import SwiftUI

/// Favourites tab — starred stations, split by fuel type. Each fuel type
/// with at least one favourite appears as its own tab; that tab's favourites
/// are ranked cheapest-first for that fuel, with the cheapest called out.
/// Favourites are fuel-scoped entries — starring a station only pins it for
/// the fuel you were viewing, so an Unleaded favourite never shows as Diesel.
struct FavouritesView: View {
    let favourites: [FavouriteEntry]
    /// The app's currently selected fuel — used only as the initial tab.
    let selectedFuel: FuelType
    let location: Coordinate?
    let distanceUnit: DistanceUnit
    var onToggleFavourite: (FuelStation, FuelType) -> Void = { _, _ in }

    /// Fuel types that currently have at least one favourite — these are the
    /// only tabs shown (a fuel with no favourites gets no tab).
    private var availableFuels: [FuelType] {
        FuelType.allCases.filter { fuel in
            favourites.contains { $0.fuel == fuel }
        }
    }

    /// The active tab. Falls back if the chosen fuel ever loses all its
    /// favourites (e.g. the last one is un-starred while viewing it).
    @State private var fuel: FuelType = .e10

    private var activeFuel: FuelType {
        availableFuels.contains(fuel) ? fuel : (availableFuels.first ?? .e10)
    }

    /// Stations favourited for the active fuel, cheapest first (distance tiebreak).
    private var ranked: [FuelStation] {
        let available = favourites.filter { $0.fuel == activeFuel }.map(\.station)
        return available.sorted { lhs, rhs in
            let lPrice = lhs.prices[activeFuel]!
            let rPrice = rhs.prices[activeFuel]!
            if lPrice != rPrice { return lPrice < rPrice }
            guard let location else { return false }
            return lhs.distanceKM(to: location.lat, lng2: location.lng) <
                   rhs.distanceKM(to: location.lat, lng2: location.lng)
        }
    }

    /// IDs favourited for the active fuel — the star state on each row.
    private var activeFuelFavouriteIDs: Set<String> {
        Set(favourites.filter { $0.fuel == activeFuel }.map(\.station.id))
    }

    private var cheapest: FuelStation? { ranked.first }
    private var cheapestPrice: Double? { cheapest?.prices[activeFuel] }

    init(favourites: [FavouriteEntry],
         selectedFuel: FuelType,
         location: Coordinate?,
         distanceUnit: DistanceUnit,
         onToggleFavourite: @escaping (FuelStation, FuelType) -> Void = { _, _ in }) {
        self.favourites = favourites
        self.selectedFuel = selectedFuel
        self.location = location
        self.distanceUnit = distanceUnit
        self.onToggleFavourite = onToggleFavourite
        _fuel = State(initialValue: selectedFuel)
    }

    var body: some View {
        NavigationStack {
            List {
                if favourites.isEmpty {
                    Section {
                        VStack(spacing: 10) {
                            Image(systemName: "star")
                                .font(.system(size: 40))
                                .foregroundStyle(.secondary)
                            Text("No favourites yet")
                                .font(.headline)
                            Text("Tap the star on any station in the Stations tab to pin it here, ranked by which is cheapest for that fuel.")
                                .font(.caption)
                                .foregroundStyle(.secondary)
                                .multilineTextAlignment(.center)
                        }
                        .frame(maxWidth: .infinity)
                        .padding(.vertical, 24)
                    }
                } else {
                    Section("Fuel type") {
                        FuelTypeSegmentedPicker(
                            selection: $fuel,
                            fuels: availableFuels
                        )
                    }

                    Section("\(activeFuel.displayName) favourites — cheapest first") {
                        ForEach(Array(ranked.enumerated()), id: \.element.id) { index, station in
                            StationRow(
                                station: station,
                                fuel: activeFuel,
                                location: location,
                                distanceUnit: distanceUnit,
                                baselinePrice: cheapestPrice,
                                isTopResult: index == 0,
                                isFavourite: activeFuelFavouriteIDs.contains(station.id),
                                onToggleFavourite: { onToggleFavourite(station, activeFuel) }
                            )
                        }
                    }

                    if let cheapest = cheapest {
                        Section {
                            Text("\(cheapest.name) is your cheapest \(activeFuel.displayName) favourite. Favourites are monitored automatically — switch to the Alerts tab and enable alerts to get pinged when you approach one that's the cheapest within the radius.")
                                .font(.caption)
                                .foregroundStyle(.secondary)
                        }
                    }
                }
            }
            .navigationTitle("Favourites")
        }
    }
}