import SwiftUI
import WidgetKit

/// Stations tab — the main list with fuel picker, distance filter, sort, pagination.
struct StationsView: View {
    let stations: [FuelStation]
    let totalCount: Int
    let isLoading: Bool
    @Binding var selectedFuel: FuelType
    @Binding var sortMode: SortMode
    @Binding var stationLimit: Int
    let distanceUnit: DistanceUnit
    let priceDisplayStyle: PriceDisplayStyle
    let baselinePrice: Double?
    let topStationID: String?
    let location: Coordinate?
    let favouriteIDs: Set<String>
    let stationTapAction: StationTapAction
    let fuelSavingsSettings: FuelSavingsSettings
    @Binding var fuelSavingsAmount: FuelSavingsFillAmount
    var savingsForStation: (FuelStation) -> FuelSavingsAvailability? = { _ in nil }
    var journeyDiagnosticsForStation: (FuelStation) -> FuelSavingsJourneyDiagnostics? = { _ in nil }
    var onToggleFavourite: (FuelStation, FuelType) -> Void = { _, _ in }
    var onRefresh: () async -> Void = {}

    /// Pagination: one page = 10 rows, reset whenever the underlying list
    /// changes (new fetch, fuel/sort/radius switch). RAG/TOP/deltas still come
    /// from the whole radius pool via `cheapestPrice` — paging never changes
    /// which station is "best".
    @State private var pageSize = 10
    @State private var visibleCount = 10

    /// Price-rating legend (the "Key") lives in a sheet, opened from the
    /// info button in the header — keeps the list focused on stations.
    @State private var showKey = false

    /// The "How Best Value works" sheet. The explanation used to be a section in
    /// the Key sheet; it is its own sheet now, opened from a row at the bottom of
    /// the Key sheet and from the More info sheet.
    @State private var showBestValueInfo = false

    /// Station picked for the More-info sheet (tap action = showDetails).
    @State private var detailStation: FuelStation?

    /// Bottom-of-tab explainer (moved from the top 2026-08-16): mode/radius/
    /// directions context + "N/TOTAL stations updated". The numerator is the
    /// current list pool; the denominator is the full UK station total from
    /// the last chain fetch (8,022 mirror snapshot) — hidden until known.
    private var footerCaption: String {
        let miles = distanceUnit.displayMiles(stationLimit)
        let unit = distanceUnit.label(for: Double(miles))
        let fuel = selectedFuel.displayName
        let mode: String
        switch sortMode {
        case .closest: mode = "Closest"
        case .cheapest: mode = "Cheapest"
        case .worthTheDrive: mode = "Best Value"
        }
        let tapHint = stationTapAction == .openMap
            ? "tap a station for directions."
            : "tap a station for details."
        let ratio: String
        if let total = FuelStore.loadStationCount() {
            ratio = "\(totalCount)/\(total)"
        } else {
            ratio = "\(totalCount)"
        }
        if location != nil {
            if sortMode == .closest {
                return "\(mode) \(fuel) stations — closest first, lowest price within \(miles) \(unit) · \(ratio) stations updated"
            }
            return "\(mode) \(fuel) within \(miles) \(unit) — \(ratio) stations updated · \(tapHint)"
        }
        return "\(mode) \(fuel) — \(ratio) stations updated · \(tapHint)"
    }

    var body: some View {
        NavigationStack {
            List {
                Section("Fuel type") {
                    FuelTypeSegmentedPicker(selection: $selectedFuel) { newValue in
                        FuelStore.saveSelectedFuel(newValue)
                        WidgetCenter.shared.reloadAllTimelines()
                    }
                }

