import UIKit

enum RDEPUBPendingPageMapUpdateKind {
    case reconcileFullMap
    case extendPartial(currentPageNumber: Int, currentLocation: RDEPUBLocation?)
    case appendForward
}

struct RDEPUBPendingPageMapUpdate {
    let pageMap: RDEPUBBookPageMap
    let kind: RDEPUBPendingPageMapUpdateKind
}

final class RDEPUBPresentationRuntime {

    private unowned let context: RDEPUBReaderContext

    private unowned let locationCoordinator: RDEPUBReaderLocationCoordinator

    private unowned let jumpSessionManager: RDEPUBJumpSessionManager

    private unowned let reconciliationCoordinator: RDEPUBPageMapReconciliationCoordinator

    private var pendingCommitRetryWorkItem: DispatchWorkItem?

    init(
        context: RDEPUBReaderContext,
        locationCoordinator: RDEPUBReaderLocationCoordinator,
        jumpSessionManager: RDEPUBJumpSessionManager,
        reconciliationCoordinator: RDEPUBPageMapReconciliationCoordinator
    ) {
        self.context = context
        self.locationCoordinator = locationCoordinator
        self.jumpSessionManager = jumpSessionManager
        self.reconciliationCoordinator = reconciliationCoordinator
    }

    func applyBookPageMap(
        _ bookPageMap: RDEPUBBookPageMap,
        restoreLocation: RDEPUBLocation?,
        finishPagination: (RDEPUBLocation?) -> Void
    ) {
        context.textBook = nil
        context.bookPageMap = bookPageMap
        context.pendingPageMapUpdates.removeAll()
        context.replaceActiveSnapshot(makeSnapshot(from: bookPageMap))
        finishPagination(restoreLocation)
    }

    func refreshBookPageMapInPlace(_ bookPageMap: RDEPUBBookPageMap) {
        RDEPUBBackgroundTrace.log(
            "Reconciliation",
            "refreshBookPageMapInPlace: enqueued reconcileFullMap chapters=\(bookPageMap.totalChapters) pages=\(bookPageMap.totalPages)"
        )
        enqueuePendingPageMapUpdate(
            RDEPUBPendingPageMapUpdate(
                pageMap: bookPageMap,
                kind: .reconcileFullMap
            )
        )
    }

    func commitPendingPageMapUpdateIfNeeded() {
        guard let readerView = context.readerView,
              let controller = context.controller else { return }

        guard !context.pendingPageMapUpdates.isEmpty else {
            cancelPendingCommitRetry()
            return
        }

        guard !controller.isRepaginating else {
            schedulePendingCommitRetry()
            return
        }
        guard !readerView.isPageCurlTransitioning else {
            schedulePendingCommitRetry()
            return
        }

        cancelPendingCommitRetry()

        let rankedUpdates = rankedPendingPageMapUpdates()
        for (index, update) in rankedUpdates {
            if commitPendingPageMapUpdate(
                update,
                at: index,
                readerView: readerView,
                controller: controller
            ) {
                if !context.pendingPageMapUpdates.isEmpty {
                    schedulePendingCommitRetry()
                }
                return
            }
        }

        if !context.pendingPageMapUpdates.isEmpty {
            schedulePendingCommitRetry()
        }
    }

    func queueExtendedPartialPageMap(
        _ bookPageMap: RDEPUBBookPageMap,
        currentPageNumber: Int,
        currentLocation: RDEPUBLocation?
    ) {
        enqueuePendingPageMapUpdate(
            RDEPUBPendingPageMapUpdate(
                pageMap: bookPageMap,
                kind: .extendPartial(
                    currentPageNumber: currentPageNumber,
                    currentLocation: currentLocation
                )
            )
        )
    }

    func applySettingsPreviewPageMap(_ bookPageMap: RDEPUBBookPageMap) {
        context.bookPageMap = bookPageMap
        context.replaceActiveSnapshot(makeSnapshot(from: bookPageMap))
    }

    func queueForwardAppendedPageMap(_ bookPageMap: RDEPUBBookPageMap) {
        enqueuePendingPageMapUpdate(
            RDEPUBPendingPageMapUpdate(
                pageMap: bookPageMap,
                kind: .appendForward
            )
        )
    }

