ReadViewSDK/Sources/RDReaderView/ReaderView/Paging/RDReaderSpreadResolver.swift

//
//  RDReaderSpreadResolver.swift
//  ReadViewSDK
//
//  文件职责：页面展开（spread）解析器，负责计算双页模式下的页面配对和翻页目标。
//

import Foundation

/// 页面展开解析器
/// 处理横屏双页模式下的页码配对逻辑，包括封面页独占、双页对齐、
/// 以及相邻页面跳转等计算。
struct RDReaderSpreadResolver {
    /// 判断指定页码是否为全屏独占页面（如封面页在双页模式下独占一屏）
    func isFullScreenPage(
        _ pageNum: Int,
        landscapeDualPageEnabled: Bool,
        isLandscape: Bool,
        coverPageIndex: Int?
    ) -> Bool {
        guard landscapeDualPageEnabled, isLandscape, let coverIndex = coverPageIndex else { return false }
        return pageNum == coverIndex
    }

    /// 计算指定页码在双页模式下的左右页配对，返回 (左页, 右页?)。
    /// 封面页独占时单独处理，后续页面按两页一组配对。
    func dualPagePair(
        for pageNum: Int,
        totalPages: Int,
        coverPageIndex: Int?
    ) -> (left: Int, right: Int?) {
        if let coverIndex = coverPageIndex {
            if pageNum == coverIndex {
                return (coverIndex, nil)
            }
            let adjustedIndex = pageNum - (coverIndex + 1)
            let pairStart = coverIndex + 1 + (adjustedIndex / 2) * 2
            let left = pairStart
            let right = pairStart + 1 < totalPages ? pairStart + 1 : nil
            return (left, right)
        }

        let left = (pageNum / 2) * 2
        let right = left + 1 < totalPages ? left + 1 : nil
        return (left, right)
    }

    /// 计算从指定页码出发的下一个（或上一个）双页展开的起始页码
    func adjacentDualPage(
        from pageNum: Int,
        totalPages: Int,
        coverPageIndex: Int?,
        forward: Bool
    ) -> Int? {
        let pair = dualPagePair(for: pageNum, totalPages: totalPages, coverPageIndex: coverPageIndex)
        if forward {
            let nextStart = (pair.right ?? pair.left) + 1
            return nextStart < totalPages ? nextStart : nil
        }

        let prevEnd = pair.left - 1
        guard prevEnd >= 0 else { return nil }
        return dualPagePair(for: prevEnd, totalPages: totalPages, coverPageIndex: coverPageIndex).left
    }

    /// 计算下一页（或上一页）的页码，自动根据单页/双页模式选择合适的算法
    func nextPage(
        from currentPage: Int,
        totalPages: Int,
        pagesPerScreen: Int,
        coverPageIndex: Int?,
        forward: Bool
    ) -> Int? {
        if pagesPerScreen > 1 {
            return adjacentDualPage(
                from: currentPage,
                totalPages: totalPages,
                coverPageIndex: coverPageIndex,
                forward: forward
            )
        }

        let target = currentPage + (forward ? 1 : -1)
        guard target >= 0, target < totalPages else { return nil }
        return target
    }
}