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openspec/changes/enterprise-gaming-game-loop/proposal.md

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+# OpenSpec Change: enterprise-gaming-game-loop
+
+- **Change ID**: enterprise-gaming-game-loop
+- **Package**: `@suaveplan/game-loop`
+- **Category**: gaming/engine
+- **Target path**: `gaming/engine/game-loop`
+- **Status**: PROPOSED
+- **Date**: 2026-04-24
+- **Reference spec**: `/mnt/s/web/suaveplan/enterprise/game-loop.md`
+
+---
+
+## Summary
+
+Deliver `@suaveplan/game-loop` on the genesis branch from scratch at enterprise quality. The package provides a fixed-timestep game loop with tick scheduling — the heartbeat that drives `@suaveplan/ecs` world updates and all time-sensitive gameplay systems. The loop implements the classic "fix your timestep" accumulator pattern: unlimited render interpolation alpha with a capped fixed physics step, lag compensation, frame-time smoothing, and panic detection. Genesis adds structured errors, injectable clock and AbortSignal cancellation, OTel frame-time histograms, and deterministic test harness via FakeClock.
+
+---
+
+## Current-state audit (genesis)
+
+Package does not exist on genesis yet — fresh build.
+
+---
+
+## Available workspace deps on genesis
+
+| Package | Role |
+|---|---|
+| `@suaveplan/error` (`workspace:^`) | `GameLoopError`; `GAME_LOOP.*` error codes |
+| `@suaveplan/types` (`workspace:^`) | `Clock` injectable |
+| `@suaveplan/events` (`workspace:^`) | `loop:tick`, `loop:panic`, `loop:stop` events |
+| `zod` (`catalog:forms`) | `GameLoopOptions` schema |
+| `@opentelemetry/api` (`catalog:observability`) | Frame-time histogram, tick counter |
+
+---
+
+## Public API surface
+
+```ts
+function createGameLoop(options: GameLoopOptions): GameLoop
+
+interface GameLoopOptions {
+  fixedStep: number           // ms per physics tick, e.g. 16.666
+  maxFrameTime?: number       // ms cap to prevent spiral of death, default 250
+  onTick: (dt: number) => void
+  onRender?: (alpha: number) => void
+  clock?: Clock
+  logger?: LoggerLike
+}
+
+interface GameLoop {
+  start(signal?: AbortSignal): void
+  stop(): void
+  readonly isRunning: boolean
+  readonly tickCount: number
+  readonly fps: number           // rolling average over last 60 frames
+  readonly frameBudgetMs: number
+}
+
+// Scheduler — attach one-shot or recurring callbacks to loop ticks
+function createTickScheduler(loop: GameLoop): TickScheduler
+
+interface TickScheduler {
+  scheduleOnce(cb: () => void, afterTicks?: number): ScheduledHandle
+  scheduleEvery(cb: () => void, everyTicks: number): ScheduledHandle
+  cancel(handle: ScheduledHandle): void
+}
+
+type ScheduledHandle = number & { readonly __brand: 'ScheduledHandle' }
+```
+
+### Error codes
+
+```ts
+type GameLoopErrorCode =
+  | 'GAME_LOOP_ALREADY_RUNNING'
+  | 'GAME_LOOP_NOT_RUNNING'
+  | 'GAME_LOOP_PANIC'
+  | 'GAME_LOOP_INVALID_STEP'
+```
+
+---
+
+## New additions for genesis
+
+1. **Accumulator + interpolation alpha** — `alpha = accumulator / fixedStep` passed to `onRender` for smooth rendering between physics steps.
+2. **Panic detection** — if `accumulator > maxFrameTime * 3` the loop fires `loop:panic` event and clamps accumulator.
+3. **FakeClock integration** — `clock` option accepts `FakeClock` from `@suaveplan/testing`; tick advances deterministically in tests.
+4. **OTel** — `game_loop.frame_time_ms` histogram, `game_loop.tick_total` counter, `game_loop.fps` gauge.
+5. **AbortSignal** — `start(signal)` — loop exits cleanly when signal fires.
+
+---
+
+## File layout
+
+```text
+gaming/engine/game-loop/
+  package.json
+  tsconfig.json
+  vitest.config.ts
+  src/
+    index.ts
+    error-codes.ts
+    constants/
+      index.ts
+    types/
+      common.ts
+      common.md
+    core/
+      game-loop/
+        game-loop.ts
+        game-loop.md
+        game-loop.types.ts
+        game-loop.test.ts
+        index.ts
+      tick-scheduler/
+        tick-scheduler.ts
+        tick-scheduler.md
+        tick-scheduler.types.ts
+        tick-scheduler.test.ts
+        index.ts
+    utils/
+      frame-stats.ts
+      frame-stats.md
+      frame-stats.test.ts
+  tests/
+    integration.test.ts
+```
+
+---
+
+## Impact
+
+**Downstream**: `@suaveplan/ecs` world.tick() is driven by this loop; `@suaveplan/r3f-*` rendering uses interpolation alpha. Prerequisites: `@suaveplan/error`, `@suaveplan/types`, `@suaveplan/events`.