scaffold-reducer

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

Scaffold Reducer

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

Pattern

Every state transition is a pure function:

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.