# Ludii

Digital Ludeme Project (Maastricht / Cameron Browne, ERC-funded) · Java ·
`checked` that it exists and what it claims; **licence `unverified`**

## What it optimises for

**Generality and concision of game representation.** Games are structures
of **ludemes** — high-level, human-understandable game concepts — which
gives concise descriptions and a very large game library, with historical
reconstruction as a founding motivation. `checked`

## How a game is defined

A game is a **quintuple of ludemes** — Name, Players, Mode, Equipment,
Rules. Games are modified by adjusting ludemes or parameters such as board
size. **Each game's description fits in a QR code.** `checked`

**Syntax formal, semantics not** `checked` — the sharp point, and it
answers "can we derive a formal language from it": **no, there is nothing
semantic to extract.**

- The grammar is a **class grammar, automatically derived from Ludii's
  source**, so keywords and their argument types come from the class
  hierarchy. That is more formal than most hand-written grammars.
- But *"Ludii effectively makes its programming language (Java) the game
  description language"*: **a ludeme means what its Java class does.**
  Compilation produces a forward model (legal moves, apply, copy).

Contrast **GDL**, which has a logical semantics independent of any
interpreter and pays for it in speed and verbosity. See
[`../research/CB-RES-0009-extensive-form-is-the-lingua-franca.md`](../research/CB-RES-0009-extensive-form-is-the-lingua-franca.md).

## What it can analyse

Playouts, agent play, and a published set of **general board game
concepts** used to measure *distance between games*. `checked` The
concept vocabulary is the interesting part for us — it is a shared
feature space over games.

Depth of design-facing analysis relative to Ai Ai: `open`.

## The game-theoretic mapping

**Proven, and it is the extensive-form game.** *The Ludii Game
Description Language is Universal* shows the language can represent an
equivalent game for **any finite, non-deterministic,
imperfect-information game**, extending earlier work limited to finite
deterministic fully-observable EFGs. `checked`

That matters more than it looks: **EFG is also OpenSpiel's object**, so
the same formalism connects description to analysis.

## What it does not do

- It is **a description someone writes**, not a claim about another
  party's artifact. There is no notion of a game *owner* who rules on an
  underdetermined case, no finding queue, no provenance binding to a
  publisher's data. `unverified` as an absolute, but nothing in the
  material we read suggests otherwise.
- No evidence-admissibility machinery: its outputs are data, not claims
  under review.

## Relevance to clay-borg

**Track A, and it is the benchmark.** If clay-borg's answer to *"why not
Ludii"* is "ours is in Rust", there is no answer
([`../specs/Positioning.md`](../specs/Positioning.md) §1).

The serious question Track A must answer is **whether we need a game
description language at all**, or should consume Ludii's and contribute
the evidence layer on top.

## What to steal

- **Ludemes as the decomposition unit.** Our kernel has commands, events
  and rules; a ludeme is a *game idea* that composes. That is a better
  fit for "define a popular game efficiently".
- **The concept vocabulary** as a comparison space between games.
- **Concision as a discipline** — a description that fits in a QR code is
  a description a designer can read.

## What to avoid

Breadth as the goal. Ludii's value is that it covers a thousand games;
ours would have to be that it says something trustworthy about one.

## Open questions

- Licence, and whether it permits embedding or only interoperation.
- ~~Can a Ludii description express simultaneous commit/reveal and hidden
  role state?~~ **Answered: yes.** Universality covers finite
  non-deterministic imperfect-information games, and simultaneity is the
  standard EFG information-set encoding. **The live question is whether a
  binding multi-round state machine like DARVO stays *readable*** —
  universality says expressible, not concise, and concision is Ludii's
  actual selling point.
- What its analysis output actually contains.

## Sources

- *Ludii — The Ludemic General Game System*, ECAI 2020
  (https://ludii.games/publications/ECAI2020.pdf)
- *The Ludii Game Description Language is Universal*
  (https://arxiv.org/pdf/2205.00451)
- *A note on the empirical comparison of RBG and Ludii*
  (https://arxiv.org/pdf/1910.00309)