//
//  BeforeAfterComparisonViewController.swift
//  suixinkan
//

import Kingfisher
import SnapKit
import UIKit

/// 暗色全屏前后图片对比页，通过可拖拽分隔线查看原图与效果图。
final class BeforeAfterComparisonViewController: UIViewController {
    private let viewModel: BeforeAfterComparisonViewModel
    private let backButton = UIButton(type: .system)
    private let comparisonContainer = UIView()
    private let afterImageView = UIImageView()
    private let beforeImageView = UIImageView()
    private let beforeRevealMask = CAShapeLayer()
    private let dividerLine = UIView()
    private let dividerHandle = BeforeAfterComparisonHandleView()
    private let beforeLabel = BeforeAfterComparisonLabel()
    private let afterLabel = BeforeAfterComparisonLabel()
    private let statusContainer = UIView()
    private let activityIndicator = UIActivityIndicatorView(style: .large)
    private let statusLabel = UILabel()
    private let retryButton = UIButton(type: .system)

    private var dividerFraction: CGFloat = 0.5
    private var loadGeneration = 0
    private var beforeLoaded = false
    private var afterLoaded = false
    private var isReady = false
    private var displayLink: CADisplayLink?
    private var animationStartTime: CFTimeInterval?
    private var isDraggingDivider = false
    private var imageAspectRatio = BeforeAfterComparisonLayout.fallbackAspectRatio
    private var beforeImageSize: CGSize?
    private var afterImageSize: CGSize?

    /// 创建指定前后图片内容的全屏对比页。
    init(viewModel: BeforeAfterComparisonViewModel) {
        self.viewModel = viewModel
        super.init(nibName: nil, bundle: nil)
        modalPresentationStyle = .fullScreen
    }

    @available(*, unavailable)
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override var preferredStatusBarStyle: UIStatusBarStyle { .lightContent }

    override func viewDidLoad() {
        super.viewDidLoad()
        setupUI()
        setupConstraints()
        bindActions()
        loadImages()
    }

    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        layoutComparisonContainer()
        layoutDivider()
    }

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        startAutomaticMotion(from: dividerFraction)
    }

    override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)
        stopAutomaticMotion()
    }

    override func viewDidDisappear(_ animated: Bool) {
        super.viewDidDisappear(animated)
        displayLink?.invalidate()
        displayLink = nil
    }

    deinit {
        displayLink?.invalidate()
        NotificationCenter.default.removeObserver(self)
    }

    private func setupUI() {
        view.backgroundColor = UIColor(hex: 0x05070D)
        view.accessibilityIdentifier = "travelAlbum.beforeAfterComparison"

        var backConfiguration = UIButton.Configuration.filled()
        backConfiguration.image = UIImage(systemName: "chevron.left")
        backConfiguration.preferredSymbolConfigurationForImage = UIImage.SymbolConfiguration(
            pointSize: 19,
            weight: .semibold
        )
        backConfiguration.baseForegroundColor = .white
        backConfiguration.baseBackgroundColor = UIColor.white.withAlphaComponent(0.12)
        backConfiguration.background.cornerRadius = 24
        backConfiguration.contentInsets = .zero
        backButton.configuration = backConfiguration
        backButton.accessibilityLabel = "返回"
        backButton.accessibilityIdentifier = "travelAlbum.beforeAfterBackButton"

        comparisonContainer.backgroundColor = UIColor(hex: 0x111827)
        comparisonContainer.layer.cornerRadius = 0
        comparisonContainer.layer.borderColor = UIColor(hex: 0x263244).cgColor
        comparisonContainer.layer.borderWidth = 1
        comparisonContainer.clipsToBounds = true
        comparisonContainer.accessibilityIdentifier = "travelAlbum.beforeAfterImageContainer"

