import SwiftUI

struct BannerCarouselItem: Identifiable, Hashable {
    let id: String
    let imageUrl: String?

    init(id: String, imageUrl: String?) {
        self.id = id
        self.imageUrl = imageUrl
    }
}

struct BannerCarousel: View {
    private static let maxItemCount = 20
    private static let autoScrollInterval: TimeInterval = 5
    private static let transitionDuration: TimeInterval = 0.35

    let items: [BannerCarouselItem]
    let action: (BannerCarouselItem) -> Void

    @State private var displayIndex = 1
    @GestureState private var dragOffset: CGFloat = 0
    @State private var autoScrollTimer: Timer?
    @State private var isSwiping = false

    init(
        items: [BannerCarouselItem],
        height: CGFloat = 120,
        action: @escaping (BannerCarouselItem) -> Void = { _ in }
    ) {
        self.items = Array(items.prefix(BannerCarousel.maxItemCount))
        self.action = action
    }

    var body: some View {
        if !items.isEmpty {
            GeometryReader { proxy in
                let itemWidth = proxy.size.width - (SodaSpacing.s20 * 2)
                let itemHeight = itemWidth
                let step = itemWidth + SodaSpacing.s8
                let leadingInset = (proxy.size.width - itemWidth) / 2
                let effectiveIndex = items.count > 1 ? displayIndex : 0
                let offset = leadingInset - (CGFloat(effectiveIndex) * step) + dragOffset

                HStack(spacing: SodaSpacing.s8) {
                    ForEach(Array(visibleItems.enumerated()), id: \.offset) { _, item in
                        Button {
                            guard !isSwiping else { return }
                            action(item)
                        } label: {
                            DownsampledKFImage(
                                url: URL(string: item.imageUrl ?? ""),
                                size: CGSize(width: itemWidth, height: itemHeight)
                            )
                            .background(Color.gray800)
                            .clipShape(RoundedRectangle(cornerRadius: SodaSpacing.s14, style: .continuous))
                        }
                        .buttonStyle(.plain)
                        .frame(width: itemWidth, height: itemHeight)
                    }
                }
                .offset(x: offset)
                .frame(width: proxy.size.width, height: itemHeight, alignment: .leading)
                .animation(.easeOut(duration: BannerCarousel.transitionDuration), value: displayIndex)
                .clipped()
                .overlay(alignment: .topTrailing) {
                    pageIndicator
                        .padding(.top, SodaSpacing.s14)
                        .padding(.trailing, SodaSpacing.s20 + SodaSpacing.s14)
                }
                .contentShape(Rectangle())
                .simultaneousGesture(dragGesture(step: step))
                .onChange(of: items) { _ in
                    displayIndex = items.count > 1 ? 1 : 0
                    restartAutoScrollTimer()
                }
            }
            .frame(height: UIScreen.main.bounds.width - (SodaSpacing.s20 * 2))
            .onAppear {
                displayIndex = items.count > 1 ? 1 : 0
                restartAutoScrollTimer()
            }
            .onDisappear {
                stopAutoScrollTimer()
            }
        }
    }

    private var visibleItems: [BannerCarouselItem] {
        guard items.count > 1, let first = items.first, let last = items.last else {
            return items
        }

        return [last] + items + [first]
    }

    private var currentPage: Int {
        guard items.count > 1 else { return items.isEmpty ? 0 : 1 }

        if displayIndex == 0 {
            return items.count
        }

        if displayIndex == items.count + 1 {
            return 1
        }

        return displayIndex
    }

    private var pageIndicator: some View {
        HStack(alignment: .center, spacing: 1) {
            Text(String(format: "%02d", currentPage))
                .foregroundColor(.white)
                .frame(width: 20)

            Text("/")
                .foregroundColor(.gray400)

            Text(String(format: "%02d", items.count))
                .foregroundColor(.gray400)
                .frame(width: 20)
        }
        .appFont(.body3)
        .padding(.horizontal, SodaSpacing.s6)
        .padding(.vertical, SodaSpacing.s4)
        .background(Color.black.opacity(0.7))
        .clipShape(Capsule())
    }

    private func dragGesture(step: CGFloat) -> some Gesture {
        DragGesture(minimumDistance: 10)
            .updating($dragOffset) { value, state, _ in
                state = value.translation.width
            }
            .onChanged { _ in
                if !isSwiping {
                    isSwiping = true
                }
            }
            .onEnded { value in
                defer {
                    DispatchQueue.main.async {
                        isSwiping = false
                    }
                }

                guard items.count > 1 else { return }

                let threshold = step * 0.2
                if value.translation.width < -threshold {
                    displayIndex += 1
                } else if value.translation.width > threshold {
                    displayIndex -= 1
                }

                wrapIfNeeded()
                restartAutoScrollTimer()
            }
    }

    private func restartAutoScrollTimer() {
        stopAutoScrollTimer()
        guard items.count > 1 else { return }

        autoScrollTimer = Timer.scheduledTimer(
            withTimeInterval: BannerCarousel.autoScrollInterval,
            repeats: true
        ) { _ in
            displayIndex += 1
            wrapIfNeeded()
        }
    }

    private func stopAutoScrollTimer() {
        autoScrollTimer?.invalidate()
        autoScrollTimer = nil
    }

    private func wrapIfNeeded() {
        guard items.count > 1 else { return }

        if displayIndex == 0 {
            DispatchQueue.main.asyncAfter(deadline: .now() + BannerCarousel.transitionDuration) {
                var transaction = Transaction()
                transaction.disablesAnimations = true
                withTransaction(transaction) {
                    displayIndex = items.count
                }
            }
        } else if displayIndex == items.count + 1 {
            DispatchQueue.main.asyncAfter(deadline: .now() + BannerCarousel.transitionDuration) {
                var transaction = Transaction()
                transaction.disablesAnimations = true
                withTransaction(transaction) {
                    displayIndex = 1
                }
            }
        }
    }
}

struct BannerCarousel_Previews: PreviewProvider {
    static var previews: some View {
        BannerCarousel(
            items: [
                BannerCarouselItem(id: "1", imageUrl: "https://picsum.photos/500/500"),
                BannerCarouselItem(id: "2", imageUrl: "https://picsum.photos/500/500"),
                BannerCarouselItem(id: "3", imageUrl: "https://picsum.photos/500/500")
            ]
        )
        .background(Color.black)
        .previewLayout(.sizeThatFits)
    }
}