# bgm-tabletop — Implementation Plan / Status

> **Scope:** A standalone r3f component library that renders [bgm](./bgm-format.md)
> board games: a state store, surface mounting, part placement with stacking,
> and per-part meshes. Design: [`bgm-tabletop.md`](./bgm-tabletop.md).
> **Status:** planning — no code yet.

## Goal

A library (new `packages/tabletop`) that takes a bgm package and renders it as
an interactive 3D table: enabled surfaces mounted in world/HUD space, parts
placed on their routes, stacked per the format's stacking strategy. The web
app's bgm inspector routes are one consumer; the library must not depend on the
web app.

## Stack

`react`, `react-router` (types only), `tailwind` (styles only), `r3f`
(`@react-three/fiber`), `drei`, `postprocessing`, `zustand`, `three`,
`@tts/bgm` (types), `@tts/mesh` (geometry).

## Package layout

```
packages/tabletop/
  package.json            # @tts/tabletop
  tsconfig.json
  vitest.config.ts
  src/
    index.ts              # public exports
    state.ts              # zustand store + derived render state
    setup.ts              # SetupLoader: seed state from a setup
    mount.ts              # resolve surface mount tree (table/hud/child)
    stacking.ts           # useStacking hook
    placement.ts          # PartPlacement
    partView.tsx          # PartView: mesh from a part definition
    surfaces/
      WorldSurfaceView.tsx
      HudSurfaceView.tsx
    *.test.ts             # colocated unit tests
```

## Work items

### 1. Package scaffold

- New `packages/tabletop` workspace package (`pnpm-workspace.yaml` already
  globs `packages/*`).
- Deps: `@tts/bgm`, `@tts/mesh`, `three`, `@react-three/fiber`, `@react-three/drei`,
  `@react-three/postprocessing`, `zustand`. Dev: `vitest`, `typescript`,
  `@types/three`.
- `tsconfig.json` mirroring `packages/bgm`'s (strict, ESM, `dist` output).

### 2. Part meshes + export + web integration

First deliverable: `PartView` renders a single part's mesh from its definition,
reusing `@tts/mesh` geometry (not the web app's viewers). This is the smallest
useful slice and unblocks the web app's part inspection route immediately.

- `PartView` (`partView.tsx`): creates a mesh from a `Part` definition:
  - `size` → world dimensions; `fillet` → corner radius.
  - `face`/`faceCrop`/`back`/`backCrop` → textures (drei `useTexture`), sprite
    UVs from `faceCrop`/`backCrop` (a `[col,row,cols,rows]` grid cell).
  - `shape` → traced silhouette (via the proxy `/trace`, like the web token
    viewer) or a fallback rect/rounded-rect.
  - `extrudeShapeParts` from `@tts/mesh` for the mesh.
- Shared geometry/material caching (module-level `Map`s) so repeated parts
  reuse buffers, mirroring the web viewers' `sharedResources`.
- Export `PartView` from `index.ts`.
- **Web integration**: replace the web app's part inspection route
  (`/bgm/:id/parts/:type/:part`) to render `PartView` from the library, proving
  it end-to-end.

### 3. State store (`state.ts`)

Source-of-truth game state per `bgm-tabletop.md` §2:

```ts
interface GameState {
  surfaces: Record<string, boolean>; // enabled per surface id
  paths: Record<string, string[]>;   // path -> part list
}
```

- A zustand store holding `GameState`.
- **Derived render state**: `game state + surface routes => map of piece id to
  `{ surface, route, candidate, index, stackSize }``, per enabled surface.
  Computed with a selector/memo so the render list is stable.
- **Assumption**: each piece id is unique within a path (documented in
  `bgm-tabletop.md`); the render map is keyed by piece id.

### 4. Setup seeding (`setup.ts`)

- `SetupLoader`: side-effect-only component that seeds the store from a
  `Setup` — enables its `surfaces` (or all when omitted) and places parts on
  `setup` paths.
- `setup` value expansion: a bare `type` (no id) expands to all parts of that
  type (documented in `bgm-format.md` §3; the loader doesn't do this — it's a
  game-state init concern, so it lives here).

### 5. Surface mounting (`mount.ts`)

- Resolve the surface mount tree from `Surface.mount` + `Surface.children`:
  - `kind: table` — root, world space.
  - `kind: hud` — HUD area (`mount.area`).
  - `kind: child` — mounted relative to a parent that lists it in `children`.
- `WorldSurfaceView` / `HudSurfaceView` mount an enabled surface; a disabled
  surface isn't rendered. Child surfaces mount relative to their parent's
  anchor (`x`/`y`/`rotation`).

### 6. Part placement (`placement.ts`)

- `PartPlacement`: stable per-part component that positions a part on a surface
  location from the derived render state (route anchor + candidate anchor).
- Applies the route's stacking strategy via `useStacking`.

### 7. Stacking (`stacking.ts`)

- `useStacking(route.stacking, index, stackSize)` → `{ offset, rotation }`.
- Implements the format's positioning process (`bgm-format.md` §4): step
  length from curve length / `max(steps, count-1)`, alignment (`start`/`end`/
  `center`), and `limit` (`0` all, `n` first n, `-n` last n).
- Curve length from an SVG path string (small helper; no new dep).

### 8. Public API (`index.ts`)

Export `SetupLoader`, `WorldSurfaceView`, `HudSurfaceView`, `PartPlacement`,
`PartView`, `useStacking`, and the store hooks. The web app consumes these; the
library never imports from `apps/*`.

## Reuse from `@tts/mesh`

- `extrudeShapeParts` / `extrudeShape` — front/back/walls geometry.
- `rectShape`, `roundedRectShape`, `circleShape`, `polygonShape`, `hexShape`,
  `frameShape`, `scaleShape` — shape generators for parts without a `shape`
  sprite.
- `shapeFromThree` — author shapes with the three.js path API.
- `ExtrudedGeometry` / `UVBounds` — raw typed arrays + UV framing.

The web viewers (`TokenViewer`/`CardViewer`) contain logic we'll mirror rather
than import: trace-to-shape conversion, sprite UV math, texture flipping. These
are candidates to lift into `@tts/mesh` or `@tts/tabletop` later so both
consumers share them (see Open decisions).

## Testing

- `state.ts` — derived render state: enabled surfaces, route matching,
  candidate selection, stacking index/stackSize.
- `stacking.ts` — positioning process: step length, alignment, limit.
- `setup.ts` — seeding + bare-type expansion.
- `mount.ts` — mount tree resolution (table/hud/child, children refs).
- `partView.tsx` — geometry from a part def (size/fillet/crop), sprite UVs.
- A real `vite build` integration test (mirroring `packages/bgm/src/vite.test.ts`)
  proving the library bundles against a fixture package.

## Validation

- `pnpm --filter @tts/tabletop build` / `typecheck` / `test`.
- Root `pnpm test` stays green.
- `pnpm --filter @tts/web build` — the part inspection route renders `PartView`
  from the library (work item 2), proving it end-to-end.

## Open decisions (defaults in bold)

- **Where the trace/sprite helpers live** — **lift into `@tts/mesh`** (shared
  by web viewers + tabletop) vs duplicate in `@tts/tabletop`. Lifting is
  cleaner but touches the web viewers; decide when `PartView` (work item 2)
  needs them.
- **HUD rendering** — **drei `Html`/orthographic overlay** vs a second
  `Canvas`. Default to an overlay so world + HUD share one scene.
- **Curve length** — **small internal SVG-path length helper** vs a dependency
  (e.g. `svg-path-properties`). Prefer the helper to avoid a dep.