import UIKit

extension RDPDFReaderView: UICollectionViewDataSource, UICollectionViewDelegateFlowLayout {
    public func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        pageCount()
    }

    public func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let view = dataSource?.pageContentView(readerView: self, pageNum: indexPath.item) ?? UIView()
        view.tag = indexPath.item
        configureSpreadAlignment(for: view, page: indexPath.item)
        configureInteraction(for: view)
        if let cell = view.superview?.superview as? RDPDFReaderContentCell { return cell }
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "fallback", for: indexPath) as! RDPDFReaderContentCell
        cell.hostedView = view
        return cell
    }

    public func collectionView(
        _ collectionView: UICollectionView,
        layout collectionViewLayout: UICollectionViewLayout,
        sizeForItemAt indexPath: IndexPath
    ) -> CGSize {
        // 手机横屏竖滑：cell 高度=宽度适配后的纸张高度，超出一屏的内容由
        // collectionView 自身的连续滚动承接；页面未加载时先按一屏高占位。
        if usesWidthFitVerticalScroll,
           let height = verticalScrollWidthFitPageHeightProvider?(indexPath.item, bounds.width),
           height > 0 {
            return CGSize(width: bounds.width, height: height)
        }
        return usesLandscapeSpread
            ? CGSize(width: bounds.width / 2, height: bounds.height)
            : bounds.size
    }

    public func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
        updateCurrentPageAfterScroll(scrollView)
    }

    public func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
        // 竖滑是连续滚动（isPagingEnabled = false），轻推后直接停下不会触发
        // 减速回调；此处兜底，否则页码停留在旧值。
        if !decelerate { updateCurrentPageAfterScroll(scrollView) }
    }

    private func updateCurrentPageAfterScroll(_ scrollView: UIScrollView) {
        guard currentDisplayType != .pageCurl else { return }
        if currentDisplayType == .verticalScroll {
            // 宽度适配下各 cell 高度不同，页码取视口中心命中的 cell。
            // 顶部/底部回弹时中心点会越出内容范围，钳制回内容内再取 cell。
            let centerY = scrollView.contentOffset.y + scrollView.bounds.height / 2
            let center = CGPoint(
                x: scrollView.bounds.width / 2,
                y: min(max(0, centerY), max(0, collectionView.contentSize.height - 1))
            )
            if let indexPath = collectionView.indexPathForItem(at: center) {
                currentPage = indexPath.item
            } else if let first = collectionView.indexPathsForVisibleItems.min(by: { $0.item < $1.item }) {
                currentPage = first.item
            }
            return
        }
        let value = scrollView.contentOffset.x / max(bounds.width, 1)
        currentPage = Int(value.rounded()) * (usesLandscapeSpread ? 2 : 1)
    }
}