tts-workshop/docs/bgm/engine.md

# bgm-engine

The message layer that drives [bgm](./format.md) board games, built into
[`@tts/engine`](../architecture.md). It unifies the two halves of scripted
interaction — *declaring* what should happen and *executing* it — into a single
reactive loop: **messages** flow through a **queue**, and **handlers** react to
them.

This doc covers the message model (what flows), the queue and its tick (how it
flows), and the handlers (who reacts). Command *execution* — the async
lifecycle, run contexts, and tap interaction — is specified in
[`commands.md`](./commands.md); this doc is the layer above it.

## package split

The engine is a **pure** package: the message bus, queue, tick, trigger
registry, and the handler runner. It has no r3f, no React, and no store, so it
is node-testable in isolation (mirroring `@tts/extract`'s isomorphic, zero-dep
style). It defines the contract — `Message`, the handler registry, `Trigger`,
`Orchestrator`, and `RunContext`.

[`@tts/tabletop`](./tabletop.md) is one consumer of that contract: it
registers the built-in commands (`move`, `focus`, `caption`, `enableSurface`,
...) that mutate the tabletop store and drive the render layer. The engine
never imports tabletop; tabletop depends on the engine for the message types
and the handler registry. A headless sim or bot harness can consume the engine
without the render layer.

## 1. messages

A **message** is the unit of communication. It is both an *event* (something
happened) and an *intent* (something should happen) — the two are the same
thing. A message is dispatched to the handlers registered for its `type`; a
handler may emit new messages in response.

Messages are a **discriminated union** on `type`. The engine defines the
generic shapes; the host's concrete union extends them with its own command
types.

```ts
interface TapMessage {
  type: 'tap';
  data: TapEvent; // part, position, trigger
}

interface CommandMessage<Name extends string, Args> {
  type: Name;
  data: Args;
}
```

A message is identified by `type`, matching the format's `type#id`
convention. A `move` message both *runs* the move command and is *observable*
as an event; the command's completion is itself a message, which is what
triggers match and orchestrators await.

The loop is just: **message → handler → message**. Handlers consume messages
and emit new ones; the queue serializes them.

## 2. the queue and ticking

Messages are not processed inline. They are **enqueued** and handled on the
next **tick**. This kills reentrancy (a handler cannot cause unbounded
recursion), gives a natural debounce, and makes the whole system a
deterministic frame.

### tick contract

The engine is pure — it has no render loop and must stay node-testable. It
exposes `tick()`, and the host calls it:

- In `@tts/tabletop`, a `useFrame` drives `tick()`.
- In tests, `tick()` is called manually.

The engine never assumes a render loop.

### drain semantics

- **Snapshot-and-drain.** At `tick()`, snapshot the queue and process it.
  Messages emitted *during* the drain go to the *next* tick. This guarantees
  no reentrancy within a drain and makes ordering deterministic.
- **FIFO within a tick.** Simple and predictable.
- **One tick drains the whole snapshot** (not one message per tick), so a
  burst of messages all resolve in one frame.

### awaiting

A handler suspends on `await ctx.wait(pred)` and resumes when a matching
message is processed during a drain. Its own emissions go to the next tick, so
it cannot re-enter itself.

## 3. message types

### interaction messages

Interaction is the player's input, reported to the engine as messages. Only
tap interaction is supported (see `commands.md` §4).

```ts
interface TapMessage {
  type: 'tap';
  data: TapEvent;
}
```

A tap on a part is reported with the nearest trigger point (or `null` on a
miss). The handler decides how to react — resolve, reject with a "wrong spot"
shake, or ignore. The runtime stays dumb; the handler owns the UX.

### command messages

A command message names a command to run. Its handler is the command
implementation; its completion is emitted as a result message. A command's
result is a **discriminated union on the type suffix**, carrying the terminal
state:

```ts
type CommandResult<Name extends string, R = void> =
  | { type: `${Name}:done`; data: R }
  | { type: `${Name}:cancel` }
  | { type: `${Name}:error`; error: Error };

// e.g. move:done { data: MoveResult } | move:cancel | move:error
```

The command-id-as-key convention means a message both *is* the intent and
*observes* the result. `move:done`, `focus:done`, etc. are the messages that
triggers match and orchestrators await. A cancelled command emits `:cancel`, an
errored one `:error` — a trigger matching `move:done` does not fire on a
cancel.

