# Implementation Plan: Advance Turn

**Branch**: `001-advance-turn` | **Date**: 2026-03-03 | **Spec**: [spec.md](./spec.md)
**Input**: Feature specification from `/specs/001-advance-turn/spec.md`

## Summary

Implement the AdvanceTurn domain operation as a pure function that
transitions an Encounter to the next combatant, wrapping rounds and
emitting TurnAdvanced / RoundAdvanced domain events. Stand up the
pnpm monorepo skeleton, Biome tooling, and Vitest test harness so
that all constitution merge-gate requirements are satisfied from the
first commit.

## Technical Context

**Node**: 22 LTS (pinned via `.nvmrc`)
**Language/Version**: TypeScript 5.8 (strict mode)
**Primary Dependencies**: React 19 (pin to major; minor upgrades
allowed), Vite 6.2
**Storage**: In-memory only (MVP baseline)
**Testing**: Vitest 3.0
**Lint/Format**: Biome 2.0.0 (exact version, single tool — no
Prettier, no ESLint)
**Package Manager**: pnpm 10.6 (pinned via `packageManager` field
in root `package.json`)
**Target Platform**: Static web app (modern browsers)
**Project Type**: Monorepo — library packages + web app
**Performance Goals**: N/A (walking skeleton)
**Constraints**: Domain package must have zero React/Vite imports
**Scale/Scope**: Single feature, ~5 source files, ~1 test file

## Constitution Check

| Principle | Status | Notes |
|-----------|--------|-------|
| I. Deterministic Domain Core | PASS | `advanceTurn` is a pure function; no I/O, randomness, or clocks |
| II. Layered Architecture | PASS | `packages/domain` → `packages/application` → `apps/web`; strict dependency direction enforced by automated import check |
| III. Agent Boundary | N/A | Agent layer out of scope for this feature |
| IV. Clarification-First | PASS | No ambiguous decisions remain; spec fully clarified |
| V. Escalation Gates | PASS | Scope strictly limited to spec; out-of-scope items listed |
| VI. MVP Baseline Language | PASS | No permanent bans; "MVP baseline does not include" used |
| VII. No Gameplay Rules | PASS | No gameplay mechanics in plan or constitution |
| Merge Gate | PASS | `pnpm check` script runs format, lint, typecheck, test |

## Project Structure

### Documentation (this feature)

```text
specs/001-advance-turn/
├── spec.md
└── plan.md
```

### Source Code (repository root)

```text
packages/
├── domain/
│   ├── package.json
│   ├── tsconfig.json
│   └── src/
│       ├── types.ts          # CombatantId, Combatant, Encounter
│       ├── events.ts         # TurnAdvanced, RoundAdvanced, DomainEvent
│       ├── advance-turn.ts   # advanceTurn pure function
│       └── index.ts          # public barrel export
├── application/
│   ├── package.json
│   ├── tsconfig.json
│   └── src/
│       ├── ports.ts          # EncounterStore port interface
│       ├── advance-turn-use-case.ts
│       └── index.ts
apps/
└── web/
    ├── package.json
    ├── tsconfig.json
    ├── vite.config.ts
    ├── index.html
    └── src/
        ├── main.tsx
        ├── App.tsx
        └── hooks/
            └── use-encounter.ts

# Testing (co-located with domain package)
packages/domain/
└── src/
    └── __tests__/
        └── advance-turn.test.ts

# Root config
├── .nvmrc                    # pins Node 22
├── pnpm-workspace.yaml
├── biome.json
├── tsconfig.base.json
└── package.json              # packageManager field pins pnpm; root scripts
```

**Structure Decision**: pnpm workspace monorepo with two packages
(`domain`, `application`) and one app (`web`). Domain is
framework-agnostic TypeScript. Application imports domain only.
Web app (React + Vite) imports both.

## Tooling & Merge Gate

### Scripts (root package.json)

```jsonc
{
  "scripts": {
    "format": "biome format --write .",
    "format:check": "biome format .",
    "lint": "biome lint .",
    "lint:fix": "biome lint --write .",
    "typecheck": "tsc --build",
    "test": "vitest run",
    "test:watch": "vitest",
    "check": "biome check . && tsc --build && vitest run"
  }
}
```

`pnpm check` is the single merge gate: format + lint + typecheck +
test. The layer boundary check runs as a Vitest test (see below),
so it executes as part of `vitest run` — no separate script needed.