        [afterImageView, beforeImageView].forEach { imageView in
            imageView.contentMode = .scaleAspectFit
            imageView.clipsToBounds = true
            imageView.backgroundColor = UIColor(hex: 0x111827)
            imageView.isHidden = true
        }
        afterImageView.accessibilityLabel = viewModel.content.afterLabel
        beforeImageView.accessibilityLabel = viewModel.content.beforeLabel
        beforeImageView.layer.mask = beforeRevealMask

        dividerLine.backgroundColor = UIColor.white.withAlphaComponent(0.72)
        dividerLine.layer.shadowColor = UIColor.black.cgColor
        dividerLine.layer.shadowOpacity = 0.35
        dividerLine.layer.shadowRadius = 2
        dividerLine.layer.shadowOffset = .zero
        dividerLine.isHidden = true
        dividerLine.isUserInteractionEnabled = false
        dividerHandle.isHidden = true
        dividerHandle.accessibilityIdentifier = "travelAlbum.beforeAfterDivider"

        beforeLabel.text = viewModel.content.beforeLabel
        beforeLabel.accessibilityIdentifier = "travelAlbum.beforeLabel"
        afterLabel.text = viewModel.content.afterLabel
        afterLabel.accessibilityIdentifier = "travelAlbum.afterLabel"
        beforeLabel.isHidden = true
        afterLabel.isHidden = true

        statusContainer.backgroundColor = UIColor(hex: 0x111827).withAlphaComponent(0.86)
        activityIndicator.color = .white
        activityIndicator.hidesWhenStopped = true
        statusLabel.textColor = UIColor.white.withAlphaComponent(0.78)
        statusLabel.font = .systemFont(ofSize: 14, weight: .medium)
        statusLabel.textAlignment = .center
        statusLabel.numberOfLines = 0
        statusLabel.text = "正在加载对比图片"
        statusLabel.accessibilityIdentifier = "travelAlbum.beforeAfterStatusLabel"
        var retryConfiguration = UIButton.Configuration.filled()
        retryConfiguration.title = "重试"
        retryConfiguration.baseForegroundColor = .white
        retryConfiguration.baseBackgroundColor = UIColor(hex: 0x1677FF)
        retryConfiguration.background.cornerRadius = 12
        retryButton.configuration = retryConfiguration
        retryButton.accessibilityIdentifier = "travelAlbum.beforeAfterRetryButton"
        retryButton.isHidden = true

        view.addSubview(backButton)
        view.addSubview(comparisonContainer)
        comparisonContainer.addSubview(afterImageView)
        comparisonContainer.addSubview(beforeImageView)
        comparisonContainer.addSubview(dividerLine)
        comparisonContainer.addSubview(dividerHandle)
        comparisonContainer.addSubview(beforeLabel)
        comparisonContainer.addSubview(afterLabel)
        comparisonContainer.addSubview(statusContainer)
        statusContainer.addSubview(activityIndicator)
        statusContainer.addSubview(statusLabel)
        statusContainer.addSubview(retryButton)
    }

    private func setupConstraints() {
        backButton.snp.makeConstraints { make in
            make.top.equalTo(view.safeAreaLayoutGuide).offset(20)
            make.leading.equalToSuperview().offset(18)
            make.size.equalTo(48)
        }
        afterImageView.snp.makeConstraints { make in
            make.edges.equalToSuperview()
        }
        beforeImageView.snp.makeConstraints { make in
            make.edges.equalToSuperview()
        }
        beforeLabel.snp.makeConstraints { make in
            make.leading.bottom.equalToSuperview().inset(10)
            make.height.equalTo(26)
        }
        afterLabel.snp.makeConstraints { make in
            make.trailing.bottom.equalToSuperview().inset(10)
            make.height.equalTo(26)
        }
        statusContainer.snp.makeConstraints { make in
            make.edges.equalToSuperview()
        }
        activityIndicator.snp.makeConstraints { make in
            make.centerX.equalToSuperview()
            make.centerY.equalToSuperview().offset(-28)
        }
        statusLabel.snp.makeConstraints { make in
            make.top.equalTo(activityIndicator.snp.bottom).offset(14)
            make.leading.trailing.equalToSuperview().inset(32)
        }
        retryButton.snp.makeConstraints { make in
            make.top.equalTo(statusLabel.snp.bottom).offset(14)
            make.centerX.equalToSuperview()
            make.width.equalTo(96)
            make.height.equalTo(44)
        }
    }