## 4. handlers

There are three kinds of handler. All three consume messages and emit
messages; they differ in how they're declared and how they run.

| Handler | Declared | Runs | Purpose |
| --- | --- | --- | --- |
| **Trigger** | data (yaml) | synchronously on match | declarative reactive glue |
| **Orchestrator** | code (`main.ts`) | async, awaits | imperative flow |
| **Command** | code (built-in) | async, on its message | atomic execution |

### triggers — declarative reactive glue

A trigger matches a message by `type` and named params, and emits messages in
response. It is declared as data, keyed by `role+type+id` like other defs, and
collision-checked the same way.

```yaml
role: trigger
type: tap
id: draw
match:
  part: carcassonne:tile#a
  trigger: draw
emit:
  - move: { part: carcassonne:tile#a, to: /grid/5/5 }
  - focus: { path: /grid/5/5 }
```

- `type` selects the message kind; `match` binds named params from the
  payload (like a route's candidates).
- `emit` uses the command-id-as-key convention.
- Multiple triggers can match the same message — both fire, which is usually
  what you want.
- A trigger is a **pre-registered handler**: it's a message consumer that
  emits commands. An orchestrator can do the same thing imperatively with
  `ctx.on(...)`.

### orchestrators — imperative async flow

An orchestrator is the code counterpart to a trigger: an async function that
emits messages and awaits matching ones. It is a proper TS module, declared
per folder as `main.ts` — unique per folder like `package.yaml`.

```ts
// main.ts
export default async function main(ctx: RunContext): Promise<void> {
  await ctx.focus({ path: '/deck' });
  await ctx.caption({ text: 'Draw a tile' });
  const tap = await ctx.wait((m) => m.type === 'tap');
  await ctx.move({ part: tap.data.part, to: '/grid/5/5' });
}
```

- **`main.ts` is executable code, loaded by the host, not the engine.** The
  engine defines the contract (the orchestrator type and runner); the host
  dynamically imports `main.ts` and hands the exported orchestrator to the
  engine. The engine never imports user code.
- **A default export async function.** `main.ts` exports a single async
  function as its default export, taking the `RunContext`. It is the folder's
  orchestrator.
- **Trigger control lives here.** The orchestrator toggles triggers at runtime
  by their `type#id`:

  ```ts
  ctx.enableTrigger('tap', 'draw');
  ctx.disableTrigger('tap', 'draw');
  ```

  Declaration is data; activation is code. The orchestrator owns game-flow
  logic ("no more placements this turn" → disable the trigger), while the
  trigger stays a dumb declarative mapping.

### commands — atomic execution

A command is an async function, the same shape as an orchestrator. It takes a
`RunContext` (with its `args`), returns its result, and throws on error. The
engine wraps it: it builds the context from the message, runs the function, and
emits the result message — `:done` on resolve, `:cancel` on abort, `:error` on
throw.

```ts
type Command<Args, Result> = (ctx: RunContext & { args: Args }) => Promise<Result>;
```

Commands are the **single mutation path** — the only way state changes.
Triggers and orchestrators never mutate state directly; they emit command
messages, and the command handlers execute them.

## 5. run context

Every handler runs against a `RunContext`, the handle to everything it can
affect and the unit of cancellation.

```ts
interface RunContext {
  signal: AbortSignal; // cancellation: superseded, skipped, surface disabled
  emit(msg: Message): void;
  wait(pred: (m: Message) => boolean): Promise<Message>; // rejects on abort
  enableTrigger(type: string, id?: string): void;
  disableTrigger(type: string, id?: string): void;
}
```

- **Cancellation** is an `AbortSignal`. A superseded command or a disabled
  surface aborts the signal; a `wait` rejects on abort, and the command's
  `:cancel` result is emitted.
- **Errors** are thrown. A command that throws emits `:error`; an orchestrator
  that throws surfaces loudly.
- Commands and orchestrators are the same shape: an async function taking the
  context. An orchestrator is a command that returns `void` and is never
  awaited by a parent.

## 6. solo-only

This design is **solo-only** — no multiplayer. Other players either don't
exist or are automated with an automata. An automata is just another message
consumer that emits commands: a stateful trigger or orchestrator. The engine
doesn't care whether a `tap` message came from a human or a bot decision —
same queue, same handlers. Solo-only simplifies the design: no network, no
sync, no authoritative-server concerns. "Other players" are just more message
producers.

## Open decisions

- **`main.ts` loading.** The host dynamically imports `main.ts`; the exact
  loading boundary (Vite dynamic import, error handling, HMR) is deferred to
  implementation. The engine defines the orchestrator type; the host loads the
  module and hands the exported orchestrator to the engine.