import SwiftUI
import FlixyDesign
import FlixyDownloads
import FlixyPlayback

/// Was offline ist und was gerade geladen wird.
struct DownloadsView: View {
    @Environment(DownloadCoordinator.self) private var downloads
    @Environment(PlaybackCoordinator.self) private var playback
    @Environment(NetworkConditions.self) private var network
    @Environment(\.flixyAccent) private var accent

    @State private var itemPendingRemoval: DownloadItem?

    var body: some View {
        Group {
            if downloads.items.isEmpty {
                ContentUnavailableView {
                    Label("downloads.empty.title", systemImage: "arrow.down.circle")
                } description: {
                    Text("downloads.empty.description")
                }
            } else {
                list
            }
        }
        .background(Color.flixyCanvas)
        .navigationTitle(Text("sidebar.downloads"))
        .navigationBarTitleDisplayMode(.inline)
        .confirmationDialog(
            Text("downloads.removeConfirm"),
            isPresented: Binding(
                get: { itemPendingRemoval != nil },
                set: { if !$0 { itemPendingRemoval = nil } }
            ),
            titleVisibility: .visible
        ) {
            Button("action.delete", role: .destructive) {
                if let item = itemPendingRemoval {
                    Task { await downloads.cancel(item) }
                }
                itemPendingRemoval = nil
            }
            Button("action.cancel", role: .cancel) { itemPendingRemoval = nil }
        }
    }

    /// Was die Übertragung gerade aufhält.
    private var hold: TransferHold {
        TransferHold.current(
            isWiFiOnly: downloads.isWiFiOnly,
            isReachable: network.isReachable,
            isExpensive: network.isExpensive
        )
    }

    private var list: some View {
        List {
            StorageSection()

            // Erst was läuft, dann was fertig ist. Wer die Ansicht öffnet,
            // während etwas lädt, will meistens genau das sehen.
            if !active.isEmpty {
                Section("downloads.active") {
                    // Die Hintergrund-Sitzung wartet in diesen Fällen von
                    // sich aus: kein Fehler, nur Stillstand. Ohne diesen
                    // Hinweis sieht ein wartender Download genauso aus wie
                    // ein hängender.
                    if hold.isHolding { HoldNotice(hold: hold) }

                    ForEach(active) { item in
                        DownloadRow(
                            item: item,
                            hold: hold,
                            onRemove: { itemPendingRemoval = item }
                        )
                    }
                }
            }

            ForEach(completedGroups, id: \.title) { group in
                Section(group.title) {
                    ForEach(group.items) { item in
                        // Fertige Downloads wartet nichts mehr auf.
                        DownloadRow(item: item, hold: .none, onRemove: { itemPendingRemoval = item })
                            .contentShape(.rect)
                            .onTapGesture { play(item) }
                    }
                }
            }
        }
        .scrollContentBackground(.hidden)
        .listRowBackground(Color.flixySurface)
    }

    private var active: [DownloadItem] {
        downloads.items.filter { $0.state != .completed }
    }

    /// Fertige Downloads, nach Serie gruppiert; Filme stehen zusammen.
    private var completedGroups: [(title: String, items: [DownloadItem])] {
        let completed = downloads.items.filter { $0.state == .completed }
        let grouped = Dictionary(grouping: completed) {
            $0.seriesTitle ?? String(localized: "downloads.movies")
        }
        return grouped
            .map { (title: $0.key, items: $0.value.sorted(by: episodeOrder)) }
            .sorted { $0.title.localizedStandardCompare($1.title) == .orderedAscending }
    }

    private func episodeOrder(_ lhs: DownloadItem, _ rhs: DownloadItem) -> Bool {
        (lhs.seasonIndex ?? 0, lhs.episodeIndex ?? 0) < (rhs.seasonIndex ?? 0, rhs.episodeIndex ?? 0)
    }

    private func play(_ item: DownloadItem) {
        guard item.state == .completed else { return }
        playback.request = PlaybackRequest(
            ratingKey: item.ratingKey,
            title: item.title,
            mediaIndex: 0,
            startTime: .zero
        )
    }
}

/// Die Kopfzeile mit Speicherbelegung und Schaltern.
private struct StorageSection: View {
    @Environment(DownloadCoordinator.self) private var downloads
    @Environment(\.flixyAccent) private var accent

    var body: some View {
        @Bindable var downloads = downloads

        Section {
            HStack {
                Text("downloads.used")
                    .foregroundStyle(Color.flixyTextSecondary)
                Spacer()
                Text(downloads.usedBytes.formatted(.byteCount(style: .file)))
                    .foregroundStyle(Color.flixyTextPrimary)
            }

            if let limit = downloads.storageLimitBytes, limit > 0 {
                ProgressView(value: min(Double(downloads.usedBytes) / Double(limit), 1))
                    .tint(accent)
                    .accessibilityLabel(Text("downloads.used"))
            }

            Toggle("downloads.wifiOnly", isOn: $downloads.isWiFiOnly)
        }
    }
}

/// Eine Zeile in der Downloads-Ansicht.
private struct DownloadRow: View {
    let item: DownloadItem
    let hold: TransferHold
    let onRemove: () -> Void