    private func bindActions() {
        backButton.addTarget(self, action: #selector(backTapped), for: .touchUpInside)
        retryButton.addTarget(self, action: #selector(retryTapped), for: .touchUpInside)
        let scrubberGesture = UILongPressGestureRecognizer(target: self, action: #selector(dividerDragged(_:)))
        scrubberGesture.minimumPressDuration = 0
        scrubberGesture.allowableMovement = .greatestFiniteMagnitude
        comparisonContainer.addGestureRecognizer(scrubberGesture)
        dividerHandle.onIncrement = { [weak self] in self?.adjustDivider(by: 0.1) }
        dividerHandle.onDecrement = { [weak self] in self?.adjustDivider(by: -0.1) }
        NotificationCenter.default.addObserver(
            self,
            selector: #selector(reduceMotionStatusChanged),
            name: UIAccessibility.reduceMotionStatusDidChangeNotification,
            object: nil
        )
    }

    private func loadImages() {
        loadGeneration += 1
        let generation = loadGeneration
        beforeImageView.kf.cancelDownloadTask()
        afterImageView.kf.cancelDownloadTask()
        beforeImageView.image = nil
        afterImageView.image = nil
        beforeLoaded = false
        afterLoaded = false
        beforeImageSize = nil
        afterImageSize = nil
        imageAspectRatio = BeforeAfterComparisonLayout.fallbackAspectRatio
        isReady = false
        dividerFraction = BeforeAfterComparisonMotion.automaticCenter
        stopAutomaticMotion()
        setComparisonVisible(false)
        statusContainer.isHidden = false
        statusLabel.text = "正在加载对比图片"
        retryButton.isHidden = true
        activityIndicator.startAnimating()

        guard viewModel.isValid,
              let beforeURL = viewModel.beforeImageURL,
              let afterURL = viewModel.afterImageURL
        else {
            showLoadFailure()
            return
        }

        beforeImageView.kf.setImage(with: beforeURL) { [weak self] result in
            self?.handleImageResult(result, isBefore: true, generation: generation)
        }
        afterImageView.kf.setImage(with: afterURL) { [weak self] result in
            self?.handleImageResult(result, isBefore: false, generation: generation)
        }
    }

    private func handleImageResult(
        _ result: Result<RetrieveImageResult, KingfisherError>,
        isBefore: Bool,
        generation: Int
    ) {
        guard generation == loadGeneration else { return }
        switch result {
        case let .success(value):
            if isBefore {
                beforeLoaded = true
                beforeImageSize = value.image.size
            } else {
                afterLoaded = true
                afterImageSize = value.image.size
            }
            guard beforeLoaded && afterLoaded else { return }
            let resolvedSize = afterImageSize ?? beforeImageSize ?? .zero
            imageAspectRatio = BeforeAfterComparisonLayout.aspectRatio(for: resolvedSize)
            view.setNeedsLayout()
            view.layoutIfNeeded()
            isReady = true
            activityIndicator.stopAnimating()
            statusContainer.isHidden = true
            setComparisonVisible(true)
            layoutDivider()
            startAutomaticMotion(from: dividerFraction)
            UIAccessibility.post(notification: .announcement, argument: "对比图片已加载")
        case .failure:
            showLoadFailure()
        }
    }

    private func showLoadFailure() {
        isReady = false
        activityIndicator.stopAnimating()
        statusContainer.isHidden = false
        statusLabel.text = "图片加载失败，请重试"
        retryButton.isHidden = false
        setComparisonVisible(false)
        UIAccessibility.post(notification: .announcement, argument: statusLabel.text)
    }

