Niner: the daily sudoku

A clean, calm sudoku game for Android. One daily puzzle, five modes, zero ads, zero tracking.

Why Niner exists

Most sudoku apps on the Play Store bury the puzzle under interstitial ads, "remove ads" upsells, and analytics SDKs that track every cell tap. The puzzle itself is fine. Everything around it isn't.

Niner is the opposite. A puzzle, a board, and the time it takes to solve it.

• No ads. Not even one. No upsells.
• No accounts. No login, no email, no cloud.
• No internet permission. The AndroidManifest.xml declares zero <uses-permission> entries. The app cannot make a network call.
• No third-party SDKs. No analytics, no crash reporters, no ad networks.

Your puzzles, stats, and achievements live in a single SharedPreferences file on your device. Uninstall the app and they're gone. No server-side data, no shadow copy.

Features

Game

• 5 difficulties. Beginner, Easy, Medium, Hard, Expert. Each has its own clue-count target and hint cap.
• 5 modes
  • Classic. 3 mistakes, normal hint count
  • Strict. 1 mistake ends the game
  • Coach. Unlimited hints and mistakes (great for learning)
  • Speed. Countdown clock; every correct cell adds 4 seconds
  • Killer. Cages with target sums layered on top of standard sudoku rules
• Daily puzzle. One shared puzzle for every player, every day, with a 12-week activity heatmap and streak tracking
• Per-mode-per-difficulty best-time tracking + average win time + win rate
• 16 achievements with bundled badge artwork

Polish

• Five hand-tuned colour themes × light/dark variants (Ember, Electric, Neon Night, Citrus Punch, Hyperdrive)
• Six celebration styles. Confetti, Fireworks, Bubbles, Cherry Blossoms, Emoji Rain, Minimal
• Wrong entries don't stick to the board. They shake and vanish, so the board always reflects what you actually know.
• Peer + same-digit highlighting for quick visual scanning
• Daily-streak heatmap with empty-state preview grid
• Animated mini-grid loader during puzzle generation
• Sequenced sound chimes via Android's ToneGenerator (no bundled audio assets)
• Loss-reason-aware celebrations. Gave-up, mistake-limit, and Speed-timeout each get their own title, subtitle, and emoji.
• Cage-aware hints in Killer. When the cage constraint alone narrows a cell to one digit, the hint dialog explains in cage terms instead of pretending it's a regular naked single.

Accessibility

• Colour-blind palette option (amber for hints, dark orange for errors, dot pattern in the heatmap)
• Large-text toggle for cell values + notes
• Centered-notes layout option for users who prefer single-row pencil marks
• TalkBack semantic overlay on the board canvas. Every cell announces row, column, current value, and (in Killer) cage sum.
• Light + dark mode controlled by system or pinned to either

Tech stack

Architecture overview

app/src/main/java/com/ninersudoku/
├── MainActivity.kt
├── achievements/      Achievement enum + AchievementManager (StateFlow + persistence)
├── daily/             DailyManager (deterministic per-date seed, streak tracking)
├── game/
│   ├── Board.kt       Immutable 9×9 board with conflict detection
│   ├── Cell.kt        Cell with value, given/hint flags, pencil notes
│   ├── Difficulty.kt  Per-level clue counts + hint caps
│   ├── GameMode.kt    Mode enum with display name + description
│   ├── Generator.kt   Classic puzzle generator with uniqueness check
│   ├── Solver.kt      Bitmask backtracking with MRV
│   ├── HintEngine.kt  Naked + hidden singles + cage-aware hints
│   └── Killer.kt      2- or 3-cell cage generator + Killer-aware solver
├── onboarding/        OnboardingManager (3-page first-run experience)
├── persistence/       SaveManager (in-progress game serialisation)
├── prefs/             DisplayPreferences (a11y + behaviour toggles)
├── sound/             SoundManager (ToneGenerator-based chimes)
├── stats/             StatsManager + per-mode best-time tracking
├── viewmodel/         GameViewModel — single source of state truth
└── ui/
    ├── BoardView.kt        Canvas-rendered grid + transparent semantics overlay for TalkBack
    ├── NumberPad.kt        Digit pad with remaining counts + tap/long-press
    ├── DifficultyScreen.kt Main menu (Daily, Continue, Mode picker, difficulty cards)
    ├── GameScreen.kt       Top bar + board + actions + pause + hint dialog + celebration
    ├── StatsScreen.kt      Hero card + summary + heatmap + per-difficulty cards + achievements
    ├── OnboardingScreen.kt 3-page intro
    ├── AboutScreen.kt
    ├── celebration/        Particle engine + style preview
    ├── components/         SettingsSheet (themes, modes, a11y)
    └── theme/              5 colour palettes × light + dark

