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.
struct FavouritesView: View {
    let favourites: [FuelStation]
    /// The app's currently selected fuel — used only as the initial tab.
    let selectedFuel: FuelType
    let location: Coordinate?
    let distanceUnit: DistanceUnit
    let favouriteIDs: Set<String>
    var onToggleFavourite: (FuelStation) -> Void = { _ in }

    /// Fuel types that currently have at least one favourite selling them —
    /// these are the only tabs shown.
    private var availableFuels: [FuelType] {
        FuelType.allCases.filter { fuel in
            favourites.contains { $0.prices[fuel] != nil }
        }
    }

    /// 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)
    }

    /// Favourites that sell the active fuel, cheapest first (distance tiebreak).
    private var ranked: [FuelStation] {
        let available = favourites.filter { $0.prices[activeFuel] != nil }
        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)
        }
    }

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

    init(favourites: [FuelStation],
         selectedFuel: FuelType,
         location: Coordinate?,
         distanceUnit: DistanceUnit,
         favouriteIDs: Set<String>,
         onToggleFavourite: @escaping (FuelStation) -> Void = { _ in }) {
        self.favourites = favourites
        self.selectedFuel = selectedFuel
        self.location = location
        self.distanceUnit = distanceUnit
        self.favouriteIDs = favouriteIDs
        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.")
                                .font(.caption)
                                .foregroundStyle(.secondary)
                                .multilineTextAlignment(.center)
                        }
                        .frame(maxWidth: .infinity)
                        .padding(.vertical, 24)
                    }
                } else {
                    Section("Fuel type") {
                        Picker("Fuel type", selection: $fuel) {
                            ForEach(availableFuels) { fuel in
                                Text(fuel.shortName).tag(fuel)
                            }
                        }
                        .pickerStyle(.segmented)
                    }

                    Section {
                        if let cheapest, let price = cheapestPrice {
                            HStack(spacing: 10) {
                                Image(systemName: "crown.fill")
                                    .foregroundStyle(.yellow)
                                Text("Cheapest \(activeFuel.displayName) favourite: \(cheapest.name) — \(String(format: "%.1fp", price))")
                                    .font(.footnote)
                            }
                        }
                    }

                    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: favouriteIDs.contains(station.id),
                                onToggleFavourite: { onToggleFavourite(station) }
                            )
                        }
                    }

                    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")
        }
    }
}