import UIKit

/// 页面创建、缩放状态和点击分区。与 EPUB 的 `RDEpubReaderView+ContentAccess`
/// 及 Paging 模块保持同一职责边界。
extension RDPDFReaderView {
    func makePageView(for page: Int) -> UIView {
        let view = dataSource?.pageContentView(readerView: self, pageNum: page) ?? UIView()
        view.tag = page
        configureInteraction(for: view)
        return view
    }

    func configureInteraction(for view: UIView) {
        guard let page = view as? RDPDFReaderPageInteractable else { return }
        page.readerSetInternalGesturesEnabled(currentDisplayType != .pageCurl)
        page.readerContentTapHandler = { [weak self, weak view] point in
            guard let self, let view else { return }
            self.handleContentTap(view.convert(point, to: self))
        }
        page.readerZoomStateChangedHandler = { [weak self, weak view] zoomed in
            guard let self, view?.tag == self.currentPage else { return }
            self.isCurrentPageZoomed = zoomed
            self.updatePagingState()
        }
        page.readerSelectionStateChangedHandler = { [weak self, weak view] active in
            guard let self, view?.tag == self.currentPage else { return }
            self.isCurrentPageTextSelectionActive = active
            self.updatePagingState()
        }
    }

    func synchronizePageState() {
        isCurrentPageZoomed = (pageContentView(pageNum: currentPage) as? RDPDFReaderPageInteractable)?.readerIsZoomed ?? false
        isCurrentPageTextSelectionActive = (pageContentView(pageNum: currentPage) as? RDPDFReaderPageInteractable)?.readerHasActiveTextSelection ?? false
        updatePagingState()
    }

    func updatePagingState() {
        let enabled = isPagingEnabled && !isCurrentPageZoomed && !isCurrentPageTextSelectionActive
        collectionView.isScrollEnabled = enabled
        curlPanGestureRecognizer.isEnabled = currentDisplayType == .pageCurl
            && isPagingEnabled
            && !isCurrentPageTextSelectionActive
    }

    func transitionCurl(to page: Int, animated: Bool) {
        if isTransitioning {
            pendingPage = (page, animated)
            return
        }
        if page == currentPage, curlContentView != nil {
            synchronizePageState()
            return
        }

        let previousPage = currentPage
        let nextView = makePageView(for: page)
        nextView.frame = curlHostView.bounds
        nextView.autoresizingMask = [.flexibleWidth, .flexibleHeight]

        let installPage = { [weak self] in
            guard let self else { return }
            self.curlContentView?.removeFromSuperview()
            self.curlHostView.addSubview(nextView)
            self.curlContentView = nextView
            self.currentPage = page
        }

        guard animated, curlContentView != nil, previousPage != page else {
            installPage()
            return
        }

        isTransitioning = true
        curlPanInteraction = .idle
        let animation: UIView.AnimationOptions = page > previousPage ? .transitionCurlUp : .transitionCurlDown
        UIView.transition(
            with: curlHostView,
            duration: 0.38,
            options: [animation, .allowAnimatedContent],
            animations: installPage
        ) { [weak self] _ in
            self?.finishCurlTransition()
        }
    }

    @objc func handleCurlPan(_ gesture: UIPanGestureRecognizer) {
        guard let page = curlContentView as? RDPDFReaderPageInteractable else { return }
        switch gesture.state {
        case .began:
            guard !page.readerHasActiveTextSelection else {
                curlPanInteraction = .idle
                return
            }
            if page.readerIsZoomed {
                curlPanInteraction = .contentPan
                page.readerBeginExternalPan()
            } else {
                curlPanInteraction = .pageTurn(startPage: currentPage)
            }
        case .changed:
            guard !page.readerHasActiveTextSelection else {
                curlPanInteraction = .idle
                return
            }
            if case .contentPan = curlPanInteraction {
                page.readerUpdateExternalPan(translation: gesture.translation(in: curlHostView))
            }
        case .ended:
            guard !page.readerHasActiveTextSelection else {
                curlPanInteraction = .idle
                return
            }
            finishCurlPan(gesture, page: page, cancelled: false)
        case .cancelled, .failed:
            finishCurlPan(gesture, page: page, cancelled: true)
        default:
            break
        }
    }

    private func finishCurlPan(
        _ gesture: UIPanGestureRecognizer,
        page: RDPDFReaderPageInteractable,
        cancelled: Bool
    ) {
        let interaction = curlPanInteraction
        curlPanInteraction = .idle
        switch interaction {
        case .contentPan:
            page.readerEndExternalPan()
        case let .pageTurn(startPage):
            guard !cancelled, currentPage == startPage else { return }
            let translation = gesture.translation(in: curlHostView)
            let velocity = gesture.velocity(in: curlHostView)
            guard abs(translation.y) > abs(translation.x) else { return }
            let crossedDistance = abs(translation.y) >= max(44, curlHostView.bounds.height * 0.12)
            let crossedVelocity = abs(velocity.y) >= 450
            guard crossedDistance || crossedVelocity else { return }
            let direction = abs(translation.y) >= 4 ? translation.y : velocity.y
            transitionToPage(pageNum: startPage + (direction < 0 ? 1 : -1), animated: true)
        case .idle:
            break
        case .pinching:
            // 捏合拥有当前触摸序列，pan 的结束事件不能改变它的状态。
            curlPanInteraction = .pinching
        }
    }

    @objc func handleCurlPinch(_ gesture: UIPinchGestureRecognizer) {
        guard currentDisplayType == .pageCurl,
              let contentView = curlContentView,
              let page = contentView as? RDPDFReaderPageInteractable else { return }
        let point = gesture.location(in: contentView)
        switch gesture.state {
        case .began:
            if case .contentPan = curlPanInteraction {
                page.readerEndExternalPan()
            }
            // 第二根手指到来后，捏合优先，已产生的单指翻页候选立即失效。
            curlPanInteraction = .pinching
            page.readerBeginExternalPinch(at: point)
        case .changed:
            if case .pinching = curlPanInteraction {
                page.readerUpdateExternalPinch(scale: gesture.scale, at: point)
            }
        case .ended, .cancelled, .failed:
            page.readerEndExternalPinch()
            // 重置 recognizer，确保捏合结束后仍留在屏幕上的单根手指不会在
            // 同一触摸序列中重新变成翻页或内容拖动。
            curlPanGestureRecognizer.isEnabled = false
            curlPanInteraction = .idle
            curlPanGestureRecognizer.isEnabled = currentDisplayType == .pageCurl && isPagingEnabled
        default:
            break
        }
    }

    func handleContentTap(_ point: CGPoint) {
        if isCurrentPageTextSelectionActive {
            (curlContentView as? RDPDFReaderPageInteractable)?.readerClearTextSelection()
            return
        }
        if isToolViewVisible {
            toggleToolbars()
        } else if point.y < bounds.height / 3 {
            transitionToPage(pageNum: currentPage - 1, animated: true)
        } else if point.y > bounds.height * 2 / 3 {
            transitionToPage(pageNum: currentPage + 1, animated: true)
        } else {
            toggleToolbars()
        }
    }
}