Add cheapest-favourite price-drop alerts

FuelBoard/ProximityMonitor.swift

@@ -79,6 +79,7 @@ struct TieChoice {
 /// entering the winner's circle — not at the forecourt.
 @MainActor
 final class ProximityMonitor: NSObject, ObservableObject, @preconcurrency CLLocationManagerDelegate, UNUserNotificationCenterDelegate {
+    private static let favouriteDropThresholdPence: Double = 1.0
     @Published var monitoredStationIDs: [String] = []
     @Published var lastAlert: String?
     @Published private(set) var lastTestResult: String?
@@ -218,6 +219,46 @@ final class ProximityMonitor: NSObject, ObservableObject, @preconcurrency CLLoca
 
     var isEnabled: Bool { enabled }
 
+    /// Compares the latest refreshed favourites against the persisted cheapest-
+    /// favourite baseline for one fuel and schedules a notification when the
+    /// winner changes or the current winner drops by a meaningful amount.
+    func evaluateFavouritePriceDropAlert(
+        favourites: [FavouriteEntry],
+        previousSnapshots: [FuelType: FavouriteAlertSnapshot],
+        monitoredFuel: FuelType,
+        enabled: Bool
+    ) {
+        var snapshots = previousSnapshots
+        guard let current = FuelStore.cheapestFavourite(in: favourites, fuel: monitoredFuel),
+              let currentPrice = current.station.prices[monitoredFuel] else {
+            snapshots.removeValue(forKey: monitoredFuel)
+            FuelStore.saveFavouriteAlertSnapshots(snapshots)
+            return
+        }
+
+        let currentSnapshot = FavouriteAlertSnapshot(
+            fuel: monitoredFuel,
+            stationID: current.station.id,
+            stationName: current.station.name,
+            price: currentPrice
+        )
+
+        defer {
+            snapshots[monitoredFuel] = currentSnapshot
+            FuelStore.saveFavouriteAlertSnapshots(snapshots)
+        }
+
+        guard enabled else { return }
+        guard let previous = previousSnapshots[monitoredFuel] else { return }
+
+        let winnerChanged = previous.stationID != currentSnapshot.stationID
+        let priceDropped = previous.stationID == currentSnapshot.stationID
+            && currentSnapshot.price <= previous.price - Self.favouriteDropThresholdPence
+
+        guard winnerChanged || priceDropped else { return }
+        fireFavouriteDropAlert(current: current, previous: previous, price: currentPrice, winnerChanged: winnerChanged)
+    }
+
     /// Records a stage in the live alert trace, newest first, capped at 8.
     private func logAlert(_ kind: AlertLogEntry.Kind, _ text: String) {
         alertLog.insert(AlertLogEntry(date: Date(), kind: kind, text: text), at: 0)
@@ -452,6 +493,31 @@ final class ProximityMonitor: NSObject, ObservableObject, @preconcurrency CLLoca
         lastAlert = "\(brand) · \(String(format: "%.1fp", price)) · \(distanceText) away"
     }
 
+    private func fireFavouriteDropAlert(
+        current: FavouriteEntry,
+        previous: FavouriteAlertSnapshot,
+        price: Double,
+        winnerChanged: Bool
+    ) {
+        let content = UNMutableNotificationContent()
+        content.sound = .default
+        let fuel = current.fuel
+        let previousPriceText = String(format: "%.1fp", previous.price)
+        let currentPriceText = String(format: "%.1fp", price)
+
+        if winnerChanged {
+            content.title = "New cheapest \(fuel.displayName.lowercased()) favourite"
+            content.body = "\(current.station.name) is now your cheapest favourite at \(currentPriceText), ahead of \(previous.stationName)."
+        } else {
+            content.title = "Cheapest favourite just dropped"
+            content.body = "\(current.station.name) fell from \(previousPriceText) to \(currentPriceText) for \(fuel.displayName.lowercased())."
+        }
+
+        addAlertRequest(content: content, station: current.station)
+        logAlert(.fired, "favourite alert scheduled — \(current.station.name) · \(currentPriceText)")
+        lastAlert = "Favourite · \(current.station.name) · \(currentPriceText)"
+    }
+
     /// Adds a notification whose tap opens directions to `station`. The
     /// station's coordinates ride in `userInfo` so the tap handler can route
     /// to Apple Maps (or an in-app fallback), and a map snapshot with a pin is