cardkit

Reusable, game-agnostic foundation for a suite of Android card-game apps.

cardkit provides the parts that every card game in the suite shares, so each game app only has to implement its own rules and screens:

Module	Platform	Contents
cardkit-core	pure Kotlin/JVM	card & deck model (supports non-standard decks), seeded RNG/shuffle, Player/PlayerView abstraction, game-loop driver scaffolding, AI Strategy interface
cardkit-ui	Android / Jetpack Compose	card rendering, hand fan, table layout, card animations, theming, generic bidding/score panels
cardkit-monetization	Android	Monetization interface + a FOSS no-op implementation (donation link). No proprietary dependencies.
cardkit-monetization-play	Android	Google Mobile Ads + Google Play Billing implementation. Depended on only by an app's play build flavor.
cardkit-net	Kotlin/JVM + Kotlin/Wasm	the online wire protocol shared by client and server — everything that is not a game's own payload — plus the WebSocket client. FOSS-only (Ktor).
cardkit-server	Kotlin/JVM	the authoritative online server for any cardkit game: rooms, invite-code lobbies, bot substitution for dropped players, reconnect by session token, games that survive a server restart. A game supplies a GameDescriptor and its own main().
templates/	—	reusable CI workflow, FUNDING.yml, PRIVACY.md, and license files for each app

Design principles

• cardkit-core has no Android dependency. The authoritative game engine is pure Kotlin so it can also run server-side — as the 500 app's online multiplayer now does, driving the engine from a hosted server over WebSockets.
• Proprietary code is quarantined in cardkit-monetization-play. Everything else is free of Google Mobile Ads / Play Billing / Firebase, so a game app's free/libre (F-Droid) flavor can exclude that one module and contain zero non-free code.
• Deterministic engine. Shuffling and AI use a seeded RNG so games are reproducible and unit-testable.

Consuming cardkit from a game app

Each game app includes this repository as a git submodule and wires it in as a Gradle composite build (built from source — no binary artifacts to publish, and F-Droid-friendly):

// <app>/settings.gradle.kts
includeBuild("cardkit")

Then app modules depend on the published coordinates, which Gradle substitutes with the local submodule build:

dependencies {
    implementation("io.github.rotundtapir.cardkit:cardkit-core")
}

Building

./gradlew build            # all modules + tests (JDK 21)
./gradlew :cardkit-core:test

cardkit-core builds with only a JDK. The Android modules require the Android SDK (compileSdk 36).

Artwork

The card faces in cardkit-ui are Byron Knoll's Vector Playing Cards (released into the public domain by the author), bundled as downscaled PNGs under cardkit-ui/src/main/res/drawable-nodpi/.

The 11/12/13-of-suit faces (used by six-handed 500's 63-card deck) do not exist in Knoll's set; they are new compositions made for this library by extracting the pip and index glyphs from Knoll's public-domain originals and laying out symmetric 11/12/13-pip grids in the same style. The card back (card_back.png) is likewise an original design (indigo lattice) drawn for this library.

Being derived from public-domain material does not by itself place a new work in the public domain, so to keep the whole card-art set uniformly reusable we dedicate these files (card_11_*, card_12_*, card_13_*, card_back.png) to the public domain under CC0 1.0. Everything else in this repository remains under the repository license below.

Audio

The sound effects in cardkit-ui (res/raw/*.ogg — card plays, deals, shuffles, trick pickups, chips) are from Kenney's Casino Audio pack, released under CC0 1.0.

License

GPLv3-or-later with a Google Mobile Ads / Play Billing linking exception — see LICENSE and LICENSE-EXCEPTION.md. This lets contributed code ship in both free/libre and ad-supported builds. See CONTRIBUTING.md (contributions require a DCO sign-off).

Namespace: the Maven group id / package prefix io.github.rotundtapir is the io.github.<account> namespace of the GitHub account rotundtapir that publishes this library.