### Layer Boundary Enforcement

Biome does not natively support cross-package import restrictions.
A lightweight `scripts/check-layer-boundaries.mjs` script will:

1. Scan `packages/domain/src/**/*.ts` — assert zero imports from
   `@initiative/application`, `apps/`, `react`, `vite`.
2. Scan `packages/application/src/**/*.ts` — assert zero imports
   from `apps/`, `react`, `vite`.
3. Exit non-zero on violation with a clear error message.

This script is invoked by a Vitest test
(`packages/domain/src/__tests__/layer-boundaries.test.ts`) so it
runs automatically as part of `vitest run` inside `pnpm check`.
No separate `check:layer` script is needed — the layer boundary
check is guaranteed to execute on every merge-gate run.

### Biome Configuration (biome.json)

```jsonc
{
  "$schema": "https://biomejs.dev/schemas/2.0.0/schema.json",
  "organizeImports": { "enabled": true },
  "formatter": {
    "enabled": true,
    "indentStyle": "tab",
    "lineWidth": 80
  },
  "linter": {
    "enabled": true,
    "rules": {
      "recommended": true
    }
  }
}
```

### TypeScript Configuration

- `tsconfig.base.json` at root: strict mode, composite projects,
  path aliases (`@initiative/domain`, `@initiative/application`).
- Each package extends `tsconfig.base.json` with its own
  `include`/`references`.
- `apps/web/tsconfig.json` references both packages.

## Milestones

### Milestone 1: Tooling & Domain (walking skeleton)

Stand up monorepo, Biome, Vitest, TypeScript project references,
and implement the complete AdvanceTurn domain logic with all tests.

**Exit criteria**: `pnpm check` passes. All 8 acceptance scenarios
green. Layer boundary check green. No React or Vite dependencies in
`packages/domain` or `packages/application`.

### Milestone 2: Application + Minimal Web Shell

Wire up the application use case and a minimal React UI that
displays the encounter state and has a single "Next Turn" button.

**Exit criteria**: `pnpm check` passes. Clicking "Next Turn" in the
browser advances the turn with correct round wrapping. The app
builds with `vite build`.

## Task List

### Phase 1: Setup (Milestone 1)

- [X] **T001** Initialize pnpm workspace and root config
  - Create `pnpm-workspace.yaml` listing `packages/*` and `apps/*`
  - Create `.nvmrc` pinning Node 22
  - Create root `package.json` with `packageManager` field pinning
    pnpm 10.6 and scripts (check, test, lint, format, typecheck)
  - Create `biome.json` at root
  - Create `tsconfig.base.json` (strict, composite, path aliases)
  - **Acceptance**: `pnpm install` succeeds; `biome check .` runs
    without config errors

- [X] **T002** [P] Create `packages/domain` package skeleton
  - `package.json` (name: `@initiative/domain`, no dependencies)
  - `tsconfig.json` extending base, composite: true
  - Empty `src/index.ts`
  - **Acceptance**: `tsc --build packages/domain` succeeds

- [X] **T003** [P] Create `packages/application` package skeleton
  - `package.json` (name: `@initiative/application`,
    depends on `@initiative/domain`)
  - `tsconfig.json` extending base, references domain
  - Empty `src/index.ts`
  - **Acceptance**: `tsc --build packages/application` succeeds

- [X] **T004** [P] Create `apps/web` package skeleton
  - `package.json` with React, Vite, depends on both packages
  - `tsconfig.json` referencing both packages
  - `vite.config.ts` (minimal)
  - `index.html` + `src/main.tsx` + `src/App.tsx` (placeholder)
  - **Acceptance**: `pnpm --filter web dev` starts; `vite build`
    succeeds

- [X] **T005** Configure Vitest
  - Add `vitest` as root dev dependency
  - Create `vitest.config.ts` at root (workspace mode) or per
    package as needed
  - Verify `pnpm test` runs (0 tests, exits clean)
  - **Acceptance**: `pnpm test` exits 0

- [X] **T006** Create layer boundary check script
  - `scripts/check-layer-boundaries.mjs`: scans domain and
    application source for forbidden imports
  - `packages/domain/src/__tests__/layer-boundaries.test.ts`:
    wraps the script as a Vitest test
  - **Acceptance**: test passes on clean skeleton; fails if a
    forbidden import is manually added (verify, then remove)

### Phase 2: Domain Implementation (Milestone 1)

- [ ] **T007** Define domain types in `packages/domain/src/types.ts`
  - `CombatantId` (branded string or opaque type)
  - `Combatant` (carries a CombatantId)
  - `Encounter` (combatants array, activeIndex, roundNumber)
  - Factory function `createEncounter` that validates INV-1, INV-2,
    INV-3
  - **Acceptance**: types compile; `createEncounter([])` returns
    error; `createEncounter([a])` returns valid Encounter

