# ItineraryConstraints API

**Location:** `SportsTime/Core/Models/Domain/ItineraryConstraints.swift`
**Verified by:** 22 tests in `SportsTimeTests/Domain/ItineraryConstraintsTests.swift`

## Overview

`ItineraryConstraints` validates item positions during drag-drop operations. It enforces:

- **Games cannot move** (CONS-01)
- **Travel segments have day range limits** (CONS-02)
- **Travel segments must respect game sortOrder on same day** (CONS-03)
- **Custom items have no constraints** (CONS-04)

## Construction

```swift
let constraints = ItineraryConstraints(
    tripDayCount: days.count,
    items: allItineraryItems  // All items including games
)
```

**Parameters:**
- `tripDayCount`: Total days in trip (1-indexed, so a 5-day trip has days 1-5)
- `items`: All itinerary items (games, travel, custom). Games are used to calculate constraints for travel items.

## Public API

### `isValidPosition(for:day:sortOrder:) -> Bool`

Check if a specific position is valid for an item.

```swift
func isValidPosition(for item: ItineraryItem, day: Int, sortOrder: Double) -> Bool
```

**Usage during drag:**
```swift
// On each drag position update
let dropPosition = calculateDropPosition(at: touchLocation)
let isValid = constraints.isValidPosition(
    for: draggedItem,
    day: dropPosition.day,
    sortOrder: dropPosition.sortOrder
)

if isValid {
    showValidDropIndicator()
} else {
    showInvalidDropIndicator()
}
```

**Returns:**
- `true`: Position is valid, allow drop
- `false`: Position is invalid, reject drop (snap back)

**Rules by item type:**
| Item Type | Day Constraint | SortOrder Constraint |
|-----------|----------------|----------------------|
| `.game` | Always `false` | Always `false` |
| `.travel` | Within valid day range | After departure games, before arrival games |
| `.custom` | Any day 1...tripDayCount | Any sortOrder |

### `validDayRange(for:) -> ClosedRange<Int>?`

Get the valid day range for a travel item (for visual feedback).

```swift
func validDayRange(for item: ItineraryItem) -> ClosedRange<Int>?
```

**Usage at drag start:**
```swift
// When drag begins, precompute valid range
guard case .travel = draggedItem.kind,
      let validRange = constraints.validDayRange(for: draggedItem) else {
    // Not a travel item or impossible constraints
    return
}

// Use range to dim invalid days
for day in 1...tripDayCount {
    if !validRange.contains(day) {
        dimDay(day)
    }
}
```

**Returns:**
- `ClosedRange<Int>`: Valid day range (e.g., `2...4`)
- `nil`: Constraints are impossible (e.g., departure game after arrival game)

### `barrierGames(for:) -> [ItineraryItem]`

Get games that constrain a travel item (for visual highlighting).

```swift
func barrierGames(for item: ItineraryItem) -> [ItineraryItem]
```

**Usage for visual feedback:**
```swift
// Highlight barrier games during drag
let barriers = constraints.barrierGames(for: travelItem)
for barrier in barriers {
    highlightAsBarrier(barrier)  // e.g., gold border
}
```

**Returns:**
- Array of game items: Last departure city game + first arrival city game
- Empty array: Not a travel item or no constraining games

## Integration Points

### ItineraryTableViewController (existing)

```swift
// In reloadData()
self.constraints = ItineraryConstraints(tripDayCount: tripDayCount, items: itineraryItems)

// In drag handling
if constraints.isValidPosition(for: draggedItem, day: targetDay, sortOrder: targetSortOrder) {
    // Allow drop
} else {
    // Reject drop, snap back
}
```

### Phase 4 Implementation Notes

1. **Drag Start:**
   - Check `item.isReorderable` (games return `false`)
   - Call `validDayRange(for:)` to precompute valid days
   - Call `barrierGames(for:)` to identify visual barriers

2. **Drag Move:**
   - Calculate target (day, sortOrder) from touch position
   - Call `isValidPosition(for:day:sortOrder:)` for real-time feedback
   - Update insertion line (valid) or red indicator (invalid)

3. **Drag End:**
   - Final `isValidPosition(for:day:sortOrder:)` check
   - Valid: Update item's day/sortOrder, animate settle
   - Invalid: Animate snap back, haptic feedback

## Test Coverage

| Requirement | Tests | Verified |
|-------------|-------|----------|
| CONS-01 (games cannot move) | 2 | Yes |
| CONS-02 (travel day range) | 3 | Yes |
| CONS-03 (travel sortOrder) | 5 | Yes |
| CONS-04 (custom flexibility) | 2 | Yes |
| Edge cases | 8 | Yes |
| Success criteria | 3 | Yes |
| Barrier games | 1 | Yes |
| **Total** | **22** | **100%** |

---
*API documented: Phase 02*
*Ready for: Phase 04 (Drag Interaction)*