import UIKit

/// UIPageViewController 数据源和代理实现
/// 处理仿真翻页模式下的页面数据供给和翻页事件
extension RDReaderView: UIPageViewControllerDataSource, UIPageViewControllerDelegate {

    /// 创建指定页码的子控制器
    /// 空白页返回空视图，正常页从缓存或数据源获取内容
    private func makeSinglePageChildVC(for pageNum: Int) -> RDReaderPageChildViewController {
        if pageNum == RDReaderView.blankPageNum || pageNum == RDReaderView.blankEndPageNum {
            return RDReaderPageChildViewController(contentView: UIView(), pageNum: pageNum)
        }
        let contentView = pageViewForDisplay(pageNum: pageNum)
        return RDReaderPageChildViewController(contentView: contentView, pageNum: pageNum)
    }

    /// 计算某页的"下一页"页码
    /// 考虑封面页独占和末尾空白页配对的情况
    private func nextPageNum(after pageNum: Int, isDualPage: Bool) -> Int? {
        let totalPages = numberOfPages()

        // 末尾空白页之后没有更多页
        if pageNum == RDReaderView.blankEndPageNum {
            return nil
        }

        if isDualPage, let coverIndex = coverPageIndex {
            if pageNum == coverIndex {
                return RDReaderView.blankPageNum
            }
            if pageNum == RDReaderView.blankPageNum {
                let firstContent = coverIndex + 1
                return firstContent < totalPages ? firstContent : nil
            }
        }

        let next = pageNum + 1
        if next < totalPages {
            return next
        }

        // pageNum 是最后一页，检查在双页模式下是否需要空白页配对
        if isDualPage {
            if let coverIndex = coverPageIndex {
                let adjustedIndex = pageNum - (coverIndex + 1)
                if adjustedIndex >= 0 && adjustedIndex % 2 == 0 {
                    return RDReaderView.blankEndPageNum
                }
            } else {
                if pageNum % 2 == 0 {
                    return RDReaderView.blankEndPageNum
                }
            }
        }
        return nil
    }

    /// 计算某页的"上一页"页码
    /// 考虑封面页独占和末尾空白页配对的情况
    private func prevPageNum(before pageNum: Int, isDualPage: Bool) -> Int? {
        if pageNum == RDReaderView.blankEndPageNum {
            let totalPages = numberOfPages()
            return totalPages > 0 ? totalPages - 1 : nil
        }
        if isDualPage, let coverIndex = coverPageIndex {
            if pageNum == RDReaderView.blankPageNum {
                return coverIndex
            }
            if pageNum == coverIndex + 1 {
                return RDReaderView.blankPageNum
            }
        }
        let prev = pageNum - 1
        return prev >= 0 ? prev : nil
    }

    /// UIPageViewController 数据源：返回当前页之前（左/上）的页面控制器
    /// RTL 模式下 before/after 语义互换：before（向右翻）= 下一页
    public func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
        guard let vc = viewController as? RDReaderPageChildViewController else { return nil }
        let isDualPage = landscapeDualPageEnabled && isLandscape
        let isRTL = pageDirection == .rightToLeft

        let targetNum = isRTL
            ? nextPageNum(after: vc.pageNum, isDualPage: isDualPage)
            : prevPageNum(before: vc.pageNum, isDualPage: isDualPage)

        guard let num = targetNum else { return nil }
        let targetVC = makeSinglePageChildVC(for: num)
        willPreviousTransitionToViewController = targetVC
        return targetVC
    }

    /// UIPageViewController 数据源：返回当前页之后（右/下）的页面控制器
    /// RTL 模式下 before/after 语义互换
    public func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
        guard let vc = viewController as? RDReaderPageChildViewController else { return nil }
        let isDualPage = landscapeDualPageEnabled && isLandscape
        let isRTL = pageDirection == .rightToLeft

        let targetNum = isRTL
            ? prevPageNum(before: vc.pageNum, isDualPage: isDualPage)
            : nextPageNum(after: vc.pageNum, isDualPage: isDualPage)

        guard let num = targetNum else { return nil }
        let targetVC = makeSinglePageChildVC(for: num)
        willNextTransitionToViewController = targetVC
        return targetVC
    }

    /// UIPageViewController 代理：翻页动画完成回调
    /// 更新当前页码，触发预加载，并检测 PageViewController 异常状态
    public func pageViewController(_ pageViewController: UIPageViewController, didFinishAnimating finished: Bool, previousViewControllers: [UIViewController], transitionCompleted completed: Bool) {
        if completed, let firstVC = pageViewController.viewControllers?.first as? RDReaderPageChildViewController {
            let pn = firstVC.pageNum
            if pn != RDReaderView.blankPageNum && pn != RDReaderView.blankEndPageNum {
                currentPage = pn
                primePageCache(around: pn, preferredForward: predictedPageDirection)
            }
        }

        predictedPageDirection = nil
        finishPageCurlTransition()
    }

    /// UIPageViewController 代理：即将开始转场动画
    /// 记录预测的翻页方向，用于优化预加载策略
    public func pageViewController(_ pageViewController: UIPageViewController, willTransitionTo pendingViewControllers: [UIViewController]) {
        pagingController.isTransitioning = true
        willTransitionToViewController = pendingViewControllers.first
        if let target = pendingViewControllers.first as? RDReaderPageChildViewController,
           target.pageNum != RDReaderView.blankPageNum,
           target.pageNum != RDReaderView.blankEndPageNum {
            predictedPageDirection = target.pageNum >= currentPage
            primePageCache(around: target.pageNum, preferredForward: predictedPageDirection)
        }
    }
}