                Section("Distance") {
                    Picker("Distance", selection: $stationLimit) {
                        ForEach(FuelStore.stationRadiusOptions, id: \.self) { miles in
                            let shown = distanceUnit.displayMiles(miles)
                            Text("\(shown) \(distanceUnit.label(for: Double(shown)))").tag(miles)
                        }
                    }
                    .pickerStyle(.segmented)
                    // In Closest mode the whole country is the pool — the
                    // radius is meaningless ("closest" must never return an
                    // empty state), so the picker is disabled but its value is
                    // kept for when the user switches back to Cheapest.
                    .disabled(sortMode == .closest)
                    .opacity(sortMode == .closest ? 0.5 : 1)
                    .onChange(of: stationLimit) { _, newValue in
                        FuelStore.saveStationLimit(newValue)
                    }
                    .padding(.vertical, 2)
                }

                Section("Stations") {
                    Picker("Sort by", selection: $sortMode) {
                        ForEach(availableSortModes) { mode in
                            Text(mode.displayName).tag(mode)
                        }
                    }
                    .pickerStyle(.segmented)
                    .onChange(of: sortMode) { _, newValue in
                        FuelStore.saveSortMode(newValue)
                    }
                    .padding(.vertical, 2)

                    // Only offered while the feature is actually configured — a
                    // stale stored sort mode must not keep the control on screen
                    // for a switched-off feature.
                    if sortMode == .worthTheDrive && fuelSavingsSettings.isConfigured {
                        FuelSavingsAdjustmentView(amount: $fuelSavingsAmount)
                    }

                    if fuelSavingsSettings.isConfigured && !stations.isEmpty {
                        Text(String(localized: "Price colours show pump price competitiveness. Savings compare with your closest station, including journey costs.", comment: "Station listing price and savings explanation"))
                            .font(.footnote)
                            .foregroundStyle(.secondary)
                            .fixedSize(horizontal: false, vertical: true)
                    }

                    if isLoading && stations.isEmpty {
                        HStack(spacing: 10) {
                            ProgressView()
                            Text("Fetching prices…")
                        }
                    } else if stations.isEmpty {
                        let miles = distanceUnit.displayMiles(stationLimit)
                        let unit = distanceUnit.label(for: Double(miles))
                        VStack(alignment: .leading, spacing: 6) {
                            Text(String(format: NSLocalizedString("No %@ stations within %lld %@.", comment: "Empty state when the radius finds nothing nearby"), selectedFuel.displayName, miles, unit))
                                .foregroundStyle(.secondary)
                            Text("FuelBoard covers the whole UK — try Closest to browse every station.")
                                .font(.caption)
                                .foregroundStyle(.secondary)
                            Button("Browse all UK stations") {
                                sortMode = .closest
                            }
                            .font(.caption)
                        }
                        .padding(.vertical, 4)
                    } else {
                        ForEach(Array(stations.prefix(visibleCount)), id: \.id) { station in
                            stationRow(station)
                        }

                        if visibleCount < stations.count {
                            Button {
                                visibleCount += pageSize
                            } label: {
                                HStack {
                                    Spacer()
                                    Text("Show \(min(pageSize, stations.count - visibleCount)) more (\(stations.count - visibleCount) remaining)")
                                    Spacer()
                                }
                            }
                        } else {
                            Text("All \(stations.count) stations shown")
                                .font(.caption2)
                                .foregroundStyle(.secondary)
                                .frame(maxWidth: .infinity)
                        }
                    }
                }

                // Bottom-of-tab explainer (moved from the top 2026-08-16),
                // styled like the Favourites tab's cheapest callout: a caption
                // paragraph in its own section cell.
                Section {
                    Text(footerCaption)
                        .font(.caption)
                        .foregroundStyle(.secondary)
                }

