CBG Pro — Clavierhaus Backgammon

powered by GNU Backgammon

CBG Pro 1.0 is a milestone for backgammon on Android: the complete GNU Backgammon engine — the same code, the same neural nets, the same numbers as the desktop reference — under a modern touch interface, with an analysis, training, and record-keeping suite that has no counterpart on the platform. Entirely offline: no server, no account, no ads, no telemetry — the app does not request the network permission at all. Free software, GPL-3.0, source in front of you.

We do not ask to be believed. Every claim in this README is checkable against this repository — the vendored engine source, the verification harnesses, the reproducible-build tooling — and the strongest claims ship with the tools to check them.

Why there is nothing similar

• The engine is not "based on" GNU Backgammon — it is GNU Backgammon. Version 1.08.003, vendored verbatim, with the neural-net weights byte-identical to the upstream release. Every move, cube decision, equity, and verdict on screen is the engine's own; the app renders and never re-derives. There are no app-side backgammon heuristics anywhere.
• Regulation rollouts on a phone. 1296-trial, cubeful, variance-reduced, quasi-random rollouts — the same discipline desktop analysts use — seed-reproducible, with per-game logging in gnubg's own format. The comparison harness that verifies our rollout core against desktop GNU Backgammon, trial by trial and dice roll by dice roll, ships in this repository (tools/rollout_harness/).
• A Coach that is correct or silent. Every explanation is licensed by the engine's numbers; when the numbers don't justify a sentence, the Coach says less rather than inventing a reason. Tap any better alternative and watch it play out on the board against your own move.
• Your career, signed. Finished matches enter a tamper-evident, cryptographically chained record — your results, provable, in plain files you own. The verification code is right here, open, like everything else.
• Your Personal Stats. Match statistics computed by gnubg's own analysis — error rates, luck, cube handling — with every number labeled by what it is and where it came from. No invented metrics.
• A tournament match clock, written as a clean C module against tournament regulation (delay clock), and offered upstream to the GNU Backgammon project itself (docs/upstream/).
• Reproducible builds. Two independent builds of a release commit produce byte-identical APKs, and tools/verify_reproducible.sh proves it on demand (see docs/REPRODUCIBLE_BUILD.md).

What it does

Play — Full matches against the engine at any match length, across seven strength levels from Beginner to Grandmaster (3-ply). The full doubling cube (offer, take, drop, redouble, resign), tournament rules (Crawford, Jacoby, automatic doubles, beavers), and the canonical match equity tables — Kazaross-XG2, Woolsey, Jacobs & Trice, Snowie, and the rest of gnubg's set. A live tutor can show the engine's own equity as you play, and the match clock brings tournament time pressure when you want it.

Train with the Coach — Play with the engine judging every move. Each move gets one of three honest verdicts — best, fine-but-not-best (with what beat it), or flagged (with gnubg's severity and the equity it cost) — and a two-tap before/after explorer steps you through your move and the engine's better ones, arrow by arrow, on the board. In matches past one point the Coach judges your cube actions too. Undo is a teaching tool: take the move back, try the better idea, feel the difference.

Analyse — Paste a GNU BG ID, Match ID, or XGID from a forum, a book, or another app — or build any position by hand on the set-up editor board. With dice, the engine ranks the chequer plays; without dice, it gives the cube decision — double / take / drop and the equities behind it, exactly as desktop gnubg's edit mode does. And when ranking isn't enough, roll the position out: regulation rollouts, right on the device.

Review — Save whole matches to standard .sgf at any point, then step through game by game and move by move on the engine's own game record: what was played, what gnubg preferred, the equity cost, the verdict. Saved files open unchanged in desktop GNU Backgammon and Backgammon Studio.

Honesty by construction

One principle underlies the whole app: GNU Backgammon is the sole authority, and the interface never substitutes its own judgement. Every number on screen is the engine's, every explanation is bounded by what the engine's numbers actually support, and where the engine is silent the app is silent too. The worst thing an analysis tool can do is sound confident about something it doesn't know; this one is built so it can't.

Your data is yours

Matches, careers, and settings live in a folder you choose through Android's file picker — plain .sgf and plain files, readable by desktop gnubg today and by anything else in twenty years. Move the folder, back it up, point a new phone at it: the same folder works across installs. Nothing is locked in, because nothing is ever sent anywhere — there is no "anywhere" in this app.

One board, every device

The board is drawn from a single geometry computed once from the screen size, so a tap lands exactly where the eye says it will — verified from tablet (16:10) to tall phone (20:9). Nothing scrolls; what does not fit is made to fit. Three hand-tuned themes (Ocean, Classic, Forest) plus a System option that follows Material You. Built for Android 12+ (minSdk 31), landscape, tap or drag to move.

Built to be checked

• The engine source is vendored in engine-core/ — diff it against upstream GNU Backgammon 1.08.003 yourself.
• tools/rollout_harness/ builds the port's exact engine subset on a desktop and verifies rollouts against desktop gnubg — same seed, same dice, same games, logged in gnubg's own format by gnubg's own code.
• tools/verify_reproducible.sh builds any commit twice, independently, and compares the APKs byte for byte.
• The build recipes are in the tree; F-Droid builds from them and verifies the published APK against them.

Building

./build_native_android.sh     # NDK build of the engine (glib included)
cd gnubg-app && ./gradlew assembleDebug

Full requirements and the deterministic-build details: docs/REPRODUCIBLE_BUILD.md.

License and lineage

GPL-3.0-or-later, like the GNU Backgammon it carries. CBG Pro exists because gnubg's authors built something extraordinary and licensed it freely; this project's additions — the interface, the Coach, the career record, the clock, the harnesses — are offered back under the same terms. The match-clock module has been submitted upstream.

CBG Pro. The whole game, in your pocket, owing nothing to anyone.