    @Environment(DownloadCoordinator.self) private var downloads
    @Environment(\.flixyAccent) private var accent

    var body: some View {
        VStack(alignment: .leading, spacing: FlixySpacing.snug) {
            HStack(spacing: FlixySpacing.regular) {
                VStack(alignment: .leading, spacing: 2) {
                    Text(item.title)
                        .font(.flixyCardTitle)
                        .foregroundStyle(Color.flixyTextPrimary)
                        .lineLimit(1)
                    if let subtitle = item.subtitle {
                        Text(subtitle)
                            .font(.flixyCaption)
                            .foregroundStyle(Color.flixyTextMuted)
                            .lineLimit(1)
                    }
                }

                Spacer()
                action
            }

            if item.state != .completed {
                progress
            }

            // Nur bei dem, was tatsächlich warten würde. Ein vom Nutzer
            // angehaltener Download wartet nicht auf WLAN, sondern auf ihn.
            if hold.isHolding, item.state == .downloading || item.state == .queued {
                Label(
                    hold == .offline ? "downloads.hold.offline" : "downloads.hold.wifi",
                    systemImage: hold == .offline ? "wifi.slash" : "wifi.exclamationmark"
                )
                .font(.flixyCaption)
                .foregroundStyle(Color.flixyTextMuted)
            }

            if let message = item.failureMessage {
                Text(message)
                    .font(.flixyCaption)
                    .foregroundStyle(Color.flixyDestructive)
                    .lineLimit(2)
            }
        }
        .padding(.vertical, FlixySpacing.tight)
        .listRowBackground(Color.flixySurface)
        .swipeActions(edge: .trailing) {
            Button("action.delete", role: .destructive, action: onRemove)
        }
    }

    @ViewBuilder
    private var action: some View {
        switch item.state {
        case .downloading:
            Button {
                Task { await downloads.pause(item) }
            } label: {
                Image(systemName: "pause.circle").font(.title3)
            }
            .buttonStyle(.plain)
            .foregroundStyle(accent)
            .accessibilityLabel(Text("downloads.pause"))

        case .paused, .failed:
            Button {
                Task { await downloads.resume(item) }
            } label: {
                Image(systemName: "arrow.clockwise.circle").font(.title3)
            }
            .buttonStyle(.plain)
            .foregroundStyle(accent)
            .accessibilityLabel(Text("downloads.resume"))

        case .completed:
            Image(systemName: "checkmark.circle.fill")
                .foregroundStyle(Color.flixyTextMuted)
                .accessibilityLabel(Text("downloads.available"))

        case .queued, .waiting:
            Image(systemName: "clock")
                .foregroundStyle(Color.flixyTextMuted)
                .accessibilityLabel(Text("downloads.queued"))
        }
    }

    private var progress: some View {
        VStack(alignment: .leading, spacing: FlixySpacing.tight) {
            // Ohne bekannte Gesamtgröße ein unbestimmter Balken: ein Balken bei
            // null Prozent, der sich nie bewegt, sieht aus wie ein hängender
            // Download.
            if let value = item.progress {
                ProgressView(value: value).tint(accent)
            } else {
                ProgressView().progressViewStyle(.linear).tint(accent)
            }

            Text(statusLine)
                .font(.flixyCaption)
                .foregroundStyle(Color.flixyTextMuted)
        }
    }

    private var statusLine: String {
        switch item.state {
        case .downloading where item.totalBytes > 0:
            "\(item.receivedBytes.formatted(.byteCount(style: .file))) / \(item.totalBytes.formatted(.byteCount(style: .file)))"
        case .downloading:
            item.receivedBytes.formatted(.byteCount(style: .file))
        case .paused:
            String(localized: "downloads.paused")
        case .queued, .waiting:
            String(localized: "downloads.queued")
        case .failed:
            String(localized: "downloads.failed")
        case .completed:
            item.totalBytes.formatted(.byteCount(style: .file))
        }
    }
}


/// Der Hinweis über der Liste, wenn nichts läuft und nichts kaputt ist.
private struct HoldNotice: View {
    let hold: TransferHold

    @Environment(\.flixyAccent) private var accent

    var body: some View {
        HStack(alignment: .top, spacing: FlixySpacing.regular) {
            Image(systemName: hold == .offline ? "wifi.slash" : "wifi.exclamationmark")
                .font(.title3)
                .foregroundStyle(accent)

            VStack(alignment: .leading, spacing: 2) {
                Text(hold == .offline ? "downloads.hold.offline" : "downloads.hold.wifi")
                    .font(.flixyCardTitle)
                    .foregroundStyle(Color.flixyTextPrimary)

                Text(hold == .offline
                    ? "downloads.hold.offline.description"
                    : "downloads.hold.wifi.description")
                    .font(.flixyCaption)
                    .foregroundStyle(Color.flixyTextSecondary)
            }
        }
        .padding(.vertical, FlixySpacing.tight)
        .listRowBackground(Color.flixySurface)
        .accessibilityElement(children: .combine)
    }
}