//
//  RemoteConfigDiskCache.swift
//  RevenueCat
//
//  Created by Rick van der Linden.
//  Copyright © 2026 RevenueCat, Inc. All rights reserved.

import Foundation

/// Read-only access to a topic's persisted item index (metadata only, no blob bytes or waiting).
protocol RemoteConfigTopicStoreType: AnyObject {

    /// The saved items for `topic`, or `nil` when nothing has been persisted yet.
    func topic(_ topic: RemoteConfigTopic) -> RemoteConfiguration.ConfigTopic?

}

protocol RemoteConfigDiskCacheType: RemoteConfigTopicStoreType {

    func read() -> PersistedRemoteConfiguration?

    @discardableResult
    func write(_ configuration: PersistedRemoteConfiguration) -> Bool

    func clear()

}

struct PersistedRemoteConfiguration: Codable, Equatable {

    let domain: String
    let manifest: String
    let activeTopics: [String]
    let prefetchBlobs: [String]
    let topics: RemoteConfiguration.Topics

    /// The most recent `X-RevenueCat-Request-Time` supplied by the main API, stored as epoch milliseconds.
    ///
    /// This value is replayed as `X-RC-Last-Refresh-Time` on the next config request.
    private(set) var lastRefreshTimeMilliseconds: UInt64?

    init(
        domain: String = RemoteConfiguration.defaultDomain,
        manifest: String,
        activeTopics: [String] = [],
        prefetchBlobs: [String] = [],
        topics: RemoteConfiguration.Topics = .init(),
        lastRefreshTimeMilliseconds: UInt64? = nil
    ) {
        self.domain = domain
        self.manifest = manifest
        self.activeTopics = activeTopics
        self.prefetchBlobs = prefetchBlobs
        self.topics = topics
        self.lastRefreshTimeMilliseconds = lastRefreshTimeMilliseconds
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        self.init(
            domain: try container.decodeIfPresent(String.self, forKey: .domain) ?? RemoteConfiguration.defaultDomain,
            manifest: try container.decode(String.self, forKey: .manifest),
            activeTopics: try container.decodeIfPresent([String].self, forKey: .activeTopics) ?? [],
            prefetchBlobs: try container.decodeIfPresent([String].self, forKey: .prefetchBlobs) ?? [],
            topics: try container.decodeIfPresent(RemoteConfiguration.Topics.self, forKey: .topics) ?? .init(),
            lastRefreshTimeMilliseconds: try container.decodeIfPresent(
                UInt64.self,
                forKey: .lastRefreshTimeMilliseconds
            )
        )
    }

    private enum CodingKeys: String, CodingKey {
        case domain
        case manifest
        case activeTopics
        case prefetchBlobs
        case topics
        case lastRefreshTimeMilliseconds
    }

}

extension PersistedRemoteConfiguration {

    /// The persisted main-server request time in the `Date` representation used by the request layer.
    var lastRefreshTime: Date? {
        return self.lastRefreshTimeMilliseconds.map(Date.init(millisecondsSince1970:))
    }

    /// Returns a copy whose replayed refresh time is updated from a successful main-server response.
    func withLastRefreshTime(_ date: Date) -> Self {
        var copy = self
        copy.lastRefreshTimeMilliseconds = date.millisecondsSince1970

        return copy
    }

}

final class RemoteConfigDiskCache: RemoteConfigDiskCacheType {

    private let cache: SynchronizedLargeItemCache

    private let snapshot: Atomic<Snapshot> = .init(.notLoaded)

    init(
        cache: SynchronizedLargeItemCache = .init(
            cache: FileManager.default,
            basePath: RemoteConfigDiskCache.basePath,
            directoryType: RemoteConfigDiskCache.directoryType
        )
    ) {
        self.cache = cache
    }

    func read() -> PersistedRemoteConfiguration? {
        return self.snapshot.modify { snapshot in
            switch snapshot {
            case .notLoaded:
                let configuration = self.readFromDisk()
                snapshot = .loaded(configuration)
                return configuration
            case .loaded(let configuration):
                return configuration
            }
        }
    }

    @discardableResult
    func write(_ configuration: PersistedRemoteConfiguration) -> Bool {
        return self.snapshot.modify { snapshot in
            let didWrite = self.cache.set(codable: configuration, forKey: Self.fileName)
            if didWrite {
                snapshot = .loaded(configuration)
            } else {
                Logger.error(Strings.remoteConfig.failedToWriteCache)
            }

            return didWrite
        }
    }

    func clear() {
        self.snapshot.modify { snapshot in
            self.cache.clear()
            snapshot = .loaded(nil)
        }
    }

    private func readFromDisk() -> PersistedRemoteConfiguration? {
        do {
            return try self.cache.value(forKey: Self.fileName, decoder: .default)
        } catch {
            Logger.error(Strings.remoteConfig.failedToReadCache(error))
            return nil
        }
    }

}

extension RemoteConfigDiskCache {

    /// In-memory snapshot of the persisted configuration, so `read()` only hits disk once.
    /// `.loaded(nil)` means we've already checked and nothing is persisted, avoiding repeated
    /// disk reads while no configuration exists.
    private enum Snapshot {
        case notLoaded
        case loaded(PersistedRemoteConfiguration?)
    }

}

extension RemoteConfigDiskCache: RemoteConfigTopicStoreType {

    func topic(_ topic: RemoteConfigTopic) -> RemoteConfiguration.ConfigTopic? {
        return self.read()?.topics.entries[topic.wireName]
    }

}

extension RemoteConfigDiskCache {

    static let basePath = "remote_config"
    static let fileName = "remote_config.json"

    static var directoryType: DirectoryHelper.DirectoryType {
        #if os(tvOS)
        return .cache
        #else
        return .applicationSupport()
        #endif
    }

}