// RDEPUBReadingSession.swift
// EPUB 阅读会话（状态机）
// 管理阅读器的运行时状态：导航状态（initializing/loading/idle/jumping/moving/repaginating）、
// 活跃页面列表、待应用的分页快照、挂起的导航请求、当前视口和阅读上下文。
// 提供分页快照生成、位置计算、导航队列、阅读上下文更新等核心业务方法。

import Foundation

/// EPUB 阅读会话，管理导航状态、页面列表和阅读进度
/// 状态机：initializing → loading → idle ↔ jumping/moving/repaginating
public final class RDEPUBReadingSession {
    /// 分页快照类型别名（页面数组 + 章节信息数组）
    public typealias PaginationSnapshot = (pages: [EPUBPage], chapters: [EPUBChapterInfo])

    /// 关联的出版物
    public let publication: RDEPUBPublication

    /// 当前导航状态
    public private(set) var navigatorState: RDEPUBNavigatorState = .initializing
    /// 当前生效的页面列表
    public private(set) var activePages: [EPUBPage] = []
    /// 当前生效的章节信息列表
    public private(set) var activeChapters: [EPUBChapterInfo] = []
    /// 暂存的待应用页面列表（分页计算完成后暂存，等状态允许时才生效）
    public private(set) var stagedPages: [EPUBPage]?
    /// 暂存的待应用章节信息列表
    public private(set) var stagedChapters: [EPUBChapterInfo]?
    /// 暂存快照对应的恢复位置
    public private(set) var stagedRestoreLocation: RDEPUBLocation?
    /// 挂起的导航目标位置（用于 fragment 精确定位）
    public private(set) var pendingNavigationLocation: RDEPUBLocation?
    /// 挂起的导航目标页码
    public private(set) var pendingNavigationPageNum: Int?
    /// 挂起的高亮 Range 信息（仅 Web 渲染路径使用）
    public private(set) var pendingNavigationHighlightRangeInfo: String?
    /// 当前视口信息
    public private(set) var currentViewport: RDEPUBViewport?
    /// 当前阅读上下文（位置 + 视口 + 页码 + 章节）
    public private(set) var currentReadingContext: RDEPUBReadingContext?

    public init(publication: RDEPUBPublication) {
        self.publication = publication
    }

    /// 资源解析器快捷访问
    public var resourceResolver: RDEPUBResourceResolver {
        publication.resourceResolver
    }

    /// 状态机转换
    public func transition(to state: RDEPUBNavigatorState) {
        navigatorState = state
    }

    /// 重置所有运行时状态（回到 initializing）
    public func resetRuntimeState() {
        navigatorState = .initializing
        activePages = []
        activeChapters = []
        clearPendingNavigation()
        clearStagedSnapshot()
        currentViewport = nil
        currentReadingContext = nil
    }

    /// 设置当前生效的分页快照
    public func setActiveSnapshot(_ snapshot: PaginationSnapshot) {
        activePages = snapshot.pages
        activeChapters = snapshot.chapters
    }

    /// 暂存分页快照（待状态允许时才生效）
    public func stageSnapshot(_ snapshot: PaginationSnapshot, restoreLocation: RDEPUBLocation?) {
        stagedPages = snapshot.pages
        stagedChapters = snapshot.chapters
        stagedRestoreLocation = restoreLocation
    }

    /// 获取暂存的分页快照
    public func stagedSnapshot() -> PaginationSnapshot? {
        guard let stagedPages, let stagedChapters else {
            return nil
        }
        return (stagedPages, stagedChapters)
    }

    /// 如果状态稳定且有暂存快照，则消费并返回（原子操作）
    public func consumeStagedSnapshotIfAllowed() -> (snapshot: PaginationSnapshot, restoreLocation: RDEPUBLocation?)? {
        guard navigatorState.isStableForSnapshotApplication,
              let stagedPages,
              let stagedChapters else {
            return nil
        }
        let restoreLocation = stagedRestoreLocation
        clearStagedSnapshot()
        return ((stagedPages, stagedChapters), restoreLocation)
    }

    /// 清除暂存快照
    public func clearStagedSnapshot() {
        stagedPages = nil
        stagedChapters = nil
        stagedRestoreLocation = nil
    }

    /// 清除挂起的导航请求
    public func clearPendingNavigation() {
        pendingNavigationLocation = nil
        pendingNavigationPageNum = nil
        pendingNavigationHighlightRangeInfo = nil
    }

    /// 检查指定页码和 spine 索引是否有挂起的导航位置
    public func pendingLocation(forPageNumber pageNumber: Int, spineIndex: Int?) -> RDEPUBLocation? {
        guard pendingNavigationPageNum == pageNumber,
              let pendingNavigationLocation else {
            return nil
        }
        guard let spineIndex else {
            return pendingNavigationLocation
        }
        let pageHref = resourceResolver.href(forSpineIndex: spineIndex)
        guard resourceResolver.normalizedHref(pageHref ?? "") == resourceResolver.normalizedHref(pendingNavigationLocation.href) else {
            return nil
        }
        return pendingNavigationLocation
    }

