LeStorage/LeStorage/BaseCollection.swift

//
//  BaseCollection.swift
//  LeStorage
//
//  Created by Laurent Morvillier on 02/02/2024.
//

import Foundation

public protocol CollectionHolder {
    associatedtype Item: Storable

    var items: [Item] { get }
    func reset()
}

public protocol SomeCollection: CollectionHolder, Identifiable {
    
    var resourceName: String { get }
    var hasLoaded: Bool { get }
    var inMemory: Bool { get }
    var type: any Storable.Type { get }

    func referenceCount<S: Storable>(type: S.Type, id: String) -> Int

    func findById(_ id: Item.ID) -> Item?
}

public class BaseCollection<T: Storable>: SomeCollection, CollectionHolder {

    /// Doesn't write the collection in a file
    fileprivate(set) public var inMemory: Bool = false

    /// The list of stored items
    @Published public fileprivate(set) var items: [T] = []

    /// The reference to the Store
    fileprivate(set) var store: Store

    /// Provides fast access for instances if the collection has been instanced with [indexed] = true
    fileprivate var _indexes: [T.ID: T]? = nil

    /// A PendingOperationManager instance that manages operations while the collection is not loaded
    fileprivate(set) var pendingOperationManager: PendingOperationManager<T>? = nil
    
    /// Indicates whether the collection has changed, thus requiring a write operation
    fileprivate var _hasChanged: Bool = false {
        didSet {
            if self._hasChanged == true {

                self._scheduleWrite()
                DispatchQueue.main.async {
                    NotificationCenter.default.post(
                        name: NSNotification.Name.CollectionDidChange, object: self)
                }
                self._hasChanged = false
            }
        }
    }

    /// Indicates if the collection has loaded locally, with or without a file
    fileprivate(set) public var hasLoaded: Bool = false

    /// Sets a max number of items inside the collection
    fileprivate(set) var limit: Int? = nil

    init(store: Store, indexed: Bool = false, inMemory: Bool = false, limit: Int? = nil, synchronousLoading: Bool = false) {
        if indexed {
            self._indexes = [:]
        }
        self.inMemory = inMemory
        self.store = store
        self.limit = limit

        if synchronousLoading {
            Task {
                await self.loadFromFile()
            }
        } else {
            Task(priority: .high) {
                await self.load()
            }
        }
    }

    init(store: Store) {
        self.store = store
    }

    var storeCenter: StoreCenter { return self.store.storeCenter }
    
    /// Returns the name of the managed resource
    public var resourceName: String {
        return T.resourceName()
    }

    public var storeId: String? {
        return self.store.identifier
    }

    // MARK: - Loading

    /// Sets the collection as changed to trigger a write
    func setChanged() {
        self._hasChanged = true
    }

    /// Migrates if necessary and asynchronously decodes the json file
    func load() async {
        if !self.inMemory {
            await self.loadFromFile()
        } else {
            await MainActor.run {
                self.setAsLoaded()
            }
        }
    }

    /// Starts the JSON file decoding synchronously or asynchronously
    func loadFromFile() async {
        do {
            try await self._decodeJSONFile()
        } catch {
            Logger.error(error)
        }
    }

    /// Decodes the json file into the items array
    fileprivate func _decodeJSONFile() async throws {

        let fileURL = try self.store.fileURL(type: T.self)

        if FileManager.default.fileExists(atPath: fileURL.path()) {
            let jsonString: String = try FileUtils.readFile(fileURL: fileURL)
            let decoded: [T] = try jsonString.decodeArray() ?? []
            self.setItems(decoded)
        }
        await MainActor.run {
            self.setAsLoaded()
        }

    }

    /// Sets the collection as loaded
    /// Send a CollectionDidLoad event
    @MainActor
    func setAsLoaded() {
        self.hasLoaded = true
        self._mergePendingOperations()

        NotificationCenter.default.post(
            name: NSNotification.Name.CollectionDidLoad, object: self)
    }
    
    /// Sets a collection of items and indexes them
    func setItems(_ items: [T]) {
        for item in items {
            item.store = self.store
        }
        self.items = items
        self._updateIndexIfNecessary()
    }

    /// Updates the whole index with the items array
    fileprivate func _updateIndexIfNecessary() {
        if self._indexes != nil {
            self._indexes = self.items.dictionary { $0.id }
        }
    }

    // MARK: - Basic operations

    /// Adds or updates the provided instance inside the collection
    /// Adds it if its id is not found, and otherwise updates it
    public func addOrUpdate(instance: T) {
        self.addOrUpdateItem(instance: instance)
    }