            }
            .refreshable {
                // Manual override for the twice-a-day cache policy.
                await onRefresh()
            }
            .onChange(of: stations) { _, _ in
                // New fetch or filter switch → back to the first page.
                visibleCount = pageSize
            }
            // Native large-title behaviour (Mail-style): the system shows the
            // large title at rest, scrolls it away and fades the compact
            // inline title in. No custom toolbar needed — the native mechanism
            // only renders text, which is exactly what we want here.
            // No numeric field is left on this screen — the Worth the Drive
            // fill-up control is a slider — so no keyboard should appear here.
            // The Done bar and interactive dismissal stay as the escape hatch
            // for any field that returns.
            .keyboardDoneButton()
            .scrollDismissesKeyboard(.interactively)
            .navigationTitle("FuelBoard")
            .navigationBarTitleDisplayMode(.large)
            .onAppear {
#if DEBUG
                // QA hook: launch with `-showKeySheet` to verify the legend
                // sheet without driving a tap (simctl has no tap command).
                if ProcessInfo.processInfo.arguments.contains("-showKeySheet") {
                    showKey = true
                }
                // QA hook: `-showBestValueInfo` opens the Key sheet and then the
                // explanation on top of it, so the copy can be screenshotted
                // without a tap. It has to chain: the explanation's `.sheet` lives
                // inside the Key sheet's content, so it cannot present while the
                // Key sheet is shut.
                if BestValueInfoQA.isRequested {
                    showKey = true
                    Task { @MainActor in
                        try? await Task.sleep(nanoseconds: 1_200_000_000)
                        showBestValueInfo = true
                    }
                }
#endif
            }
            .onReceive(NotificationCenter.default.publisher(for: .fuelBoardOpenBestValueSettings)) { _ in
                // "Setup Best Value" from the explanation sheet. That sheet closes
                // itself and ContentView switches to Settings, but both of the
                // sheets this view can have open are presented from HERE and each
                // survived the tab switch, floating over Settings: the Key sheet
                // and the More info sheet. Close whichever is up.
                showKey = false
                detailStation = nil
            }
            .task(id: stations.count) {
#if DEBUG
                // QA hook: `-showDetail` opens the More info sheet for the first
                // listed station, so the sheet can be screenshotted without a
                // tap. Waits for the list to arrive first.
                guard ProcessInfo.processInfo.arguments.contains("-showDetail"),
                      detailStation == nil,
                      let first = stations.first else { return }
                try? await Task.sleep(nanoseconds: 2_000_000_000)
                detailStation = first
#endif
            }
            .toolbar {
                ToolbarItem(placement: .topBarTrailing) {
                    Button {
                        showKey = true
                    } label: {
                        Image(systemName: "info.circle")
                    }
                    .accessibilityLabel("Key")
                }
            }
            .sheet(isPresented: $showKey) {
                keySheet
            }
            .sheet(item: $detailStation) { station in
                detailSheet(for: station)
            }
        }
    }

    private func detailSheet(for station: FuelStation) -> some View {
        StationDetailView(
            station: station,
            location: location,
            distanceUnit: distanceUnit,
            priceStyle: priceDisplayStyle,
            selectedFuel: selectedFuel,
            fuelSavings: savingsForStation(station),
            fuelSavingsAmount: fuelSavingsAmount,
            journeyDiagnostics: journeyDiagnosticsForStation(station),
            isFavourite: favouriteIDs.contains(station.id),
            onToggleFavourite: { onToggleFavourite(station, selectedFuel) }
        )
    }

