Gin Rummy Engine

Bots and strategy tooling for gin rummy, built on the gin-rummy mechanics crate. Where gin-rummy answers "what moves are legal?", this crate answers "which move should I make?"

The design triangle:

• Strategy: a decision procedure for one seat — take or pass the upcard, where to draw, what to shed, whether to knock, what to lay off.
• View: the information a seat may legally see. The underlying Round exposes both hands and the stock order; strategies never touch it. A View shows only the seat's own hand, the discard pile, the stock count, and what the opponent has revealed (cards taken from the pile, discards, declined upcards).
• Table: the driver. It owns the Round, tracks each seat's knowledge, asks strategies for decisions, and applies them — so information hygiene holds by construction.

Bots

• HeuristicBot: deterministic and fast. Draws from the pile only when that strictly lowers deadwood, sheds the least useful card weighted by how dangerous it is to the opponent, knocks by a configurable threshold, and lays off greedily but never breaks its own melds.
• MonteCarloBot (feature rand): determinized Monte Carlo. At each decision it samples hidden worlds consistent with the View — opponent hands containing every known card, random stock orders over the unseen cards — rolls each out with the greedy policy, and picks the action with the best expected score.
• EaaiSimpleBot (feature rand): a port of SimpleGinRummyPlayer, the baseline every entry of the EAAI-2021 Gin Rummy AI challenge was measured against. Deliberately weak and knob-free — it exists so that win rates against it are comparable across engines and papers.

Benchmarks

EaaiSimpleBot is the yardstick: arena runs under --rules eaai (the challenge's round conditions), seats and the dealer alternating, seed 7. Parenthesized ranges are 95% Wilson intervals, in percent, over 4000 rounds and over 500 (greedy) or 600 (mc:64) whole games.

The default heuristic concedes rounds by design — it hunts gin while the baseline knocks at the first opportunity — yet outscores it per round and wins the matches, which is what its knobs are tuned for. For calibration, EAAI-21 entries reported roughly 55–68% against this same baseline (metrics vary by paper); comparisons stay approximate because this crate's match play rotates the deal to each round's winner where the challenge alternated dealers. Throughput in those same runs, single-threaded: ~8500 rounds/s for greedy vs the baseline, 5.8 rounds/s at mc:64, 3.4 at mc:128.

Quick start

A bot-vs-bot round needs no features:

use gin_rummy::{Hand, Player, Round, Rules};
use gin_rummy_engine::{HeuristicBot, play_round};

let hands: [Hand; 2] = ["A23.456.789.T".parse()?, "TJQK.A23.456.".parse()?];
# let rest: Vec<_> = (Hand::ALL - (hands[0] | hands[1])).iter().collect();
let (upcard, stock) = (rest[0], rest[1..].to_vec());  // the other 32 cards
let round = Round::from_deal(Rules::default(), Player::One, hands, upcard, stock)?;
let result = play_round(round, [&mut HeuristicBot::new(), &mut HeuristicBot::new()])?;
println!("{result:?}");
# Ok::<(), Box<dyn std::error::Error>>(())

With the (default) rand feature, deal and settle whole games:

# #[cfg(feature = "rand")]
# fn main() -> Result<(), gin_rummy_engine::EngineError> {
use gin_rummy::{Game, Player, Rules};
use gin_rummy_engine::{HeuristicBot, MonteCarloBot, play_game};

let mut rules = Rules::default();
# rules.game_target = 1; // a single round settles this demo game
let mut game = Game::new(rules, Player::One);
let mut greedy = HeuristicBot::new();
let mut mc = MonteCarloBot::new(rand::rng()).samples(8);
let score = play_game(&mut game, [&mut greedy, &mut mc], &mut rand::rng())?;
println!("{} wins {} : {}", score.winner, score.totals[0], score.totals[1]);
# Ok(())
# }
# #[cfg(not(feature = "rand"))]
# fn main() {}

Writing your own bot is implementing Strategy's four decisions against a View; the driver handles all bookkeeping.

Feature flags

• rand (default): the Monte Carlo bot, Table::deal, play_game, and the examples. Disable it for a dependency-free heuristic-only build.
• parallel: Monte Carlo rollouts across the CPU cores via rayon. Decisions are bit-identical to the serial build, each just arrives faster; worthwhile at high sample counts. Off by default.

Examples

• play: play against a bot in the terminal — cargo run --example play (--bot mc, --rules classic, …)
• arena: bot-vs-bot tournaments with win-rate statistics — cargo run --release --example arena -- --rounds 1000 --p1 greedy --p2 mc:64

Alternatives

No other open-source project ships gin rummy bots as a reusable library. OpenSpiel and RLCard embed gin rummy environments for generic search and reinforcement-learning algorithms (bring your own agent), and gin-rummy-eaai is the EAAI-2021 challenge framework whose reference baseline this crate ports as EaaiSimpleBot — precisely so the numbers above stay comparable with the challenge literature.