# Engine Mixer Helpers

<!-- begin:auto-docs -->
_(no submodules)_
<!-- end:auto-docs -->

## Purpose

The `mixer.ts` module bridges `TimelineEngine` mixer public methods to the pure `mixer.ts` helpers. It manages undo pushes and adapter forwarding for every mixer mutation, keeping the engine class from growing too large.

## Features

- `engineSetMasterVolume` updates master volume and pushes undo
- `engineSetTrackVolume` delegates to pure helper and forwards to adapter
- `engineSetTrackMute` toggles mute and emits via adapter
- `engineSetTrackSolo` toggles solo and emits via adapter
- `engineSetTrackPan` clamps pan and emits via adapter

## Usage

These helpers are used internally by `TimelineEngine`. Consumers call `engine.setTrackVolume(id, 0.8)` etc.

## API Reference

| Export | Signature | Description |
|--------|-----------|-------------|
| `engineSetMasterVolume` | `(deps, volume) => void` | Master volume |
| `engineSetTrackVolume` | `(deps, trackId, volume) => void` | Track volume |
| `engineSetTrackMute` | `(deps, trackId, muted) => void` | Track mute |
| `engineSetTrackSolo` | `(deps, trackId, soloed) => void` | Track solo |
| `engineSetTrackPan` | `(deps, trackId, pan) => void` | Track pan |

## Implementation Notes

The `MixerDeps` interface isolates the helpers from the engine class. Every track mutation goes through `updateTrackInPlace`, which pushes an undo command automatically. The master volume operation is special-cased because it does not live on a track; it pushes its own undo command directly. All other mixer state lives on individual tracks and is therefore covered by the standard track-update undo path. This symmetry makes it easy to reason about what changes are reversible.