# Implementation Plan: Turn Navigation

**Branch**: `012-turn-navigation` | **Date**: 2026-03-05 | **Spec**: [spec.md](./spec.md)
**Input**: Feature specification from `/specs/012-turn-navigation/spec.md`

## Summary

Add a RetreatTurn domain operation (inverse of AdvanceTurn) and relocate both Previous/Next Turn buttons to a new turn navigation bar at the top of the encounter tracker. The domain function is a pure state transition that decrements the active index (wrapping to the last combatant and decrementing round number when crossing a round boundary), with a guard preventing retreat before the encounter start (round 1, index 0).

## Technical Context

**Language/Version**: TypeScript 5.8 (strict mode, verbatimModuleSyntax)
**Primary Dependencies**: React 19, Vite 6, Tailwind CSS v4, shadcn/ui-style components, Lucide React (icons)
**Storage**: N/A (no storage changes -- existing localStorage persistence unchanged)
**Testing**: Vitest (pure-function domain tests + component tests)
**Target Platform**: Web browser (single-user, local-first)
**Project Type**: Web application (monorepo: domain + application + web adapter)
**Performance Goals**: Standard web app -- instant UI response (<100ms)
**Constraints**: Domain layer must remain pure (no I/O, no framework imports)
**Scale/Scope**: Single-user encounter tracker, ~5-20 combatants typical

## Constitution Check

*GATE: Must pass before Phase 0 research. Re-check after Phase 1 design.*

| Principle | Status | Notes |
|-----------|--------|-------|
| I. Deterministic Domain Core | PASS | RetreatTurn is a pure function: same input = same output. No I/O, randomness, or clocks. |
| II. Layered Architecture | PASS | Domain (retreat-turn.ts) -> Application (retreat-turn-use-case.ts) -> Adapter (React hook + component). Dependency direction preserved. |
| III. Agent Boundary | N/A | No agent layer involved. |
| IV. Clarification-First | PASS | No ambiguities remain in spec. All edge cases resolved. |
| V. Escalation Gates | PASS | Implementation matches spec scope exactly. |
| VI. MVP Baseline Language | PASS | Assumptions use "MVP baseline does not include" phrasing. |
| VII. No Gameplay Rules | PASS | No gameplay mechanics in plan. |

## Project Structure

### Documentation (this feature)

```text
specs/012-turn-navigation/
├── plan.md              # This file
├── research.md          # Phase 0 output
├── data-model.md        # Phase 1 output
├── quickstart.md        # Phase 1 output
├── contracts/           # Phase 1 output
└── tasks.md             # Phase 2 output (via /speckit.tasks)
```

### Source Code (repository root)

```text
packages/domain/src/
├── retreat-turn.ts              # NEW: RetreatTurn pure function
├── events.ts                    # MODIFY: Add TurnRetreated + RoundRetreated events
├── types.ts                     # UNCHANGED
├── index.ts                     # MODIFY: Re-export retreatTurn
└── __tests__/
    └── retreat-turn.test.ts     # NEW: All acceptance scenarios

packages/application/src/
├── retreat-turn-use-case.ts     # NEW: Orchestrates retreatTurn via EncounterStore
├── ports.ts                     # UNCHANGED
├── index.ts                     # MODIFY: Re-export new use case
└── ...

apps/web/src/
├── components/
│   ├── turn-navigation.tsx      # NEW: Top-placed Previous/Next turn bar
│   └── action-bar.tsx           # MODIFY: Remove Next Turn button
├── hooks/
│   └── use-encounter.ts         # MODIFY: Add retreatTurn handler
└── App.tsx                      # MODIFY: Add TurnNavigation component above combatant list
```

**Structure Decision**: Follows existing monorepo layout (domain -> application -> web adapter). New files mirror the existing advance-turn pattern. The turn navigation component is a new UI component placed in the existing components directory.