    /// 查询指定页是否存在待消费的高亮 Range 信息。
    public func pendingHighlightRangeInfo(forPageNumber pageNumber: Int, spineIndex: Int?) -> String? {
        guard pendingNavigationPageNum == pageNumber,
              let pendingNavigationHighlightRangeInfo,
              !pendingNavigationHighlightRangeInfo.isEmpty else {
            return nil
        }
        guard let pendingNavigationLocation else {
            return pendingNavigationHighlightRangeInfo
        }
        guard let spineIndex else {
            return pendingNavigationHighlightRangeInfo
        }
        let pageHref = resourceResolver.href(forSpineIndex: spineIndex)
        guard resourceResolver.normalizedHref(pageHref ?? "") == resourceResolver.normalizedHref(pendingNavigationLocation.href) else {
            return nil
        }
        return pendingNavigationHighlightRangeInfo
    }

    /// 判断指定页面是否包含给定的 spine 索引（固定版式需检查 spread 内所有资源）
    public func pageContains(spineIndex: Int, in page: EPUBPage) -> Bool {
        if let fixedSpread = page.fixedSpread {
            return fixedSpread.resources.contains(where: { $0.spineIndex == spineIndex })
        }
        return page.spineIndex == spineIndex
    }

    /// 根据页面信息生成回退位置（用于无精确位置时的兜底）
    public func fallbackLocation(for page: EPUBPage, bookIdentifier: String?) -> RDEPUBLocation? {
        if let fixedSpread = page.fixedSpread {
            return RDEPUBLocation(
                bookIdentifier: bookIdentifier,
                href: fixedSpread.primaryResource.href,
                progression: 0,
                lastProgression: 1
            )
        }

        guard let href = resourceResolver.href(forSpineIndex: page.spineIndex) else {
            return nil
        }

        let progression: Double
        if page.totalPagesInChapter <= 1 {
            progression = 0
        } else {
            progression = Double(page.pageIndexInChapter) / Double(page.totalPagesInChapter - 1)
        }

        return RDEPUBLocation(
            bookIdentifier: bookIdentifier,
            href: href,
            progression: progression,
            lastProgression: progression
        )
    }

    /// 获取当前可见页面对应的位置
    public func currentVisibleLocation(currentPageNumber: Int, bookIdentifier: String?) -> RDEPUBLocation? {
        guard currentPageNumber > 0,
              activePages.indices.contains(currentPageNumber - 1) else {
            return nil
        }
        return fallbackLocation(for: activePages[currentPageNumber - 1], bookIdentifier: bookIdentifier)
    }

    /// 根据位置获取初始 spine 索引
    public func initialSpineIndex(for location: RDEPUBLocation?) -> Int {
        guard let location,
              let normalizedHref = resourceResolver.normalizedHref(location.href),
              let spineIndex = publication.spine.firstIndex(where: { resourceResolver.normalizedHref($0.href) == normalizedHref }) else {
            return 0
        }
        return spineIndex
    }

    /// 根据位置计算对应的页面索引（支持固定版式和重排模式）
    public func pageIndex(for location: RDEPUBLocation, bookIdentifier: String?) -> Int? {
        guard location.bookIdentifier == nil || location.bookIdentifier == bookIdentifier else {
            return nil
        }
        guard let targetHref = resourceResolver.normalizedHref(location.href) else {
            return nil
        }

        if publication.layout == .fixed {
            return activePages.firstIndex(where: { page in
                if let fixedSpread = page.fixedSpread {
                    return fixedSpread.contains(normalizedHref: targetHref, normalizer: { self.resourceResolver.normalizedHref($0) })
                }
                guard let href = self.resourceResolver.href(forSpineIndex: page.spineIndex) else {
                    return false
                }
                return self.resourceResolver.normalizedHref(href) == targetHref
            })
        }

        guard let spineIndex = publication.spine.firstIndex(where: { resourceResolver.normalizedHref($0.href) == targetHref }) else {
            return nil
        }

        let candidates = activePages.enumerated().filter { $0.element.spineIndex == spineIndex }
        guard !candidates.isEmpty else {
            return nil
        }
        if candidates.count == 1 {
            return candidates[0].offset
        }

        let navigationProgression = location.navigationProgression
        let localIndex = min(
            candidates.count - 1,
            max(0, Int(round(navigationProgression * Double(candidates.count - 1))))
        )
        return candidates[localIndex].offset
    }

