# Systems

*Core and viewer systems architecture.*

Applies to: `packages/core/src/systems/**`, `packages/viewer/src/systems/**`.

Systems own business logic, geometry generation, and constraints. They run in the Three.js frame loop and are never rendered directly.

> **For registry-driven kinds, prefer no per-kind system.** If your kind's only job is "rebuild geometry on dirty", set `def.geometry` and let the framework's `<GeometrySystem>` handle the rebuild loop. Per-kind systems remain for *extra* responsibilities — animations, cross-kind dirty cascades, named-mesh material poking. See [node-definitions.md](node-definitions.md).

## Two Kinds of Systems

### Core Systems — `packages/core/src/systems/`

Pure logic: no rendering, no Three.js objects. They read nodes from `useScene`, compute derived values (geometry, constraints), and write results back.

| System | Responsibility |
|---|---|
| `WallSystem` | Wall mitering, corner joints |
| `SlabSystem` | Polygon-based floor/roof generation |
| `CeilingSystem` | Polygon-based ceiling generation |
| `RoofSystem` | Pitched roof shape |
| `DoorSystem` | Placement constraints on walls |
| `WindowSystem` | Placement constraints on walls |
| `ItemSystem` | Item transforms, collision |

### Viewer Systems — `packages/viewer/src/systems/`

Access Three.js objects (via `useRegistry`) and manage rendering side-effects.

| System | Responsibility |
|---|---|
| `LevelSystem` | Stacked / exploded / solo / manual level positions |
| `WallCutout` | Cuts door/window holes in wall geometry |
| `ZoneSystem` | Zone display and label placement |
| `InteractiveSystem` | Item toggles and sliders in the scene |
| `GuideSystem` | Temporary helper geometry |
| `ScanSystem` | Point cloud rendering |

## Pattern

Systems are React components that render nothing (`return null`) and use `useFrame` for per-frame logic.

```tsx
// packages/core/src/systems/my-system.tsx
import { useFrame } from '@react-three/fiber'
import { useScene } from '../store/use-scene'

export function MySystem() {
  const nodes = useScene(s => s.nodes)

  useFrame(() => {
    // compute and write back derived state
  })

  return null
}
```

Core and viewer systems are mounted inside `<Viewer>` alongside renderers. See `packages/viewer/src/components/viewer/index.tsx` for the mount order.

**Systems are a customization point.** Any consumer of `<Viewer>` — the editor app, an embed, a read-only preview — can inject its own systems as children. This is how editor-specific behaviour (space detection, tool feedback) is added without touching the viewer package.

## Rules

- **Core systems must not import Three.js** — they work with plain data.
- **Viewer systems must not contain business logic** — delegate to core if the rule is domain-level.
- **Never duplicate logic** between a system and a renderer — if the renderer needs it, the system should compute and store it, and the renderer reads the result.
- Systems should be **idempotent**: given the same nodes, they produce the same output.
- Mark nodes as `dirty` in the scene store to signal that a system should re-run. Avoid running expensive logic every frame without a dirty check.

## Adding a New System

1. Decide the scope:
   - **Domain logic** → `packages/core/src/systems/`
   - **Viewer rendering side-effect** → `packages/viewer/src/systems/` — mount in `packages/viewer/src/components/viewer/index.tsx`
   - **Editor-specific or integration-specific** → keep it in the consuming app (e.g. `apps/editor/components/systems/`) and inject it as a child of `<Viewer>`

2. Create `<name>-system.tsx` in the appropriate directory.

3. Mount it in the right place:
   - Viewer-internal systems go in `packages/viewer/src/components/viewer/index.tsx`
   - App-specific systems are injected as children from outside:
     ```tsx
     // apps/editor — editor injects its own systems without modifying the viewer
     <Viewer>
       <MyEditorSystem />
       <ToolManager />
     </Viewer>
     ```

4. **Mount order matters.** Most viewer systems run *after* renderers in the JSX tree — they consume `sceneRegistry` data that renderers populate on mount. Only place a system before renderers if it explicitly does not read the registry.