    private var keySheet: some View {
                NavigationStack {
                    List {
                        Section {
                            VStack(spacing: 10) {
                                keyCard(
                                    icon: "checkmark.circle.fill", color: .green,
                                    title: "Good price", blurb: "Within 1.5p of the cheapest",
                                    threshold: "0–1.5p")
                                keyCard(
                                    icon: "equal.circle.fill", color: .orange,
                                    title: "Okay", blurb: "More than 1.5p, up to 4p above the cheapest",
                                    threshold: ">1.5–4p")
                                keyCard(
                                    icon: "exclamationmark.circle.fill", color: .red,
                                    title: "Pricey", blurb: "More than 4p above the cheapest",
                                    threshold: ">4p")
                                Rectangle()
                                    .fill(Color(.separator))
                                    .frame(height: 0.5)
                                    .padding(.vertical, 2)
                                HStack(spacing: 12) {
                                    Text("TOP")
                                        .font(.caption2.bold())
                                        .padding(.horizontal, 7)
                                        .padding(.vertical, 3)
                                        .background(Capsule().fill(.blue.opacity(0.16)))
                                        .foregroundStyle(.blue)
                                    Text("Top result for the current sort")
                                        .font(.subheadline.weight(.semibold))
                                    Spacer(minLength: 0)
                                }
                                .padding(.horizontal, 14)
                                .padding(.vertical, 12)
                                .keyCardFill(color: .blue)
                                HStack(spacing: 12) {
                                    Image(systemName: "star.fill")
                                        .font(.title3)
                                        .foregroundStyle(.yellow)
                                        .frame(width: 34)
                                    Text("Star a station to add it to Favourites")
                                        .font(.subheadline.weight(.semibold))
                                    Spacer(minLength: 0)
                                }
                                .padding(.horizontal, 14)
                                .padding(.vertical, 12)
                                .keyCardFill(color: .yellow)
                                Text("Price differences are shown in pence per litre (p/litre). Colours match each station's rating on the list.")
                                    .font(.footnote)
                                    .foregroundStyle(.secondary)
                                    .frame(maxWidth: .infinity, alignment: .center)
                                    .multilineTextAlignment(.center)
                                    .padding(.top, 4)
                            }
                            .listRowBackground(Color.clear)
                            .listRowInsets(EdgeInsets(top: 4, leading: 16, bottom: 4, trailing: 16))
                        }

                        // The explanation is its own sheet now, opened from this
                        // row. Gated exactly like the More info row and the Best
                        // Value card itself: `isConfigured` is the same condition
                        // `savingsForStation` uses, so the row appears and
                        // disappears with the feature.
                        if fuelSavingsSettings.isConfigured {
                            Section {
                                HowBestValueRow { showBestValueInfo = true }
                            }
                        }
                    }
                    .navigationTitle("Key")
                    .navigationBarTitleDisplayMode(.inline)
                    .toolbar {
                        ToolbarItem(placement: .confirmationAction) {
                            Button("Done") { showKey = false }
                        }
                    }
                    // Nested sheet: the Key sheet stays behind the explanation.
                    .sheet(isPresented: $showBestValueInfo) { HowBestValueSheet() }
                }
                .presentationDetents([.fraction(0.6)])
                .presentationBackground(Color(UIColor.systemGroupedBackground))
    }

    private func stationRow(_ station: FuelStation) -> some View {
        StationRow(
            station: station,
            fuel: selectedFuel,
            location: location,
            distanceUnit: distanceUnit,
            priceDisplayStyle: priceDisplayStyle,
            baselinePrice: baselinePrice,
            isTopResult: station.id == topStationID,
            isFavourite: favouriteIDs.contains(station.id),
            tapAction: stationTapAction,
            fuelSavings: savingsForStation(station),
            onToggleFavourite: { onToggleFavourite(station, selectedFuel) },
            onShowDetails: { detailStation = $0 }
        )
    }

    private var availableSortModes: [SortMode] {
        var modes: [SortMode] = [.closest, .cheapest]
        if fuelSavingsSettings.isConfigured {
            modes.append(.worthTheDrive)
        }
        return modes
    }

    /// One price-rating card for the Key sheet: accent icon, title + blurb,
    /// and the rating threshold in the accent colour (mirrors the tip cards).
    private func keyCard(icon: String, color: Color, title: String, blurb: String, threshold: String) -> some View {
        HStack(spacing: 12) {
            Image(systemName: icon)
                .font(.title3)
                .foregroundStyle(color)
                .frame(width: 34)
            VStack(alignment: .leading, spacing: 2) {
                Text(LocalizedStringKey(title)).font(.headline)
                Text(LocalizedStringKey(blurb))
                    .font(.caption)
                    .foregroundStyle(.secondary)
                    .fixedSize(horizontal: false, vertical: true)
            }
            Spacer(minLength: 8)
            HStack(spacing: 6) {
                Circle().fill(color).frame(width: 10, height: 10)
                Text(threshold)
                    .font(.headline.weight(.bold))
                    .foregroundStyle(color)
                    .monospacedDigit()
            }
        }
        .padding(.horizontal, 14)
        .padding(.vertical, 10)
        .frame(maxWidth: .infinity, alignment: .leading)
        .keyCardFill(color: color)
    }
}