    private func setComparisonVisible(_ visible: Bool) {
        afterImageView.isHidden = !visible
        beforeImageView.isHidden = !visible
        dividerLine.isHidden = !visible
        dividerHandle.isHidden = !visible
        beforeLabel.isHidden = !visible
        afterLabel.isHidden = !visible
    }

    private func layoutComparisonContainer() {
        let safeFrame = view.bounds.inset(by: view.safeAreaInsets)
        let topBoundary = max(safeFrame.minY, backButton.frame.maxY + 24)
        let bottomBoundary = safeFrame.maxY - 24
        let center = CGPoint(x: view.bounds.midX, y: safeFrame.midY)
        let verticalRadius = max(
            1,
            min(center.y - topBoundary, bottomBoundary - center.y)
        )
        let maximumSize = CGSize(
            width: max(1, view.bounds.width),
            height: verticalRadius * 2
        )
        let fittedSize = BeforeAfterComparisonLayout.fittedSize(
            aspectRatio: imageAspectRatio,
            maximumSize: maximumSize
        )
        comparisonContainer.frame = CGRect(
            x: center.x - fittedSize.width / 2,
            y: center.y - fittedSize.height / 2,
            width: fittedSize.width,
            height: fittedSize.height
        )
        comparisonContainer.layoutIfNeeded()
    }

    private func layoutDivider() {
        guard comparisonContainer.bounds.width > 0 else { return }
        let bounds = comparisonContainer.bounds
        let x = bounds.width * dividerFraction
        beforeRevealMask.frame = beforeImageView.bounds
        beforeRevealMask.path = UIBezierPath(
            rect: CGRect(x: 0, y: 0, width: x, height: bounds.height)
        ).cgPath
        dividerLine.frame = CGRect(x: x - 1.5, y: 0, width: 3, height: bounds.height)
        dividerHandle.frame = CGRect(x: x - 24, y: bounds.midY - 24, width: 48, height: 48)
        dividerHandle.accessibilityValue = "原图占比 \(Int((dividerFraction * 100).rounded()))%"
    }

    private func updateDivider(locationX: CGFloat) {
        guard isReady, comparisonContainer.bounds.width > 0 else { return }
        dividerFraction = BeforeAfterComparisonMotion.clampedManualFraction(
            locationX / comparisonContainer.bounds.width
        )
        layoutDivider()
    }

    private func adjustDivider(by delta: CGFloat) {
        stopAutomaticMotion()
        dividerFraction = BeforeAfterComparisonMotion.clampedManualFraction(dividerFraction + delta)
        layoutDivider()
    }

    @objc private func dividerDragged(_ gesture: UILongPressGestureRecognizer) {
        switch gesture.state {
        case .began:
            isDraggingDivider = true
            stopAutomaticMotion()
            dividerHandle.setActive(true, animated: !UIAccessibility.isReduceMotionEnabled)
            updateDivider(locationX: gesture.location(in: comparisonContainer).x)
        case .changed:
            updateDivider(locationX: gesture.location(in: comparisonContainer).x)
        case .ended, .cancelled, .failed:
            updateDivider(locationX: gesture.location(in: comparisonContainer).x)
            isDraggingDivider = false
            dividerHandle.setActive(false, animated: !UIAccessibility.isReduceMotionEnabled)
            startAutomaticMotion(from: dividerFraction)
        default:
            break
        }
    }