    func makeSnapshot(from bookPageMap: RDEPUBBookPageMap) -> RDEPUBReadingSession.PaginationSnapshot {
        let pages = bookPageMap.entries.flatMap { entry in
            (0..<entry.pageCount).map { localPageIndex in
                EPUBPage(
                    spineIndex: entry.spineIndex,
                    chapterIndex: bookPageMap.chapterIndex(forSpineIndex: entry.spineIndex) ?? 0,
                    pageIndexInChapter: localPageIndex,
                    totalPagesInChapter: entry.pageCount,
                    chapterTitle: entry.title,
                    fixedSpread: nil
                )
            }
        }
        let chapters = bookPageMap.entries.map { entry in
            EPUBChapterInfo(
                spineIndex: entry.spineIndex,
                title: entry.title,
                pageCount: entry.pageCount
            )
        }
        return (pages, chapters)
    }

    private func applyFullPageMapReplacement(
        _ newPageMap: RDEPUBBookPageMap,
        readerView: RDEpubReaderView,
        controller: RDEPUBReaderController
    ) {
        let currentLocation = locationCoordinator.currentVisibleLocation()
        let livePageBeforeApply = readerView.currentPage + 1
        // Resolved against the outgoing page map. When it round-trips to the live
        // page, the location faithfully describes what is on screen, so whatever
        // page it resolves to in the new map is authoritative even if the two maps
        // number pages differently (partial-window -> full-book takeover).
        let oldResolvedPage = currentLocation.flatMap { controller.pageNumber(for: $0) }

        context.textBook = nil
        applyPageMapToLiveModel(newPageMap)
        RDEPUBBackgroundTrace.log(
            "Reconciliation",
            "applyFullPageMapReplacement livePageBeforeApply=\(livePageBeforeApply) totalPages=\(newPageMap.totalPages) totalChapters=\(newPageMap.totalChapters)"
        )

        if let currentLocation {
            let resolvedTargetPage = controller.pageNumber(for: currentLocation)
            let shouldTrustResolvedLocation = shouldTrustFullReplaceResolvedPage(
                resolvedTargetPage,
                livePageBeforeApply: livePageBeforeApply,
                locationMatchesLivePage: oldResolvedPage == livePageBeforeApply
            )
            RDEPUBBackgroundTrace.log(
                "Reconciliation",
                "applyFullPageMapReplacement decision livePage=\(livePageBeforeApply) oldResolvedPage=\(oldResolvedPage ?? -1) resolvedTargetPage=\(resolvedTargetPage ?? -1) trustResolved=\(shouldTrustResolvedLocation) href=\(currentLocation.href)"
            )

            if shouldTrustResolvedLocation,
               rebindVisibleLocation(currentLocation, readerView: readerView, controller: controller) {
                return
            }

            let fallbackPage = max(livePageBeforeApply, 1)
            RDEPUBBackgroundTrace.log(
                "Reconciliation",
                "applyFullPageMapReplacement preserveLivePage fallbackPage=\(fallbackPage) currentReaderPage=\(readerView.currentPage + 1)"
            )
            rebindVisiblePage(
                to: fallbackPage - 1,
                readerView: readerView
            )
        } else {
            readerView.reloadPageCountOnly()
        }

        if let currentLocation,
           context.normalizedSpineIndex(for: currentLocation) != nil,
           let activeSession = jumpSessionManager.activeSession {
            let candidateIndices = Set(newPageMap.entries.map { $0.spineIndex })
            let protectedIndices = activeSession.protectedSpineIndices
            if protectedIndices.isSubset(of: candidateIndices) {
                jumpSessionManager.endSession(.coverageComplete)
            }
        }
    }

    private func enqueuePendingPageMapUpdate(_ update: RDEPUBPendingPageMapUpdate) {
        var updates = context.pendingPageMapUpdates
        if let existingIndex = updates.firstIndex(where: {
            pendingPageMapUpdateKindMatches($0.kind, update.kind)
        }) {
            let existing = updates[existingIndex]
            if shouldReplacePendingPageMapUpdate(existing, with: update) {
                updates[existingIndex] = update
            }
        } else {
            updates.append(update)
        }
        context.pendingPageMapUpdates = updates
        commitPendingPageMapUpdateIfNeeded()
    }

    private func rankedPendingPageMapUpdates() -> [(Int, RDEPUBPendingPageMapUpdate)] {
        context.pendingPageMapUpdates.enumerated().sorted { lhs, rhs in
            pendingPriority(for: lhs.element.kind) > pendingPriority(for: rhs.element.kind)
        }
    }