/// Card background shared by the Key sheet cards — subtle accent tint over
/// the grouped background, matching the tip-section card style.
private extension View {
    func keyCardFill(color: Color) -> some View {
        background(
            RoundedRectangle(cornerRadius: 12)
                .fill(Color(.secondarySystemGroupedBackground))
                .overlay(RoundedRectangle(cornerRadius: 12).fill(color.opacity(0.08)))
        )
    }
}

// MARK: - Fuel-type iconography (app target only — FuelStore.swift is Foundation-only)

extension FuelType {
    /// Short segment label ("Unleaded" / "Premium" / "Diesel") for
    /// the picker and description.
    var shortName: String {
        switch self {
        case .e10: return "Unleaded"
        case .e5: return "Premium"
        case .diesel: return "Diesel"
        }
    }
}

/// Fuel-type selector styled like a segmented control, with a coloured pump
/// icon per fuel (green = unleaded, yellow = premium, cyan = diesel). Built
/// custom because the native `.segmented` picker tints every segment the same
/// accent colour — it can't show per-fuel pump colours. Shared by the Stations
/// and Favourites tabs.
struct FuelTypeSegmentedPicker: View {
    @Binding var selection: FuelType
    /// Which fuels to show. Defaults to all; the Favourites tab passes only
    /// the fuels that actually have favourites.
    var fuels: [FuelType] = FuelType.allCases
    var onSelect: (FuelType) -> Void = { _ in }

    @Namespace private var selectionSlider

    var body: some View {
        HStack(spacing: 3) {
            ForEach(fuels) { fuel in
                let isSelected = fuel == selection
                Button {
                    withAnimation(.spring(response: 0.42, dampingFraction: 0.78)) {
                        selection = fuel
                    }
                    onSelect(fuel)
                } label: {
                    HStack(spacing: 5) {
                        Image(systemName: "fuelpump.fill")
                            .font(.caption2)
                            .foregroundStyle(fuel.tintColor)
                            .scaleEffect(isSelected ? 1.08 : 0.94)
                        Text(fuel.shortName)
                            .font(.subheadline.weight(isSelected ? .semibold : .regular))
                            .contentTransition(.opacity)
                    }
                    .foregroundStyle(isSelected ? Color.primary : Color.secondary)
                    .frame(maxWidth: .infinity)
                    .padding(.vertical, 7)
                    .background {
                        if isSelected {
                            Capsule()
                                .fill(Color(UIColor.systemBackground))
                                .overlay {
                                    Capsule()
                                        .stroke(fuel.tintColor.opacity(0.28), lineWidth: 1)
                                }
                                .shadow(color: fuel.tintColor.opacity(0.16), radius: 6, y: 2)
                                .matchedGeometryEffect(id: "fuelSelectionSlider", in: selectionSlider)
                        }
                    }
                    .scaleEffect(isSelected ? 1.02 : 0.98)
                    .contentShape(Capsule())
                }
                .buttonStyle(.plain)
            }
        }
        .padding(3)
        .background(Capsule().fill(Color(.secondarySystemBackground)))
        .animation(.spring(response: 0.42, dampingFraction: 0.78), value: selection)
    }
}

#Preview("Fuel Picker") {
    @Previewable @State var selectedFuel: FuelType = .e10
    FuelTypeSegmentedPicker(selection: $selectedFuel)
        .padding()
        .background(Color(.systemGroupedBackground))
}