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+# Phase 2: Constraint Validation - Research
+
+**Researched:** 2026-01-18
+**Domain:** Constraint validation for itinerary item positioning
+**Confidence:** HIGH
+
+## Summary
+
+This research reveals that Phase 2 is largely **already implemented**. The codebase contains a complete `ItineraryConstraints` struct with 17 XCTest test cases covering all the constraint requirements from the phase description. The Phase 1 work (SortOrderProvider, Trip day derivation) provides the foundation, and `ItineraryConstraints` already validates game immutability, travel segment positioning, and custom item flexibility.
+
+The main gap is **migrating the tests from XCTest to Swift Testing** (@Test, @Suite) to match the project's established patterns from Phase 1. The `ItineraryTableViewController` already integrates with `ItineraryConstraints` for drag-drop validation during Phase 4's UI work.
+
+**Primary recommendation:** Verify existing implementation covers all CONS-* requirements, migrate tests to Swift Testing, and document the constraint API for Phase 4's UI integration.
+
+## Codebase Analysis
+
+### Key Files
+
+| File | Purpose | Status |
+|------|---------|--------|
+| `SportsTime/Core/Models/Domain/ItineraryConstraints.swift` | Constraint validation struct | **Complete** |
+| `SportsTimeTests/ItineraryConstraintsTests.swift` | 17 XCTest test cases | Needs migration to Swift Testing |
+| `SportsTime/Core/Models/Domain/SortOrderProvider.swift` | sortOrder calculation utilities | Complete (Phase 1) |
+| `SportsTime/Core/Models/Domain/Trip.swift` | Day derivation methods | Complete (Phase 1) |
+| `SportsTime/Core/Models/Domain/ItineraryItem.swift` | Unified itinerary item model | Complete |
+| `SportsTime/Features/Trip/Views/ItineraryTableViewController.swift` | UITableView with drag-drop | Uses constraints |
+
+### Existing ItineraryConstraints API
+
+```swift
+// Source: SportsTime/Core/Models/Domain/ItineraryConstraints.swift
+struct ItineraryConstraints {
+    let tripDayCount: Int
+    private let items: [ItineraryItem]
+
+    init(tripDayCount: Int, items: [ItineraryItem])
+
+    /// Check if a position is valid for an item
+    func isValidPosition(for item: ItineraryItem, day: Int, sortOrder: Double) -> Bool
+
+    /// Get the valid day range for a travel item
+    func validDayRange(for item: ItineraryItem) -> ClosedRange<Int>?
+
+    /// Get the games that act as barriers for visual highlighting
+    func barrierGames(for item: ItineraryItem) -> [ItineraryItem]
+}
+```
+
+### ItemKind Types
+
+```swift
+// Source: SportsTime/Core/Models/Domain/ItineraryItem.swift
+enum ItemKind: Codable, Hashable {
+    case game(gameId: String)        // CONS-01: Cannot be moved
+    case travel(TravelInfo)          // CONS-02, CONS-03: Day range + ordering constraints
+    case custom(CustomInfo)          // CONS-04: No constraints
+}
+```
+
+### Integration with Drag-Drop
+
+The `ItineraryTableViewController` already creates and uses `ItineraryConstraints`:
+
+```swift
+// Source: ItineraryTableViewController.swift (lines 484-494)
+func reloadData(
+    days: [ItineraryDayData],
+    travelValidRanges: [String: ClosedRange<Int>],
+    itineraryItems: [ItineraryItem] = []
+) {
+    self.travelValidRanges = travelValidRanges
+    self.allItineraryItems = itineraryItems
+    self.tripDayCount = days.count
+
+    // Rebuild constraints with new data
+    self.constraints = ItineraryConstraints(tripDayCount: tripDayCount, items: itineraryItems)
+    // ...