Notable design decisions

• No external state library. ViewModel + StateFlow + mutableStateOf is enough for a single-activity Compose app. Adding Hilt, Koin, or Room would have inflated the APK with no real benefit for this scope.
• org.json over kotlinx.serialization. The persistence surface is small (saved game, stats, achievements). org.json ships with Android and saves an entire serialisation plugin from being added to the build.
• Wrong entries shake but don't stick. A wrong digit increments the mistake counter and triggers a board shake, but the cell stays empty. Removes the awkward "I see a 7 here but I know it's wrong, do I leave it?" loop.
• Killer cages are strictly 2 or 3 cells. Single-cell cages would double as pre-filled givens and dilute the Killer identity. Larger cages slow the solve loop without adding interesting reasoning. Strict 2- or 3-cell cages keep deductions tractable. A topology-trapped orphan triggers a regen rather than a 4-cell exception.
• Killer uses fewer starting clues than Classic at the same difficulty (32 vs 50 at Beginner, 9 vs 24 at Expert). Cage sums carry part of the deduction load.
• The Killer hint engine is cage-aware. When the cage constraint alone narrows a cell to one digit, the hint dialog explains in cage terms ("This cage needs 14. With 9 already in, the last cell must be 5") instead of pretending it's a standard naked single.
• "New puzzle" generates fresh, never restarts the same puzzle. A "restart this puzzle" button would let players pre-scout, then run again to fake a best time or unlock Perfectionist/Flawless cheaply. The overflow menu always ships a different puzzle.
• Process-death restore via SharedPreferences. Every cell entry persists the full game state to a single key (saved_game). am force-stop + relaunch lands the player on the menu with a Continue card pointing at the exact board they left.
• Adaptive launcher icon with dedicated mipmap-anydpi-v26 background colour matching the splash screen, so no flicker on cold launch.

Build

Prerequisites

• Android Studio Ladybug or newer
• JDK 21 (Gradle requires it on AGP 8.7+)
• Android SDK with platform 35 + build-tools 35.0.0

Debug build

export JAVA_HOME=$(brew --prefix openjdk@21)/libexec/openjdk.jdk/Contents/Home
export PATH=$JAVA_HOME/bin:$PATH

./gradlew assembleDebug
adb install -r app/build/outputs/apk/debug/app-debug.apk

Release build

You'll need your own keystore. Run once:

keytool -genkeypair -v \
  -keystore app/release.jks \
  -alias niner \
  -keyalg RSA -keysize 2048 \
  -validity 10000

Then add to local.properties (gitignored):

RELEASE_STORE_FILE=release.jks
RELEASE_STORE_PASSWORD=...
RELEASE_KEY_ALIAS=niner
RELEASE_KEY_PASSWORD=...

Build:

./gradlew bundleRelease       # AAB for Play Store
./gradlew assembleRelease     # APK for sideloading

Without a keystore, the release build fails loudly with SigningConfig "release" is missing required property "storeFile". That's intentional. It stops you from accidentally shipping unsigned artefacts.

Privacy

The full privacy policy is hosted at https://niner-privacy.pages.dev/privacy.

The HTML source lives in docs/privacy.html. It's deployed to Cloudflare Pages with:

wrangler pages deploy docs --project-name=niner-privacy --branch=main

Short version: zero data collection, zero permissions, zero network calls. Everything stays on the device. Uninstalling the app removes all of it.

Roadmap

Things deferred for v1.x:

• Smarter hint techniques (naked pair, X-Wing) for Hard / Expert
• Per-mode-per-difficulty win counts (the "Cage Master" achievement currently uses "won Killer at all 5 difficulties" as a proxy for "won 10 Killer puzzles")
• Tablet-optimised layout. The phone layout works on tablets but doesn't take advantage of the extra width
• Localisation. Strings are inline; needs extraction to strings.xml first

License

MIT. Fork it, learn from it, remix it.

Credits

Built solo by Rayjack CB. Made for people who love sudoku.