//
//  CachingDiskStorage.swift
//  Tusker
//
//  Created by Shadowfacts on 1/17/21.
//  Copyright © 2021 Shadowfacts. All rights reserved.
//

import Foundation
import Cache

/// This class wraps a `DiskStorage` and maintains an in-memory cache of whether objects
/// exist on disk to avoid unnecessary disk I/O when calling synchronous methods like
/// `existsObject(forKey:)`.
class CachingDiskStorage<T>: StorageAware {
    
    private let storage: DiskStorage<T>
    private var files = [String: FileState]()
    
    init(config: DiskConfig, transformer: Transformer<T>) throws {
        storage = try DiskStorage(config: config, transformer: transformer)
    }
    
    private func state(for key: String) -> FileState {
        return files[key] ?? .unknown
    }
    
    func entry(forKey key: String) throws -> Entry<T> {
        if state(for: key) == .doesNotExist {
            throw StorageError.notFound
        }
        return try storage.entry(forKey: key)
    }
    
    func existsObject(forKey key: String) throws -> Bool {
        switch state(for: key) {
        case .unknown:
            return try storage.existsObject(forKey: key)
        case .exists:
            return true
        case .doesNotExist:
            return false
        }
    }
    
    func removeObject(forKey key: String) throws {
        try storage.removeObject(forKey: key)
        files[key] = .doesNotExist
    }
    
    func setObject(_ object: T, forKey key: String, expiry: Expiry? = nil) throws {
        try storage.setObject(object, forKey: key, expiry: expiry)
        files[key] = .exists
    }
    
    func removeAll() throws {
        try storage.removeAll()
        files.removeAll()
    }
    
    func removeExpiredObjects() throws {
        try storage.removeExpiredObjects()
    }
    
}

extension CachingDiskStorage {
    private enum FileState {
        case unknown, exists, doesNotExist
    }
}