import Foundation

final class RDEPUBChapterDataCache {

    private var storage: [Int: RDEPUBRuntimeChapter] = [:]
    private var accessOrder: [Int] = []  // H-05: LRU tracking for eviction
    private let maxEntryCount: Int

    private let lock = NSLock()

    init(maxEntryCount: Int = 30) {
        self.maxEntryCount = maxEntryCount
    }

    subscript(_ spineIndex: Int) -> RDEPUBRuntimeChapter? {
        get {
            lock.lock()
            defer { lock.unlock() }
            guard let chapter = storage[spineIndex] else {
                return nil
            }
            touchLocked(spineIndex)
            return chapter
        }
        set {
            lock.lock()
            defer { lock.unlock() }
            if let newValue {
                storage[spineIndex] = newValue
                touchLocked(spineIndex)
                // Evict oldest entries if over limit
                evictIfNeededLocked()
            } else {
                storage.removeValue(forKey: spineIndex)
                accessOrder.removeAll { $0 == spineIndex }
            }
        }
    }

    var storedSpineIndices: [Int] {
        lock.lock()
        defer { lock.unlock() }
        return Array(storage.keys)
    }

    func remove(spineIndex: Int) {
        lock.lock()
        defer { lock.unlock() }
        storage.removeValue(forKey: spineIndex)
        accessOrder.removeAll { $0 == spineIndex }
    }

    func removeAll() {
        lock.lock()
        defer { lock.unlock() }
        storage.removeAll()
        accessOrder.removeAll()
    }

    /// H-05: Evict least recently used entries when cache exceeds maxEntryCount.
    /// Must be called while holding lock.
    private func evictIfNeededLocked() {
        while storage.count > maxEntryCount, let oldest = accessOrder.first {
            storage.removeValue(forKey: oldest)
            accessOrder.removeFirst()
        }
    }

    /// Marks an entry as most recently used.
    /// Must be called while holding lock.
    private func touchLocked(_ spineIndex: Int) {
        accessOrder.removeAll { $0 == spineIndex }
        accessOrder.append(spineIndex)
    }
}