    private func commitPendingPageMapUpdate(
        _ update: RDEPUBPendingPageMapUpdate,
        at index: Int,
        readerView: RDEpubReaderView,
        controller: RDEPUBReaderController
    ) -> Bool {
        switch update.kind {
        case .reconcileFullMap:
            let decision = reconciliationCoordinator.evaluateTakeover(
                candidatePageMap: update.pageMap,
                candidateSegment: nil,
                currentWindow: context.bookPageMap,
                jumpSession: jumpSessionManager.activeSession
            )

            switch decision {
            case .keepCurrentWindow:
                RDEPUBBackgroundTrace.log(
                    "Reconciliation",
                    "commitPendingPageMapUpdate: keepCurrentWindow — removed pending update, totalPages=\(update.pageMap.totalPages)"
                )
                removePendingPageMapUpdate(at: index)
                return false

            case .fullReplace(let newPageMap):
                removePendingPageMapUpdate(at: index)
                applyFullPageMapReplacement(newPageMap, readerView: readerView, controller: controller)
                return true

            case .expandWindow, .segmentReplace:
                removePendingPageMapUpdate(at: index)
                return false
            }

        case .extendPartial(let capturedPageNumber, let currentLocation):
            removePendingPageMapUpdate(at: index)
            applyPageMapToLiveModel(update.pageMap)
            let livePageNumber = max(readerView.currentPage, 0) + 1
            let shouldTrustCapturedLocation = livePageNumber == max(capturedPageNumber, 1)
            RDEPUBBackgroundTrace.log(
                "Reconciliation",
                "extendPartial commit capturedPage=\(capturedPageNumber) livePage=\(livePageNumber) trustCaptured=\(shouldTrustCapturedLocation) totalPages=\(update.pageMap.totalPages) totalChapters=\(update.pageMap.totalChapters)"
            )
            if shouldTrustCapturedLocation,
               let currentLocation,
               rebindVisibleLocation(currentLocation, readerView: readerView, controller: controller) {
                return true
            }
            // Prefer the live readerView page when the user has moved since the
            // extension request was created; otherwise a stale captured location can
            // snap pageCurl back to the previous page after the turn completes.
            let livePageIndex = max(readerView.currentPage, 0)
            rebindVisiblePage(
                to: livePageIndex,
                readerView: readerView
            )
            return true

        case .appendForward:
            removePendingPageMapUpdate(at: index)
            applyPageMapToLiveModel(update.pageMap)
            readerView.reloadPageCountOnly()
            return true
        }
    }

    private func rebindVisiblePage(to pageIndex: Int, readerView: RDEpubReaderView) {
        if pageIndex == readerView.currentPage {
            RDEPUBBackgroundTrace.log(
                "Reconciliation",
                "rebindVisiblePage pageUnchanged currentPage=\(readerView.currentPage + 1) displayType=\(readerView.currentDisplayType)"
            )
            readerView.reloadPageCountOnly()
            return
        }

        RDEPUBBackgroundTrace.log(
            "Reconciliation",
            "rebindVisiblePage targetPage=\(pageIndex + 1) currentPage=\(readerView.currentPage + 1) displayType=\(readerView.currentDisplayType) isPageCurlTransitioning=\(readerView.isPageCurlTransitioning)"
        )
        if readerView.currentDisplayType == .pageCurl {
            if readerView.isPageCurlTransitioning {
                // Defer the transition until the current page-curl animation completes,
                // and re-read the live page at that time to avoid jumping to a stale position.
                DispatchQueue.main.async { [weak readerView] in
                    guard let readerView, !readerView.isPageCurlTransitioning else { return }
                    let livePageIndex = max(readerView.currentPage, 0)
                    RDEPUBBackgroundTrace.log(
                        "Reconciliation",
                        "rebindVisiblePage deferredTransition livePage=\(livePageIndex + 1)"
                    )
                    readerView.transitionToPage(pageNum: livePageIndex, animated: false)
                }
            } else {
                readerView.transitionToPage(pageNum: pageIndex, animated: false)
            }
        } else {
            readerView.reloadPageCountOnly()
            if pageIndex != readerView.currentPage {
                readerView.transitionToPage(pageNum: pageIndex, animated: false)
            }
        }
    }

