import Foundation

/// PDF 的跨页解析规则，与 EPUB 阅读器的无封面分支保持一致。
final class RDPDFReaderSpreadResolver {
    func pair(for page: Int, totalPages: Int) -> (left: Int, right: Int?) {
        let left = max(0, page / 2 * 2)
        return (left, left + 1 < totalPages ? left + 1 : nil)
    }

    func adjacent(from page: Int, totalPages: Int, forward: Bool) -> Int? {
        let current = pair(for: page, totalPages: totalPages)
        if forward {
            let next = (current.right ?? current.left) + 1
            return next < totalPages ? next : nil
        }
        guard current.left > 0 else { return nil }
        return pair(for: current.left - 1, totalPages: totalPages).left
    }
}

/// 翻页运行时状态的归属点，避免阅读容器同时承担布局和转场状态。
final class RDPDFReaderPagingController: NSObject {
    var isTransitioning = false
    var pendingTransition: (page: Int, animated: Bool)?
}