+}
+```
+
+## Constraint Requirements Mapping
+
+### CONS-01: Games cannot be moved (fixed by schedule)
+
+**Status: IMPLEMENTED**
+
+```swift
+// ItineraryConstraints.isValidPosition()
+case .game:
+    // Games are fixed, should never be moved
+    return false
+```
+
+**Existing tests:**
+- `test_gameItem_cannotBeMoved()` - Verifies games return false for any position
+
+**UI Integration (Phase 4):**
+- `ItineraryRowItem.isReorderable` returns false for `.games`
+- No drag handle appears on game rows
+
+### CONS-02: Travel segments constrained to valid day range
+
+**Status: IMPLEMENTED**
+
+```swift
+// ItineraryConstraints.isValidTravelPosition()
+let departureGameDays = gameDays(in: fromCity)
+let arrivalGameDays = gameDays(in: toCity)
+
+let minDay = departureGameDays.max() ?? 1     // After last from-city game
+let maxDay = arrivalGameDays.min() ?? tripDayCount  // Before first to-city game
+
+guard day >= minDay && day <= maxDay else { return false }
+```
+
+**Existing tests:**
+- `test_travel_validDayRange_simpleCase()` - Chicago Day 1, Detroit Day 3 -> range 1...3
+- `test_travel_cannotGoOutsideValidDayRange()` - Day before departure invalid
+- `test_travel_validDayRange_returnsNil_whenConstraintsImpossible()` - Reversed order returns nil
+
+### CONS-03: Travel must be after from-city games, before to-city games on same day
+
+**Status: IMPLEMENTED**
+
+```swift
+// ItineraryConstraints.isValidTravelPosition()
+if departureGameDays.contains(day) {
+    let maxGameSortOrder = games(in: fromCity)
+        .filter { $0.day == day }
+        .map { $0.sortOrder }
+        .max() ?? 0
+
+    if sortOrder <= maxGameSortOrder {
+        return false  // Must be AFTER all departure games
+    }
+}
+
+if arrivalGameDays.contains(day) {
+    let minGameSortOrder = games(in: toCity)
+        .filter { $0.day == day }
+        .map { $0.sortOrder }
+        .min() ?? Double.greatestFiniteMagnitude
+
+    if sortOrder >= minGameSortOrder {
+        return false  // Must be BEFORE all arrival games
+    }
+}
+```
+
+**Existing tests:**
+- `test_travel_mustBeAfterDepartureGames()` - Before departure game invalid
+- `test_travel_mustBeBeforeArrivalGames()` - After arrival game invalid
+- `test_travel_mustBeAfterAllDepartureGamesOnSameDay()` - Between games invalid
+- `test_travel_mustBeBeforeAllArrivalGamesOnSameDay()` - Between games invalid
+- `test_travel_canBeAnywhereOnRestDays()` - No games = any position valid
+
+### CONS-04: Custom items have no constraints
+
+**Status: IMPLEMENTED**
+
+```swift
+// ItineraryConstraints.isValidPosition()
+case .custom:
+    // Custom items can go anywhere
+    return true
+```
+
+**Existing tests:**
+- `test_customItem_canGoOnAnyDay()` - Days 1-5 all valid
+- `test_customItem_canGoBeforeOrAfterGames()` - Any sortOrder valid
+
+## Standard Stack
+
+### Core
+| Library | Version | Purpose | Why Standard |
+|---------|---------|---------|--------------|
+| Swift Testing | Swift 5.10+ | Test framework | Project standard from Phase 1 |
+| Foundation | Swift stdlib | Date/Calendar | Already used throughout |
+
+### Supporting
+| Library | Version | Purpose | When to Use |
+|---------|---------|---------|-------------|
+| XCTest | iOS 26+ | Legacy tests | Being migrated away |
+
+## Architecture Patterns
+
+### Pattern 1: Pure Function Constraint Checking
+
+**What:** `ItineraryConstraints.isValidPosition()` is a pure function - no side effects, deterministic
+
+**When to use:** All constraint validation - fast, testable, no async complexity
+
+**Example:**
+```swift
+// Source: ItineraryConstraints.swift
+func isValidPosition(for item: ItineraryItem, day: Int, sortOrder: Double) -> Bool {
+    guard day >= 1 && day <= tripDayCount else { return false }
+
+    switch item.kind {
+    case .game:
+        return false
+    case .travel(let info):
+        return isValidTravelPosition(fromCity: info.fromCity, toCity: info.toCity, day: day, sortOrder: sortOrder)
+    case .custom:
+        return true
+    }
+}
+```
+
+### Pattern 2: Precomputed Valid Ranges
+
+**What:** `validDayRange(for:)` computes the valid day range once at drag start
+
+**When to use:** UI needs to quickly check many positions during drag
+
+**Example:**
+```swift
+// Source: ItineraryTableViewController.swift
+func calculateTravelDragZones(segment: TravelSegment) {
+    let travelId = "travel:\(segment.fromLocation.name.lowercased())->\(segment.toLocation.name.lowercased())"
+
+    guard let validRange = travelValidRanges[travelId] else { ... }
+
+    // Pre-calculate ALL valid row indices
+    for (index, rowItem) in flatItems.enumerated() {
+        if validRange.contains(dayNum) {
+            validRows.append(index)