    @objc private func automaticMotionTick(_ link: CADisplayLink) {
        guard isReady, !isDraggingDivider, let animationStartTime else { return }
        dividerFraction = BeforeAfterComparisonMotion.automaticFraction(
            elapsed: link.timestamp - animationStartTime
        )
        layoutDivider()
    }

    private func startAutomaticMotion(from fraction: CGFloat) {
        guard isReady,
              viewIfLoaded?.window != nil,
              !isDraggingDivider,
              !UIAccessibility.isReduceMotionEnabled
        else { return }
        animationStartTime = CACurrentMediaTime() - BeforeAfterComparisonMotion.phaseTime(for: fraction)
        if displayLink == nil {
            let link = CADisplayLink(target: self, selector: #selector(automaticMotionTick(_:)))
            link.preferredFrameRateRange = CAFrameRateRange(minimum: 30, maximum: 60, preferred: 60)
            link.add(to: .main, forMode: .common)
            displayLink = link
        }
        displayLink?.isPaused = false
    }

    private func stopAutomaticMotion() {
        displayLink?.isPaused = true
        animationStartTime = nil
    }

    @objc private func reduceMotionStatusChanged() {
        if UIAccessibility.isReduceMotionEnabled {
            stopAutomaticMotion()
        } else {
            startAutomaticMotion(from: dividerFraction)
        }
    }

    @objc private func backTapped() {
        dismiss(animated: true)
    }

    @objc private func retryTapped() {
        loadImages()
    }
}

/// 前后对比分隔线的可调节圆形手柄。
private final class BeforeAfterComparisonHandleView: UIView {
    var onIncrement: (() -> Void)?
    var onDecrement: (() -> Void)?

    override init(frame: CGRect) {
        super.init(frame: frame)
        backgroundColor = UIColor.black.withAlphaComponent(0.38)
        layer.cornerRadius = 24
        layer.borderColor = UIColor.white.withAlphaComponent(0.82).cgColor
        layer.borderWidth = 2
        layer.shadowColor = UIColor.black.cgColor
        layer.shadowOpacity = 0.32
        layer.shadowRadius = 7
        layer.shadowOffset = CGSize(width: 0, height: 3)
        isAccessibilityElement = true
        accessibilityLabel = "前后对比分隔线"
        accessibilityTraits = [.adjustable]

        let imageView = UIImageView(image: UIImage(systemName: "arrow.left.and.right"))
        imageView.tintColor = .white
        imageView.contentMode = .scaleAspectFit
        addSubview(imageView)
        imageView.snp.makeConstraints { make in
            make.center.equalToSuperview()
            make.size.equalTo(21)
        }
    }

    /// 更新手柄按压态，为拖动接管提供轻微但不改变布局的视觉反馈。
    func setActive(_ active: Bool, animated: Bool) {
        let changes = {
            self.transform = active ? CGAffineTransform(scaleX: 1.08, y: 1.08) : .identity
            self.backgroundColor = UIColor.black.withAlphaComponent(active ? 0.52 : 0.38)
        }
        guard animated else {
            changes()
            return
        }
        UIView.animate(
            withDuration: 0.18,
            delay: 0,
            options: [.curveEaseOut, .beginFromCurrentState],
            animations: changes
        )
    }

    @available(*, unavailable)
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override func accessibilityIncrement() {
        onIncrement?()
    }

    override func accessibilityDecrement() {
        onDecrement?()
    }
}

/// 前后对比图片底部的高对比度胶囊角标。
private final class BeforeAfterComparisonLabel: UILabel {
    override init(frame: CGRect) {
        super.init(frame: frame)
        textColor = .white
        font = .systemFont(ofSize: 12, weight: .medium)
        textAlignment = .center
        backgroundColor = UIColor.black.withAlphaComponent(0.5)
        layer.cornerRadius = 13
        layer.borderColor = UIColor.white.withAlphaComponent(0.2).cgColor
        layer.borderWidth = 0.5
        clipsToBounds = true
        setContentHuggingPriority(.required, for: .horizontal)
        layoutMargins = UIEdgeInsets(top: 0, left: 10, bottom: 0, right: 10)
    }

    @available(*, unavailable)
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override var intrinsicContentSize: CGSize {
        let size = super.intrinsicContentSize
        return CGSize(width: size.width + 20, height: 26)
    }
}