    private func rebindVisibleLocation(
        _ location: RDEPUBLocation,
        readerView: RDEpubReaderView,
        controller: RDEPUBReaderController
    ) -> Bool {
        guard let targetPageNumber = controller.pageNumber(for: location) else {
            RDEPUBBackgroundTrace.log(
                "Reconciliation",
                "rebindVisibleLocation failedToResolve locationHref=\(location.href) currentPage=\(readerView.currentPage + 1)"
            )
            return false
        }

        if context.bookPageMap != nil,
           context.runtime?.prepareOnDemandChapter(
                forAbsolutePageNumber: targetPageNumber,
                allowSynchronousLoad: false
           ) == false {
            RDEPUBBackgroundTrace.log(
                "Reconciliation",
                "rebindVisibleLocation prepareOnDemandBlocked targetPage=\(targetPageNumber) currentPage=\(readerView.currentPage + 1)"
            )
            return false
        }

        RDEPUBBackgroundTrace.log(
            "Reconciliation",
            "rebindVisibleLocation targetPage=\(targetPageNumber) currentPage=\(readerView.currentPage + 1) href=\(location.href)"
        )

        rebindVisiblePage(
            to: max(targetPageNumber - 1, 0),
            readerView: readerView
        )
        return true
    }

    private func applyPageMapToLiveModel(_ pageMap: RDEPUBBookPageMap) {
        context.bookPageMap = pageMap
        context.replaceActiveSnapshot(makeSnapshot(from: pageMap))
        discardSupersededPendingPageMapUpdates(afterApplying: pageMap)
    }

    private func removePendingPageMapUpdate(at index: Int) {
        var updates = context.pendingPageMapUpdates
        guard updates.indices.contains(index) else { return }
        updates.remove(at: index)
        context.pendingPageMapUpdates = updates
    }

    private func discardSupersededPendingPageMapUpdates(afterApplying liveMap: RDEPUBBookPageMap) {
        let updates = context.pendingPageMapUpdates.filter { update in
            update.pageMap.totalChapters > liveMap.totalChapters
                || (
                    update.pageMap.totalChapters == liveMap.totalChapters
                        && update.pageMap.totalPages > liveMap.totalPages
                )
        }
        context.pendingPageMapUpdates = updates
    }

    private func pendingPriority(for kind: RDEPUBPendingPageMapUpdateKind) -> Int {
        switch kind {
        case .extendPartial:
            return 3
        case .appendForward:
            return 2
        case .reconcileFullMap:
            return 1
        }
    }

    private func pendingPageMapUpdateKindMatches(
        _ lhs: RDEPUBPendingPageMapUpdateKind,
        _ rhs: RDEPUBPendingPageMapUpdateKind
    ) -> Bool {
        switch (lhs, rhs) {
        case (.reconcileFullMap, .reconcileFullMap),
             (.appendForward, .appendForward),
             (.extendPartial, .extendPartial):
            return true
        default:
            return false
        }
    }

    private func shouldReplacePendingPageMapUpdate(
        _ existing: RDEPUBPendingPageMapUpdate,
        with candidate: RDEPUBPendingPageMapUpdate
    ) -> Bool {
        candidate.pageMap.totalChapters > existing.pageMap.totalChapters
            || (
                candidate.pageMap.totalChapters == existing.pageMap.totalChapters
                    && candidate.pageMap.totalPages >= existing.pageMap.totalPages
            )
    }

    private func shouldTrustFullReplaceResolvedPage(
        _ resolvedTargetPage: Int?,
        livePageBeforeApply: Int,
        locationMatchesLivePage: Bool
    ) -> Bool {
        guard let resolvedTargetPage else { return false }
        if locationMatchesLivePage {
            return true
        }
        // The location did not round-trip to the live page in the outgoing map
        // (stale persisted location or mid-transition), so only follow it when it
        // stays next to the page the user is actually looking at.
        return abs(resolvedTargetPage - livePageBeforeApply) <= 1
    }

    private func schedulePendingCommitRetry() {
        guard pendingCommitRetryWorkItem == nil else { return }

        let workItem = DispatchWorkItem { [weak self] in
            guard let self else { return }
            self.pendingCommitRetryWorkItem = nil
            self.commitPendingPageMapUpdateIfNeeded()
        }
        pendingCommitRetryWorkItem = workItem
        DispatchQueue.main.asyncAfter(deadline: .now() + 0.05, execute: workItem)
    }

    private func cancelPendingCommitRetry() {
        pendingCommitRetryWorkItem?.cancel()
        pendingCommitRetryWorkItem = nil
    }
}