- [ ] **T008** [P] Define domain events in
  `packages/domain/src/events.ts`
  - `TurnAdvanced { previousCombatantId, newCombatantId,
    roundNumber }`
  - `RoundAdvanced { newRoundNumber }`
  - `DomainEvent = TurnAdvanced | RoundAdvanced`
  - **Acceptance**: types compile; events are plain data (no
    classes with methods)

- [ ] **T009** Implement `advanceTurn` in
  `packages/domain/src/advance-turn.ts`
  - Signature: `(encounter: Encounter) =>
    { encounter: Encounter; events: DomainEvent[] } | DomainError`
  - Implements FR-001 through FR-005
  - Returns error for empty combatant list (INV-1)
  - Emits TurnAdvanced on every call (INV-5)
  - Emits TurnAdvanced then RoundAdvanced on wrap (event order
    contract)
  - **Acceptance**: compiles; satisfies type contract

- [ ] **T010** Write tests for all 8 acceptance scenarios +
  invariants in
  `packages/domain/src/__tests__/advance-turn.test.ts`
  - Scenarios 1–8 from spec (Given/When/Then)
  - INV-1: empty encounter rejected
  - INV-2: activeIndex always in bounds (property check across
    scenarios)
  - INV-3: roundNumber never decreases
  - INV-4: determinism — same input produces same output (call
    twice, assert deep equal)
  - INV-5: every success emits at least TurnAdvanced
  - Event ordering: on wrap, events array is
    [TurnAdvanced, RoundAdvanced] in that order
  - **Acceptance**: `pnpm test` — all tests green; `pnpm check` —
    full pipeline green

- [ ] **T011** Export public API from `packages/domain/src/index.ts`
  - Re-export types, events, `advanceTurn`, `createEncounter`
  - **Acceptance**: consuming packages can
    `import { advanceTurn } from "@initiative/domain"`

**Milestone 1 checkpoint**: `pnpm check` passes (format + lint +
typecheck + test + layer boundaries). All 8 scenarios + invariants
green.

### Phase 3: Application + Web Shell (Milestone 2)

- [ ] **T012** Define port interface in
  `packages/application/src/ports.ts`
  - `EncounterStore` port: `get(): Encounter`, `save(e: Encounter)`
  - **Acceptance**: compiles; no imports from adapters or React

- [ ] **T013** Implement `AdvanceTurnUseCase` in
  `packages/application/src/advance-turn-use-case.ts`
  - Accepts `EncounterStore` port
  - Calls `advanceTurn` from domain, saves result, returns events
  - **Acceptance**: compiles; imports only from `@initiative/domain`
    and local ports

- [ ] **T014** Export public API from
  `packages/application/src/index.ts`
  - Re-export use case and port types
  - **Acceptance**: consuming app can import from
    `@initiative/application`

- [ ] **T015** Implement `useEncounter` hook in
  `apps/web/src/hooks/use-encounter.ts`
  - In-memory implementation of `EncounterStore` port (React state)
  - Exposes current encounter state + `advanceTurn` action
  - Initializes with a hardcoded 3-combatant encounter for demo
  - **Acceptance**: hook compiles; usable in a React component

- [ ] **T016** Wire up `App.tsx`
  - Display: current combatant name, round number, combatant list
    with active indicator
  - Single "Next Turn" button calling the use case
  - Display emitted events (optional, for demo clarity)
  - **Acceptance**: `vite build` succeeds; clicking "Next Turn"
    cycles through combatants and increments rounds correctly

**Milestone 2 checkpoint**: `pnpm check` passes. App runs in
browser. Full round-trip from button click → domain pure function →
UI update verified manually.

## Risks & Open Questions

| # | Item | Severity | Mitigation |
|---|------|----------|------------|
| 1 | pnpm workspace + TypeScript project references can have path resolution quirks with Vite | Low | Use `vite-tsconfig-paths` plugin if needed; test early in T004 |
| 2 | Biome config format may change across versions | Low | Pinned to exact 2.0.0; `$schema` in config validates structure |
| 3 | Layer boundary script is a lightweight grep — not a full architectural fitness function | Low | Sufficient for walking skeleton; can upgrade to a Biome plugin or `dependency-cruiser` later if needed |

## Complexity Tracking

No constitution violations. No complexity justifications needed.