# Data Model: Turn Navigation

## Existing Entities (unchanged)

### Encounter
- `combatants`: readonly array of Combatant
- `activeIndex`: number (0-based index into combatants)
- `roundNumber`: positive integer (>= 1)

### Combatant
- `id`: CombatantId (branded string)
- `name`: string
- `initiative?`: number
- `maxHp?`: number
- `currentHp?`: number

## New Domain Events

### TurnRetreated
Emitted on every successful RetreatTurn operation.

| Field | Type | Description |
|-------|------|-------------|
| type | `"TurnRetreated"` (literal) | Discriminant for event union |
| previousCombatantId | CombatantId | The combatant whose turn was active before retreat |
| newCombatantId | CombatantId | The combatant who is now active after retreat |
| roundNumber | number | The round number after the retreat |

### RoundRetreated
Emitted when RetreatTurn crosses a round boundary (activeIndex wraps from 0 to last combatant).

| Field | Type | Description |
|-------|------|-------------|
| type | `"RoundRetreated"` (literal) | Discriminant for event union |
| newRoundNumber | number | The round number after decrementing |

## State Transitions

### RetreatTurn

**Input**: Encounter
**Output**: `{ encounter: Encounter, events: DomainEvent[] }` | `DomainError`

**Rules**:
1. If `combatants.length === 0` -> DomainError("invalid-encounter")
2. If `roundNumber === 1 && activeIndex === 0` -> DomainError("no-previous-turn")
3. If `activeIndex > 0`: newIndex = activeIndex - 1, roundNumber unchanged
4. If `activeIndex === 0`: newIndex = combatants.length - 1, roundNumber - 1

**Events emitted**:
- Always: TurnRetreated
- On round boundary crossing (rule 4): TurnRetreated then RoundRetreated (order matters)

## Validation Rules

- RetreatTurn MUST NOT produce roundNumber < 1
- RetreatTurn MUST NOT produce activeIndex < 0 or >= combatants.length
- RetreatTurn is a pure function: identical input produces identical output