# Scaffold Reducer

> Scaffold a pure state reducer following the (state, action) => state pattern used in packages/game

- Skill: `dkolba/scaffold-reducer` (Agent Skill)
- Install (CLI): `npx skillmds@latest add dkolba/scaffold-reducer`
- Raw SKILL.md: https://api.skillmd.com/api/skills/dkolba/scaffold-reducer/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: dkolba (https://skillmd.com/u/dkolba)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/dkolba/scaffold-reducer

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# Scaffold Reducer

Use when adding a new state transition to the game simulation.

## Pattern

Every state transition is a pure function:

```ts
const myReducer = (state: GameState, action: MyAction): GameState => {
  switch (action.type) {
    case "CASE_A":
      return { ...state /* … */ };
    case "CASE_B":
      return { ...state /* … */ };
    default: {
      const _exhaustive: never = action;
      return _exhaustive;
    }
  }
};
```

## Rules

- **No mutation** — always return a new object via spread or structural sharing.
- **No side effects** — no logging, no DOM, no network, no randomness (pass PRNG seed through state if needed).
- **No classes, no `this`**.
- Co-locate the file in `packages/game/lib/state/` as `<verb>-<noun>.ts` (e.g. `update-player.ts`, `update-enemies.ts`).
- Export as a named or default function — follow the existing file's convention.

## Steps

1. Create `packages/game/lib/state/<verb>-<noun>.ts` with the reducer body.
2. Create `packages/game/lib/state/<verb>-<noun>.test.ts` with:
   - At least one test per `case` branch.
3. Import `GameState` from `../core/types.ts` using `import type`.
4. Wire the reducer into `packages/game/lib/state/advance-game-state.ts` if it participates in the main tick.

## Composing into the pipeline

The root deterministic step in `advance-game-state.ts` chains reducers:

```
input normalisation → updatePlayer → updateEnemies → … → frameIndex increment
```

Add new reducers to that chain inside `advanceGameState`.