# Phase 4: Runtime Integration

**Status:** ✅ Done
**Parent plan:** [PLAN.md](./PLAN.md)
**Domain:** backend

## Objective
Wire the audio filter pipeline into the runtime audio streaming system so that ProcessedAudioSources actually apply their filter chains to live audio data.

## Tasks

- [x] Task 1: Create filter pipeline executor in `core/audio/filters/pipeline.py`
  - `AudioFilterPipeline` class:
    - `__init__(filter_instances: List[FilterInstance], registry: AudioFilterRegistry)`
    - Instantiates all filters from FilterInstance specs
    - `process(analysis: AudioAnalysis) -> AudioAnalysis` — runs analysis through all filters in order
    - `reset()` — resets all stateful filters
    - `close()` — cleanup resources
  - Handles stateful filter lifecycle (create on init, reset on demand, close on cleanup)
- [x] Task 2: Update `AudioColorStripStream` in `core/processing/audio_stream.py`
  - On construction: if source is ProcessedAudioSource, resolve the full chain:
    - Walk to CaptureAudioSource for device info
    - Collect all AudioProcessingTemplates along the chain
    - Resolve all filter instances (with template expansion)
    - Create AudioFilterPipeline
  - In render loop: after getting AudioAnalysis from ManagedAudioStream, run it through the filter pipeline before visualization
  - Remove old inline channel selection and band filtering code (now handled by filters)
  - On stop: close the filter pipeline
- [x] Task 3: Update `AudioValueStream` in `core/processing/value_stream.py`
  - Same pattern: resolve ProcessedAudioSource chain, create filter pipeline, apply in get_value()
  - Remove old inline channel/band handling
- [x] Task 4: Hot-update support for filter templates
  - When an AudioProcessingTemplate is updated, running streams that use it should re-resolve their filter pipeline
  - Listen for template update events (or implement a refresh mechanism)
  - Re-create AudioFilterPipeline with updated filter instances
  - Reset stateful filter state on pipeline refresh
- [x] Task 5: Update WebSocket test endpoint in `api/routes/audio_sources.py`
  - For ProcessedAudioSource: resolve chain, create pipeline, apply filters to test stream data
  - Return filtered analysis in real-time over WebSocket
- [x] Task 6: Update any code that calls `AudioSourceStore.resolve_audio_source()` to handle the new return shape

## Files to Modify/Create
- `core/audio/filters/pipeline.py` — **create** — AudioFilterPipeline
- `core/processing/audio_stream.py` — **modify** — integrate filter pipeline
- `core/processing/value_stream.py` — **modify** — integrate filter pipeline
- `api/routes/audio_sources.py` — **modify** — update WebSocket test
- Any other consumers of `resolve_audio_source()` — **modify**

## Acceptance Criteria
- ProcessedAudioSource chains are resolved and filter pipelines created at stream start
- AudioAnalysis passes through the filter chain before visualization/value extraction
- Stateful filters maintain correct state across frames
- Hot-update of templates refreshes running filter pipelines
- WebSocket test endpoint works with processed sources
- Old inline channel/band code removed from stream classes

## Notes
- ⚠️ Temporary breakage: Removing inline channel/band code from AudioColorStripStream breaks existing MonoAudioSource/BandExtractAudioSource flows — but those types were already removed in Phase 3.
- Filter pipeline must be thread-safe: AudioColorStripStream and AudioValueStream run in background threads.
- For hot-update: consider using an event callback from the template store rather than polling.
- The filter pipeline should produce a new AudioAnalysis each time (immutability), not mutate the shared snapshot from ManagedAudioStream.

## Review Checklist
- [x] All tasks completed
- [x] Code follows project conventions
- [x] No unintended side effects
- [ ] Build passes
- [ ] Tests pass (new + existing)

## Handoff to Next Phase

### What was built
- `AudioFilterPipeline` class in `core/audio/filters/pipeline.py` — thread-safe pipeline that instantiates, processes, resets, and closes audio filters
- `build_pipeline_from_template_ids()` helper — resolves template IDs to FilterInstance lists and creates a pipeline
- `AudioColorStripStream` now builds and applies filter pipeline: `_rebuild_filter_pipeline()` called on construction and source update; `_apply_filters()` replaces `_pick_channel()` in render loop; pipeline closed on stop
- `AudioValueStream` now builds and applies filter pipeline: `_rebuild_filter_pipeline()` called on construction; filters applied in `_extract_raw()` before scalar extraction; pipeline closed on stop
- Hot-update: `ProcessorManager.refresh_audio_filter_pipelines(template_id)` dispatches to both CSS and value stream managers; called from audio processing template update/delete routes
- WebSocket test endpoint creates a filter pipeline from the resolved template chain and applies it to analysis before sending
- Dependency injection: `audio_processing_template_store` threaded through `ProcessorDependencies` -> `ProcessorManager` -> `ColorStripStreamManager` / `ValueStreamManager` -> `AudioColorStripStream` / `AudioValueStream`

### What Phase 5 needs to know
- The backend is fully wired: creating a ProcessedAudioSource that references templates with channel_extract, band_extract, gain, etc. filters will apply those filters to live audio data
- The WebSocket test endpoint shows filtered analysis in real-time
- Frontend needs to provide UI for creating/editing AudioProcessingTemplates and ProcessedAudioSources
- Filter pipeline is per-stream-instance (not shared) — stateful filters maintain independent state

### Temporary breakages resolved
- `_pick_channel()` removed from AudioColorStripStream — replaced by `_apply_filters()` which uses the filter pipeline
- Channel/band handling in AudioValueStream now goes through filter pipeline
- All "Phase 4 will wire..." comments resolved

### Known deviations from plan
- `AudioFilterPipeline.__init__` takes `List[FilterInstance]` only (not `AudioFilterRegistry` — uses the class-level registry directly via `AudioFilterRegistry.create_instance()`)
- Hot-update uses explicit method calls from the route handler rather than an event-listener pattern — simpler and avoids the need for a subscription system