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packages/media/audio/audio-controllers/src/base/connection-fsm.md

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+# connection-fsm
+
+## Purpose
+
+Builds the controller connection-state machine consumed by every
+`MidiControllerPlugin`. The FSM models the lifecycle of a hardware
+controller's MIDI access: a fresh controller starts `disconnected`, moves
+through `connecting`, settles in `connected`, and falls into `error` on
+any access failure or rejected transition. Because the FSM is driven from
+both the adapter (real connect/disconnect) and from `handleStateChange`
+(hot-plug events from `navigator.requestMIDIAccess().onstatechange`-shaped
+sources), centralising the rules here keeps every device subclass
+ignorant of state-machine details.
+
+## Features
+
+- `createConnectionMachine()` returns an `@suaveplan/fsm` interpreter with
+  the four canonical states pre-wired.
+- `sendOrThrow(machine, event)` converts the FSM's underlying
+  `INVALID_TRANSITION` into a domain-stable
+  `AudioControllersError("AUDIO_CONTROLLERS.INVALID_STATE")` with the
+  offending `from` and `event` carried in `context`.
+- Transition table mirrors the spec exactly: `DISCONNECT`, `CONNECT`,
+  `CONNECTED`, `ERROR`, `RESET`.
+
+## Usage: Basic
+
+```ts
+import { createConnectionMachine, sendOrThrow } from "@suaveplan/audio-controllers";
+
+const m = createConnectionMachine();
+sendOrThrow(m, "CONNECT");
+sendOrThrow(m, "CONNECTED");
+```
+
+## Usage: Advanced
+
+```ts
+const m = createConnectionMachine();
+m.onTransition((state) => log.debug({ value: state.value }, "controller-fsm"));
+try {
+  sendOrThrow(m, "CONNECTED"); // illegal from `disconnected`
+} catch (err) {
+  if (err instanceof AudioControllersError) {
+    expect(err.code).toBe(AUDIO_CONTROLLERS_CODES.INVALID_STATE);
+  }
+}
+```
+
+## API Reference
+
+- `createConnectionMachine()` → `FSMInterpreter<{ lastError?: string }>`.
+- `sendOrThrow(machine, event)` → `void`.
+- `ConnectionMachine` — re-exported interpreter type alias.
+
+## Implementation Notes
+
+A local `TRANSITIONS` table is consulted before delegating to the FSM's
+own dispatcher so the package surface throws an `AUDIO_CONTROLLERS.*`
+error rather than the raw `FSM.INVALID_TRANSITION` code. The state
+graph deliberately omits transitions out of `error` other than `RESET`
+to force callers to explicitly acknowledge recovery — accidental
+re-`CONNECT` from `error` would mask the original failure.