+        } else {
+            invalidRows.insert(index)
+        }
+    }
+}
+```
+
+### Pattern 3: City Extraction from Game ID
+
+**What:** Game IDs encode city: `game-CityName-xxxx`
+
+**Why:** Avoids needing to look up game details during constraint checking
+
+**Example:**
+```swift
+// Source: ItineraryConstraints.swift
+private func city(forGameId gameId: String) -> String? {
+    let components = gameId.components(separatedBy: "-")
+    guard components.count >= 2 else { return nil }
+    return components[1]
+}
+```
+
+### Anti-Patterns to Avoid
+
+- **Async constraint validation:** Constraints must be synchronous for responsive drag feedback
+- **Row-index based constraints:** Always use semantic (day, sortOrder), never row indices
+- **Checking constraints on drop only:** Check at drag start for valid range, each position during drag
+
+## Don't Hand-Roll
+
+| Problem | Don't Build | Use Instead | Why |
+|---------|-------------|-------------|-----|
+| Constraint validation | Custom per-item-type logic | `ItineraryConstraints` | Already handles all cases |
+| Day range calculation | Manual game day scanning | `validDayRange(for:)` | Handles edge cases |
+| City matching | String equality | `city(forGameId:)` helper | Game ID format is stable |
+| Position checking | Multiple conditions scattered | `isValidPosition()` | Single entry point |
+
+## Common Pitfalls
+
+### Pitfall 1: Forgetting sortOrder Constraints on Same Day
+
+**What goes wrong:** Travel placed on game day but ignoring sortOrder requirement
+
+**Why it happens:** Day range looks valid, forget to check sortOrder against games
+
+**How to avoid:** Always use `isValidPosition()` which checks both day AND sortOrder
+
+**Warning signs:** Travel placed before departure game or after arrival game
+
+### Pitfall 2: City Name Case Sensitivity
+
+**What goes wrong:** "Chicago" != "chicago" causing constraint checks to fail
+
+**Why it happens:** TravelInfo stores display-case cities, game IDs may differ
+
+**How to avoid:** The implementation already lowercases for comparison
+
+**Warning signs:** Valid travel rejected because city match fails
+
+### Pitfall 3: Empty Game Days
+
+**What goes wrong:** No games in a city means null/empty arrays
+
+**Why it happens:** Some cities might have no games yet (planning in progress)
+
+**How to avoid:** Implementation uses `?? 1` and `?? tripDayCount` defaults
+
+**Warning signs:** Constraint checks crash on cities with no games
+
+## Code Examples
+
+### Constraint Checking (Full Implementation)
+
+```swift
+// Source: SportsTime/Core/Models/Domain/ItineraryConstraints.swift
+struct ItineraryConstraints {
+    let tripDayCount: Int
+    private let items: [ItineraryItem]
+
+    func isValidPosition(for item: ItineraryItem, day: Int, sortOrder: Double) -> Bool {
+        guard day >= 1 && day <= tripDayCount else { return false }
+
+        switch item.kind {
+        case .game:
+            return false
+        case .travel(let info):
+            return isValidTravelPosition(
+                fromCity: info.fromCity,
+                toCity: info.toCity,
+                day: day,
+                sortOrder: sortOrder
+            )
+        case .custom:
+            return true
+        }
+    }
+
+    func validDayRange(for item: ItineraryItem) -> ClosedRange<Int>? {
+        guard case .travel(let info) = item.kind else { return nil }
+
+        let departureGameDays = gameDays(in: info.fromCity)
+        let arrivalGameDays = gameDays(in: info.toCity)
+
+        let minDay = departureGameDays.max() ?? 1
+        let maxDay = arrivalGameDays.min() ?? tripDayCount
+
+        guard minDay <= maxDay else { return nil }
+        return minDay...maxDay
+    }
+}
+```
+
+### Test Pattern (Swift Testing)
+
+```swift
+// Pattern from Phase 1 tests - to be applied to constraint tests
+@Suite("ItineraryConstraints")
+struct ItineraryConstraintsTests {
+
+    @Test("game: cannot be moved to any position")
+    func game_cannotBeMoved() {
+        let constraints = makeConstraints(tripDays: 5, games: [gameItem])
+
+        #expect(constraints.isValidPosition(for: gameItem, day: 2, sortOrder: 100) == false)
+        #expect(constraints.isValidPosition(for: gameItem, day: 3, sortOrder: 100) == false)
+    }
+
+    @Test("travel: must be after departure games on same day")
+    func travel_mustBeAfterDepartureGames() {
+        let constraints = makeConstraints(tripDays: 3, games: [chicagoGame])
+        let travel = makeTravelItem(from: "Chicago", to: "Detroit")
+
+        // Before departure game - invalid
+        #expect(constraints.isValidPosition(for: travel, day: 1, sortOrder: 50) == false)