    /// Adds or update an instance inside the collection and writes
    func addOrUpdateItem(instance: T) {

        defer {
            self._hasChanged = true
        }

        if let index = self.items.firstIndex(where: { $0.id == instance.id }) {
            self.updateItem(instance, index: index)
        } else {
            self.addItem(instance: instance)
        }

    }

    /// A method the treat the collection as a single instance holder
    func setSingletonNoSync(instance: T) {
        defer {
            self._hasChanged = true
        }
        self.items.removeAll()
        self.addItem(instance: instance)
    }

    /// Deletes an item by its id
    public func deleteById(_ id: T.ID) {
        if let instance = self.findById(id) {
            self.delete(instance: instance)
        }
    }

    /// Deletes the instance in the collection and sets the collection as changed to trigger a write
    public func delete(instance: T) {
        defer {
            self._hasChanged = true
        }
        self.deleteItem(instance)
    }

    /// Deletes all items of the sequence by id and sets the collection as changed to trigger a write
    public func delete(contentOfs sequence: any RandomAccessCollection<T>) {

        defer {
            self._hasChanged = true
        }

        for instance in sequence {
            self.deleteItem(instance)
        }
    }

    /// Adds or update a sequence of elements
    public func addOrUpdate(contentOfs sequence: any Sequence<T>) {
        self.addSequence(sequence)
        //        self._addOrUpdate(contentOfs: sequence)
    }

    /// Adds a sequence of objects inside the collection and performs a write
    func addSequence(_ sequence: any Sequence<T>, checkLoaded: Bool = true) {
        defer {
            self._hasChanged = true
        }

        for instance in sequence {
            if let index = self.items.firstIndex(where: { $0.id == instance.id }) {
                self.updateItem(instance, index: index, checkLoaded: checkLoaded)
            } else {  // insert
                self.addItem(instance: instance, checkLoaded: checkLoaded)
            }
        }

    }

    /// This method sets the storeId for the given instance if the collection belongs to a store with an id
    fileprivate func _affectStoreIdIfNecessary(instance: T) {
        if let storeId = self.store.identifier {
            if var altStorable = instance as? SideStorable {
                altStorable.storeId = storeId
            } else {
                fatalError("instance does not implement SideStorable, thus sync cannot work")
            }
        }
    }

    /// Adds an instance to the collection
    @discardableResult func addItem(instance: T, checkLoaded: Bool = true, shouldBeSynchronized: Bool = false) -> Bool {
        
        if checkLoaded && !self.hasLoaded {
            self._addPendingOperation(method: .addOrUpdate, instance: instance, shouldBeSynchronized: shouldBeSynchronized)
            return false
        }
        
        self._affectStoreIdIfNecessary(instance: instance)
        self.items.append(instance)
        instance.store = self.store
        self._indexes?[instance.id] = instance
        self._applyLimitIfPresent()
        return true
    }

    /// Updates an instance to the collection by index
    @discardableResult func updateItem(_ instance: T, index: Int, checkLoaded: Bool = true, shouldBeSynchronized: Bool = false) -> Bool {
        
        if checkLoaded && !self.hasLoaded {
            self._addPendingOperation(method: .addOrUpdate, instance: instance, shouldBeSynchronized: shouldBeSynchronized)
            return false
        }

        let item = self.items[index]
        if item !== instance {
            self.items[index].copy(from: instance)
        }

        instance.store = self.store
        self._indexes?[instance.id] = instance
        return true
    }

    /// Deletes an instance from the collection
    @discardableResult func deleteItem(_ instance: T, shouldBeSynchronized: Bool = false) -> Bool {
        
        if !self.hasLoaded {
            self._addPendingOperation(method: .addOrUpdate, instance: instance, shouldBeSynchronized: shouldBeSynchronized)
            return false
        }

        instance.deleteDependencies(shouldBeSynchronized: shouldBeSynchronized)
        self.items.removeAll { $0.id == instance.id }
        self._indexes?.removeValue(forKey: instance.id)
        return true
    }

    /// If the collection has more instance that its limit, remove the surplus
    fileprivate func _applyLimitIfPresent() {
        if let limit {
            self.items = self.items.suffix(limit)
        }
    }

    /// Returns the instance corresponding to the provided [id]
    public func findById(_ id: T.ID) -> T? {
        if let index = self._indexes, let instance = index[id] {
            return instance
        }
        return self.items.first(where: { $0.id == id })
    }

    /// Proceeds to "hard" delete the items without synchronizing them
    /// Also removes related API calls
    public func deleteDependencies(_ items: any Sequence<T>) {
        defer {
            self._hasChanged = true
        }
        let itemsArray = Array(items)  // fix error if items is self.items
        for item in itemsArray {
            if let index = self.items.firstIndex(where: { $0.id == item.id }) {
                self.items.remove(at: index)
            }
        }

