometimes the most valuable feature is simply what the app leaves out.

Thank you for reading, and enjoy the games!