    /// 排队导航到指定位置：规范化位置、查找页面索引、设置挂起导航、切换到 jumping 状态
    public func queueNavigation(
        to location: RDEPUBLocation,
        relativeToSpineIndex spineIndex: Int? = nil,
        bookIdentifier: String?,
        targetHighlightRangeInfo: String? = nil
    ) -> Int? {
        guard let normalizedLocation = resourceResolver.normalizedLocation(
            location,
            relativeToSpineIndex: spineIndex,
            bookIdentifier: bookIdentifier
        ), let pageIndex = pageIndex(for: normalizedLocation, bookIdentifier: bookIdentifier) else {
            return nil
        }

        let hasTargetHighlightRangeInfo = targetHighlightRangeInfo?.isEmpty == false
        let shouldKeepPendingNavigation =
            normalizedLocation.fragment != nil ||
            normalizedLocation.rangeAnchor != nil ||
            hasTargetHighlightRangeInfo
        pendingNavigationLocation = shouldKeepPendingNavigation ? normalizedLocation : nil
        pendingNavigationPageNum = shouldKeepPendingNavigation ? pageIndex + 1 : nil
        pendingNavigationHighlightRangeInfo = hasTargetHighlightRangeInfo ? targetHighlightRangeInfo : nil
        transition(to: .jumping)
        return pageIndex + 1
    }

    /// 更新阅读上下文：规范化位置、设置视口和阅读上下文、清除挂起导航、回到 idle 状态
    public func updateReadingContext(
        pageNumber: Int,
        location: RDEPUBLocation,
        spineIndex: Int,
        chapterIndex: Int?,
        bookIdentifier: String?
    ) {
        let normalizedLocation = resourceResolver.normalizedLocation(
            location,
            relativeToSpineIndex: spineIndex,
            bookIdentifier: bookIdentifier
        ) ?? RDEPUBLocation(
            bookIdentifier: bookIdentifier,
            href: location.href,
            progression: location.progression,
            lastProgression: location.lastProgression,
            fragment: location.fragment
        )

        let resource = RDEPUBViewportResource(
            href: normalizedLocation.href,
            spineIndex: spineIndex,
            progression: normalizedLocation.progression,
            lastProgression: normalizedLocation.lastProgression,
            fragment: normalizedLocation.fragment
        )
        let viewport = RDEPUBViewport(
            resources: [resource],
            visiblePageNumber: pageNumber,
            chapterIndex: chapterIndex,
            isFixedLayout: publication.layout == .fixed
        )
        currentViewport = viewport
        currentReadingContext = RDEPUBReadingContext(
            location: normalizedLocation,
            viewport: viewport,
            pageNumber: pageNumber,
            chapterIndex: chapterIndex
        )

        if pendingNavigationPageNum == pageNumber {
            clearPendingNavigation()
        }
        if navigatorState == .jumping || navigatorState == .moving {
            transition(to: .idle)
        }
    }

    /// 获取当前阅读位置（优先从阅读上下文获取，回退到可见位置）
    public func currentReadingLocation(bookIdentifier: String?) -> RDEPUBLocation? {
        currentReadingContext?.location ?? currentVisibleLocation(currentPageNumber: currentViewport?.visiblePageNumber ?? 0, bookIdentifier: bookIdentifier)
    }

    /// 根据分页计数和偏好设置生成分页快照
    /// 固定版式按 spread 分页，重排模式按 spine 索引逐章分页
    public func makePaginationSnapshot(
        pageCounts: [Int],
        preferences: RDEPUBPreferences,
        layoutContext: RDEPUBNavigatorLayoutContext
    ) -> PaginationSnapshot {
        if publication.layout == .fixed {
            let spreads = publication.makeFixedSpreads(
                preferences: preferences,
                viewportSize: layoutContext.viewportSize
            )

            let chapters = spreads.enumerated().map { spreadIndex, spread in
                EPUBChapterInfo(
                    spineIndex: spread.primaryResource.spineIndex,
                    title: spread.primaryResource.title,
                    pageCount: 1
                )
            }

            let pages = spreads.enumerated().map { spreadIndex, spread in
                EPUBPage(
                    spineIndex: spread.primaryResource.spineIndex,
                    chapterIndex: spreadIndex,
                    pageIndexInChapter: 0,
                    totalPagesInChapter: 1,
                    chapterTitle: spread.primaryResource.title,
                    fixedSpread: spread
                )
            }

            return (pages, chapters)
        }

        var pages: [EPUBPage] = []
        var chapters: [EPUBChapterInfo] = []

        for (spineIndex, pageCount) in pageCounts.enumerated() {
            guard pageCount > 0, spineIndex < publication.spine.count else { continue }

            let chapterIndex = chapters.count
            let title = publication.spine[spineIndex].title

            chapters.append(
                EPUBChapterInfo(
                    spineIndex: spineIndex,
                    title: title,
                    pageCount: pageCount
                )
            )

            for pageIndex in 0..<pageCount {
                pages.append(
                    EPUBPage(
                        spineIndex: spineIndex,
                        chapterIndex: chapterIndex,
                        pageIndexInChapter: pageIndex,
                        totalPagesInChapter: pageCount,
                        chapterTitle: title,
                        fixedSpread: nil
                    )
                )
            }
        }

        return (pages, chapters)
    }
}