import UIKit

final class RDEPUBChapterRuntimeStore {

    private let chapterDataCache = RDEPUBChapterDataCache()

    private let pageCountCache = RDEPUBPageCountCache()

    let imageCache = NSCache<NSString, UIImage>()

    let chapterLoadQueue = DispatchQueue(label: "com.rdreader.chapterload", qos: .userInitiated)

    private let chapterLoadQueueKey = DispatchSpecificKey<Void>()

    // M-02: currentSpineIndex and windowSpineIndices are accessed only from the main thread
    // (verified by audit of all 7 access points). They are not protected by locks unlike
    // navigationLock/prefetchLock/buildingLock/cfiMapLock, but this is safe as long as
    // access remains main-thread-only. Do NOT access from chapterLoadQueue.
    private(set) var currentSpineIndex: Int?

    private(set) var windowSpineIndices: [Int] = []

    private var pendingNavigationTarget: Int?

    private let navigationLock = NSLock()

    private var pendingPrefetchTargets: Set<Int> = []

    private let prefetchLock = NSLock()

    private(set) var isBuilding: Bool = false

    private let buildingLock = NSLock()

    private var buildingCFIMapSpineIndices: Set<Int> = []

    private let cfiMapLock = NSLock()

    private var pendingChapterLoadSpineIndices: Set<Int> = []

    private let pendingChapterLoadLock = NSLock()

    init() {

        imageCache.countLimit = 50
        imageCache.totalCostLimit = 104_857_600  // 100 MB

        chapterLoadQueue.setSpecific(key: chapterLoadQueueKey, value: ())
    }

    func assertNotOnChapterLoadQueue() {
        dispatchPrecondition(condition: .notOnQueue(chapterLoadQueue))
    }

    func chapterData(for spineIndex: Int) -> RDEPUBRuntimeChapter? {
        return chapterDataCache[spineIndex]
    }

    func pageCount(for key: RDEPUBChapterCacheKey) -> RDEPUBRuntimePageCount? {
        return pageCountCache[key]
    }

    func insertChapter(_ chapter: RDEPUBRuntimeChapter) {
        chapterDataCache[chapter.spineIndex] = chapter
        RDEPUBMemoryProbe.log("chapterLoaded spine=\(chapter.spineIndex) pages=\(chapter.pages.count)")
    }

    func insertPageCount(_ pc: RDEPUBRuntimePageCount, for key: RDEPUBChapterCacheKey) {
        pageCountCache[key] = pc
    }

    func setCurrentChapter(spineIndex: Int, totalSpineCount: Int, windowRadius: Int = 1) {
        currentSpineIndex = spineIndex
        let radius = max(0, windowRadius)
        let lowerBound = max(0, spineIndex - radius)
        let upperBound = min(totalSpineCount - 1, spineIndex + radius)
        guard lowerBound <= upperBound else {
            windowSpineIndices = [spineIndex]
            return
        }
        windowSpineIndices = Array(lowerBound...upperBound)
    }

    func evictableSpineIndices() -> [Int] {
        let windowSet = Set(windowSpineIndices)
        return chapterDataCache.storedSpineIndices.filter { !windowSet.contains($0) }
    }

    func evict(spineIndex: Int) {
        chapterDataCache.remove(spineIndex: spineIndex)
        pageCountCache.remove(forSpineIndex: spineIndex)
    }

    func evictAllExceptCurrent() {
        guard let current = currentSpineIndex else {
            chapterDataCache.removeAll()
            pageCountCache.removeAll()
            return
        }
        let currentChapter = chapterDataCache[current]
        chapterDataCache.removeAll()
        if let ch = currentChapter {
            chapterDataCache[current] = ch
        }

        pageCountCache.removeAll()
    }

    func handleMemoryWarning() {
        evictAllExceptCurrent()
        imageCache.removeAllObjects()
    }

    func setNavigationTarget(spineIndex: Int) {
        navigationLock.lock()
        pendingNavigationTarget = spineIndex
        navigationLock.unlock()
    }

    func consumeNavigationTarget() -> Int? {
        navigationLock.lock()
        let target = pendingNavigationTarget
        pendingNavigationTarget = nil
        navigationLock.unlock()
        return target
    }

    func addPrefetchTarget(_ spineIndex: Int) {
        prefetchLock.lock()
        pendingPrefetchTargets.insert(spineIndex)
        prefetchLock.unlock()
    }

    func removePrefetchTarget(_ spineIndex: Int) {
        prefetchLock.lock()
        pendingPrefetchTargets.remove(spineIndex)
        prefetchLock.unlock()
    }

    func clearPrefetchTargets() {
        prefetchLock.lock()
        pendingPrefetchTargets.removeAll()
        prefetchLock.unlock()
    }

    func hasPrefetchTarget(_ spineIndex: Int) -> Bool {
        prefetchLock.lock()
        let has = pendingPrefetchTargets.contains(spineIndex)
        prefetchLock.unlock()
        return has
    }

    func markBuilding(_ building: Bool) {
        buildingLock.lock()
        isBuilding = building
        buildingLock.unlock()
    }

    func beginPendingChapterLoad(for spineIndex: Int) -> Bool {
        pendingChapterLoadLock.lock()
        defer { pendingChapterLoadLock.unlock() }
        return pendingChapterLoadSpineIndices.insert(spineIndex).inserted
    }

    func endPendingChapterLoad(for spineIndex: Int) {
        pendingChapterLoadLock.lock()
        pendingChapterLoadSpineIndices.remove(spineIndex)
        pendingChapterLoadLock.unlock()
    }

    func hasPendingChapterLoad(for spineIndex: Int) -> Bool {
        pendingChapterLoadLock.lock()
        let hasPendingLoad = pendingChapterLoadSpineIndices.contains(spineIndex)
        pendingChapterLoadLock.unlock()
        return hasPendingLoad
    }

    func beginBuildingCFIMap(for spineIndex: Int) -> Bool {
        cfiMapLock.lock()
        defer { cfiMapLock.unlock() }
        let inserted = buildingCFIMapSpineIndices.insert(spineIndex).inserted
        return inserted
    }

    func endBuildingCFIMap(for spineIndex: Int) {
        cfiMapLock.lock()
        buildingCFIMapSpineIndices.remove(spineIndex)
        cfiMapLock.unlock()
    }

    func invalidateAllForSettingsChange() {
        RDEPUBMemoryProbe.log("settingsChangeInvalidateAll")
        chapterDataCache.removeAll()
        pageCountCache.removeAll()
        imageCache.removeAllObjects()
        cfiMapLock.lock()
        buildingCFIMapSpineIndices.removeAll()
        cfiMapLock.unlock()
        pendingChapterLoadLock.lock()
        pendingChapterLoadSpineIndices.removeAll()
        pendingChapterLoadLock.unlock()
    }
}