// core/bus.js (Lane F) — event bus. Event names are a contract: TECH.md §Bus.

export function createBus() {
  const map = new Map();
  return {
    on(name, fn) {
      if (!map.has(name)) map.set(name, new Set());
      map.get(name).add(fn);
      return () => map.get(name)?.delete(fn);
    },
    off(name, fn) { map.get(name)?.delete(fn); },
    emit(name, payload) {
      const set = map.get(name);
      if (!set) return;
      // ISOLATED. Listeners run bare here until now, so one throw in (say) the HUD aborted the
      // rest of the emit — and since player:state is emitted every frame from inside step(),
      // a single bad frame in one subscriber took down combat, audio and the run loop with it.
      // A UI module failing should cost its own readout and nothing else, which is the same
      // principle boot already applies when CONSTRUCTING them.
      for (const fn of [...set]) {
        try { fn(payload); } catch (e) { console.error(`[bus] listener for "${name}" threw —`, e); }
      }
    },
  };
}