+
+        // After departure game - valid
+        #expect(constraints.isValidPosition(for: travel, day: 1, sortOrder: 150) == true)
+    }
+
+    @Test("custom: can go anywhere")
+    func custom_canGoAnywhere() {
+        let constraints = makeConstraints(tripDays: 5)
+        let custom = makeCustomItem()
+
+        for day in 1...5 {
+            #expect(constraints.isValidPosition(for: custom, day: day, sortOrder: 50) == true)
+        }
+    }
+}
+```
+
+## State of the Art
+
+| Old Approach | Current Approach | When Changed | Impact |
+|--------------|------------------|--------------|--------|
+| Row-index validation | Semantic (day, sortOrder) validation | Phase 1 | Stable across reloads |
+| XCTest framework | Swift Testing (@Test, @Suite) | Phase 1 | Modern, cleaner assertions |
+| Constraint checking on drop | Precompute at drag start | Already implemented | Smoother drag UX |
+
+## Open Questions
+
+### Resolved by Research
+
+1. **Where does constraint validation live?**
+   - Answer: `ItineraryConstraints` struct in Domain models
+   - Confidence: HIGH - already implemented and integrated
+
+2. **How will Phase 4 call it?**
+   - Answer: `ItineraryTableViewController` already integrates via `self.constraints`
+   - Confidence: HIGH - verified in codebase
+
+### Minor Questions (Claude's Discretion per CONTEXT.md)
+
+1. **Visual styling for invalid zones?**
+   - Current: Alpha 0.3 dimming, gold border on barrier games
+   - CONTEXT.md says: "Border highlight color for valid drop zones" is Claude's discretion
+   - Recommendation: Keep current implementation, refine in Phase 4 if needed
+
+2. **sortOrder precision exhaustion handling?**
+   - Current: `SortOrderProvider.needsNormalization()` and `normalize()` exist
+   - CONTEXT.md says: "Renormalize vs. block" is Claude's discretion
+   - Recommendation: Renormalize proactively when detected
+
+## Test Strategy
+
+### Migration Required
+
+The existing 17 XCTest tests need migration to Swift Testing:
+
+| XCTest Method | Swift Testing Equivalent |
+|---------------|--------------------------|
+| `XCTestCase` class | `@Suite` struct |
+| `func test_*` | `@Test("description") func` |
+| `XCTAssertTrue(x)` | `#expect(x == true)` |
+| `XCTAssertFalse(x)` | `#expect(x == false)` |
+| `XCTAssertEqual(a, b)` | `#expect(a == b)` |
+| `XCTAssertNil(x)` | `#expect(x == nil)` |
+
+### Test Categories to Verify
+
+| Category | Current Count | Coverage |
+|----------|---------------|----------|
+| Game immutability (CONS-01) | 1 test | Complete |
+| Travel day range (CONS-02) | 4 tests | Complete |
+| Travel sortOrder constraints (CONS-03) | 4 tests | Complete |
+| Custom item flexibility (CONS-04) | 2 tests | Complete |
+| Edge cases | 1 test | Impossible constraints |
+| Barrier games | 1 test | Visual highlighting |
+
+## Sources
+
+### Primary (HIGH confidence)
+- `SportsTime/Core/Models/Domain/ItineraryConstraints.swift` - Full implementation reviewed
+- `SportsTimeTests/ItineraryConstraintsTests.swift` - 17 test cases reviewed
+- `SportsTime/Features/Trip/Views/ItineraryTableViewController.swift` - Integration verified
+- Phase 1 research and summaries - Pattern consistency verified
+
+### Secondary (MEDIUM confidence)
+- CONTEXT.md decisions - Visual styling details are Claude's discretion
+- CLAUDE.md test patterns - Swift Testing is project standard
+
+## Metadata
+
+**Confidence breakdown:**
+- Constraint implementation: HIGH - Code reviewed, all CONS-* requirements met
+- Test coverage: HIGH - 17 existing tests cover all requirements
+- UI integration: HIGH - Already used in ItineraryTableViewController
+- Migration path: HIGH - Clear XCTest -> Swift Testing mapping
+
+**Research date:** 2026-01-18
+**Valid until:** Indefinite (constraint logic is stable)
+
+## Recommendations for Planning
+
+### Phase 2 Scope (Refined)
+
+Given that `ItineraryConstraints` is already implemented and tested:
+
+1. **Verify existing tests cover all requirements** - Compare against CONS-01 through CONS-04
+2. **Migrate tests to Swift Testing** - Match Phase 1 patterns
+3. **Add any missing edge case tests** - e.g., empty trip, single-day trip
+4. **Document constraint API** - For Phase 4 UI integration reference
+
+### What NOT to Build
+
+- The constraint checking logic already exists and works
+- The UI integration already exists in `ItineraryTableViewController`
+- The visual feedback (dimming, barriers) already exists
+
+### Minimal Work Required
+
+Phase 2 is essentially a **verification and standardization phase**, not a building phase:
+- Verify implementation matches requirements
+- Standardize tests to project patterns
+- Document for downstream phases