    }

    /// Proceeds to delete all instance of the collection, properly cleaning up dependencies and sending API calls
    public func deleteAll() throws {
        self.delete(contentOfs: self.items)
    }

    // MARK: - Pending operations
    
    fileprivate func _addPendingOperation(method: StorageMethod, instance: T, shouldBeSynchronized: Bool) {
        if self.pendingOperationManager == nil {
            self.pendingOperationManager = PendingOperationManager<T>(store: self.store, inMemory: self.inMemory)
        }
        self.addPendingOperation(method: method, instance: instance, shouldBeSynchronized: false)
    }
    
    func addPendingOperation(method: StorageMethod, instance: T, shouldBeSynchronized: Bool) {
        self.pendingOperationManager?.addPendingOperation(method: method, instance: instance, shouldBeSynchronized: shouldBeSynchronized)
    }

    fileprivate func _mergePendingOperations() {
        
        guard let manager = self.pendingOperationManager, manager.items.isNotEmpty else { return }

        Logger.log(">>> Merge pending: \(manager.items.count)")
        for item in manager.items {
            let data = item.data
            switch (item.method, item.shouldBeSynchronized) {
            case (.addOrUpdate, true):
                self.addOrUpdate(instance: data)
            case (.addOrUpdate, false):
                self.addOrUpdateItem(instance: data)
            case (.delete, true):
                self.delete(instance: data)
            case (.delete, false):
                self.deleteItem(data)
            }
        }
        
//        let methodGroups = manager.items.group { $0.method }
//        for (method, group) in methodGroups {
//            let dataArray = group.map { $0.data }
//            switch method {
//            case .addOrUpdate:
//                self.addOrUpdate(contentOfs: dataArray)
//            case .delete:
//                self.delete(contentOfs: dataArray)
//            }
//        }
        self.pendingOperationManager = nil
    }

    // MARK: - File access

    /// Schedules a write operation
    fileprivate func _scheduleWrite() {

        guard !self.inMemory else { return }

        DispatchQueue(label: "lestorage.queue.write", qos: .utility).asyncAndWait {  // sync to make sure we don't have writes performed at the same time
            self._write()
        }
    }

    /// Writes all the items as a json array inside a file
    fileprivate func _write() {
        do {
            let jsonString: String = try self.items.jsonString()
            try self.store.write(content: jsonString, fileName: T.fileName())
        } catch {
            Logger.error(error)
            self.storeCenter.log(
                message: "write failed for \(T.resourceName()): \(error.localizedDescription)")
        }
    }

    /// Simply clears the items of the collection
    public func clear() {
        self.items.removeAll()
    }

    /// Removes the items of the collection and deletes the corresponding file
    public func reset() {
        self.items.removeAll()
        self.store.removeFile(type: T.self)
        setChanged()
    }

    public var type: any Storable.Type { return T.self }

    // MARK: - Reference count

    /// Counts the references to an object - given its type and id - inside the collection
    public func referenceCount<S: Storable>(type: S.Type, id: String) -> Int {
        let relationships = T.relationships().filter { $0.type == type }
        guard relationships.count > 0 else { return 0 }

        return self.items.reduce(0) { count, item in
            count
                + relationships.filter { relationship in
                    (item[keyPath: relationship.keyPath] as? String) == id
                }.count
        }
    }

}

public class StoredCollection<T: Storable>: BaseCollection<T>, RandomAccessCollection {
    
    /// Returns a dummy StoredCollection instance
    public static func placeholder() -> StoredCollection<T> {
        return StoredCollection<T>(store: Store(storeCenter: StoreCenter.main))
    }

    // MARK: - RandomAccessCollection

    public var startIndex: Int { return self.items.startIndex }

    public var endIndex: Int { return self.items.endIndex }

    public func index(after i: Int) -> Int {
        return self.items.index(after: i)
    }

    open subscript(index: Int) -> T {
        get {
            return self.items[index]
        }
        set(newValue) {
            self.items[index] = newValue
            self._hasChanged = true
        }
    }

}

extension SyncedCollection: RandomAccessCollection {
    
    public var startIndex: Int { return self.items.startIndex }

    public var endIndex: Int { return self.items.endIndex }

    public func index(after i: Int) -> Int {
        return self.items.index(after: i)
    }

    public subscript(index: Int) -> T {
        get {
            return self.items[index]
        }
        set(newValue) {
            self.items[index] = newValue
            self._hasChanged = true
        }
    }
}