import SwiftUI

import Kingfisher

struct SubBannerCarousel: View {
    let items: [SubBanner]
    let onTap: (SubBanner) -> Void

    @Environment(\.scenePhase) private var scenePhase
    @State private var displayIndex = 1
    @State private var itemsRevision = 0
    @State private var containerWidth: CGFloat = 0
    @State private var imageRatios: [String: CGFloat] = [:]
    @State private var lastKnownRatio: CGFloat?
    @GestureState private var isSwiping = false
    @GestureState private var dragOffset: CGFloat = 0

    private static let autoScrollNanoseconds: UInt64 = 5_000_000_000
    private static let transitionDuration: TimeInterval = 0.35

    private var safeDisplayIndex: Int {
        items.count > 1 ? min(max(displayIndex, 0), items.count + 1) : 0
    }

    private var safeIndex: Int {
        items.count > 1 ? (safeDisplayIndex + items.count - 1) % items.count : 0
    }

    private var pages: [(id: String, item: SubBanner)] {
        let originals = items.map { (id: "item-\($0.id)", item: $0) }
        guard items.count > 1, let first = items.first, let last = items.last else { return originals }
        return [(id: "leading-\(last.id)", item: last)] + originals + [(id: "trailing-\(first.id)", item: first)]
    }

    private var currentRatio: CGFloat? {
        guard !items.isEmpty else { return nil }
        return imageRatios[items[safeIndex].imageUrl]
    }

    private var height: CGFloat {
        guard !items.isEmpty, let ratio = currentRatio ?? lastKnownRatio else { return 0 }
        return containerWidth / ratio
    }

    var body: some View {
        GeometryReader { geometry in
            HStack(spacing: 0) {
                ForEach(pages, id: \.id) { page in
                    Button {
                        onTap(page.item)
                    } label: {
                        KFImage(page.item.imageURL)
                            .placeholder { Color.gray800 }
                            .onSuccess { result in
                                let size = result.image.size
                                guard size.width > 0, size.height > 0 else { return }
                                imageRatios[page.item.imageUrl] = size.width / size.height
                            }
                            .startLoadingBeforeViewAppear()
                            .resizable()
                            .scaledToFit()
                            .frame(width: geometry.size.width, height: height)
                            .background(Color.gray800)
                    }
                    .buttonStyle(SubBannerTapStyle(drag: dragGesture(width: geometry.size.width)))
                    .accessibilityLabel(Text("\(safeIndex + 1) / \(items.count)"))
                    .accessibilityHidden(pages[safeDisplayIndex].id != page.id)
                }
            }
            .offset(x: -CGFloat(safeDisplayIndex) * geometry.size.width + dragOffset)
            .animation(.easeOut(duration: Self.transitionDuration), value: displayIndex)
            .frame(width: geometry.size.width, height: height, alignment: .leading)
            .contentShape(Rectangle())
            .clipped()
            .onAppear { containerWidth = geometry.size.width }
            .onChange(of: geometry.size.width) { containerWidth = $0 }
        }
        .frame(height: height)
        .onChange(of: currentRatio) { ratio in
            if let ratio { lastKnownRatio = ratio }
        }
        .onChange(of: items) { _ in
            itemsRevision += 1
            normalize(to: items.count > 1 ? 1 : 0)
            if items.isEmpty { lastKnownRatio = nil }
        }
        .onDisappear {
            itemsRevision += 1
            normalize(to: items.count > 1 ? safeIndex + 1 : 0)
        }
        .task(id: scenePhase == .active && items.count > 1 ? items : []) {
            guard scenePhase == .active, items.count > 1 else { return }
            while !Task.isCancelled {
                do {
                    try await Task.sleep(nanoseconds: Self.autoScrollNanoseconds)
                } catch {
                    return
                }
                guard !Task.isCancelled else { return }
                if !isSwiping { move(by: 1) }
            }
        }
        .accessibilityAdjustableAction { direction in
            switch direction {
            case .increment: move(by: 1)
            case .decrement: move(by: -1)
            @unknown default: break
            }
        }
    }

    private func dragGesture(width: CGFloat) -> some Gesture {
        DragGesture(minimumDistance: SodaSpacing.s12, coordinateSpace: .global)
            .updating($dragOffset) { value, offset, _ in
                if items.count > 1, abs(value.translation.width) > abs(value.translation.height) {
                    offset = value.translation.width
                }
            }
            .updating($isSwiping) { _, swiping, _ in swiping = true }
            .onEnded { value in
                guard abs(value.translation.width) > abs(value.translation.height) else { return }
                if value.translation.width < -width * 0.2 {
                    move(by: 1)
                } else if value.translation.width > width * 0.2 {
                    move(by: -1)
                }
            }
    }

    private func move(by step: Int) {
        guard items.count > 1, (1...items.count).contains(displayIndex) else { return }
        let next = displayIndex + step
        displayIndex = next
        guard next == 0 || next == items.count + 1 else { return }
        let revision = itemsRevision
        DispatchQueue.main.asyncAfter(deadline: .now() + Self.transitionDuration) {
            guard itemsRevision == revision, displayIndex == next else { return }
            normalize(to: next == 0 ? items.count : 1)
        }
    }

    private func normalize(to index: Int) {
        var transaction = Transaction()
        transaction.disablesAnimations = true
        withTransaction(transaction) { displayIndex = index }
    }
}

private struct SubBannerTapStyle<Drag: Gesture>: PrimitiveButtonStyle {
    let drag: Drag

    func makeBody(configuration: Configuration) -> some View {
        configuration.label
            .contentShape(Rectangle())
            .simultaneousGesture(drag.exclusively(before: TapGesture().onEnded { configuration.trigger() }))
            .accessibilityAction { configuration.trigger() }
    }
}