@civ-clone/core-game

Everything one game owns: 43 registries plus its turn, year and random number generator.

import { Game } from '@civ-clone/core-game';

const game = new Game();

game.players.register(new Player(game.rules));

Two Games in one process share nothing, which is what makes a host able to run two matches, a test able to skip its beforeEach reset, and a save able to load into a fresh context beside the running one.

defaultGame

The engine's registries have always been module-level singletons. defaultGame adopts them rather than constructing its own:

import { defaultGame } from '@civ-clone/core-game';
import { instance as cityRegistryInstance } from '@civ-clone/core-city/CityRegistry';

defaultGame.cities === cityRegistryInstance; // true

That is what lets packages migrate one at a time. A package still importing instance and one using game.cities see the same registry, so a half-migrated engine does not quietly disagree with itself — which would show up as a rule not firing rather than as an error.

Every other new Game() gets its own of everything.

Why this is not in core-engine

Game imports every registry, and the seventeen packages with a registerRules.ts will import Game. Putting it in a package those registries already depend on would make a cycle. Nothing Game imports imports Game: the registry owners are all core-*, the registrars are all plugins.

Plugin registries stay out for the same reason and one more — core-game should not depend on a particular ruleset.