/*
 * Copyright (C) 2022 by Claudio Cambra <claudio.cambra@nextcloud.com>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
 * or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
 * for more details.
 */

import FileProvider
import NextcloudKit
import OSLog

class FileProviderEnumerator: NSObject, NSFileProviderEnumerator {
    
    private let enumeratedItemIdentifier: NSFileProviderItemIdentifier
    private var enumeratedItemMetadata: NextcloudItemMetadataTable?
    private var enumeratingSystemIdentifier: Bool {
        return FileProviderEnumerator.isSystemIdentifier(enumeratedItemIdentifier)
    }
    private let anchor = NSFileProviderSyncAnchor(Date().description.data(using: .utf8)!) // TODO: actually use this in NCKit and server requests
    private static let maxItemsPerFileProviderPage = 100
    let ncAccount: NextcloudAccount
    let ncKit: NextcloudKit
    var serverUrl: String = ""
    var isInvalidated = false

    let syncEngine = NextcloudSyncEngine()

    private static func isSystemIdentifier(_ identifier: NSFileProviderItemIdentifier) -> Bool {
        return identifier == .rootContainer ||
            identifier == .trashContainer ||
            identifier == .workingSet
    }
    
    init(enumeratedItemIdentifier: NSFileProviderItemIdentifier, ncAccount: NextcloudAccount, ncKit: NextcloudKit) {
        self.enumeratedItemIdentifier = enumeratedItemIdentifier
        self.ncAccount = ncAccount
        self.ncKit = ncKit

        if FileProviderEnumerator.isSystemIdentifier(enumeratedItemIdentifier) {
            Logger.enumeration.debug("Providing enumerator for a system defined container: \(enumeratedItemIdentifier.rawValue, privacy: .public)")
            self.serverUrl = ncAccount.davFilesUrl
        } else {
            Logger.enumeration.debug("Providing enumerator for item with identifier: \(enumeratedItemIdentifier.rawValue, privacy: .public)")
            let dbManager = NextcloudFilesDatabaseManager.shared

            enumeratedItemMetadata = dbManager.itemMetadataFromFileProviderItemIdentifier(enumeratedItemIdentifier)
            if enumeratedItemMetadata != nil {
                self.serverUrl = enumeratedItemMetadata!.serverUrl + "/" + enumeratedItemMetadata!.fileName
            } else {
                Logger.enumeration.error("Could not find itemMetadata for file with identifier: \(enumeratedItemIdentifier.rawValue, privacy: .public)")
            }
        }

        Logger.enumeration.info("Set up enumerator for user: \(self.ncAccount.ncKitAccount, privacy: OSLogPrivacy.auto(mask: .hash)) with serverUrl: \(self.serverUrl, privacy: OSLogPrivacy.auto(mask: .hash))")
        super.init()
    }

    func invalidate() {
        Logger.enumeration.debug("Enumerator is being invalidated for item with identifier: \(self.enumeratedItemIdentifier.rawValue, privacy: .public)")
        syncEngine.invalidate()
        self.isInvalidated = true
    }

    // MARK: - Protocol methods

    func enumerateItems(for observer: NSFileProviderEnumerationObserver, startingAt page: NSFileProviderPage) {
        Logger.enumeration.debug("Received enumerate items request for enumerator with user: \(self.ncAccount.ncKitAccount, privacy: OSLogPrivacy.auto(mask: .hash)) with serverUrl: \(self.serverUrl, privacy: OSLogPrivacy.auto(mask: .hash))")
        /*
         - inspect the page to determine whether this is an initial or a follow-up request (TODO)
         
         If this is an enumerator for a directory, the root container or all directories:
         - perform a server request to fetch directory contents
         If this is an enumerator for the working set:
         - perform a server request to update your local database
         - fetch the working set from your local database
         
         - inform the observer about the items returned by the server (possibly multiple times)
         - inform the observer that you are finished with this page
         */

        let dbManager = NextcloudFilesDatabaseManager.shared

        // When enumerating items in the working set, we are expected to provide an account of the
        // whole server and all its files.
        if enumeratedItemIdentifier == .workingSet {
            if page == NSFileProviderPage.initialPageSortedByDate as NSFileProviderPage ||
                page == NSFileProviderPage.initialPageSortedByName as NSFileProviderPage {

                // We enumerate items as we get the server data for two reasons:
                // A) we avoid having a gigantic chunk of files to enumerate to the observer at the end
                // B) we don't need to worry about resolving which files are truly deleted vs moved at the end
                syncEngine.fullRecursiveScan(ncAccount: self.ncAccount, ncKit: self.ncKit, scanChangesOnly: false, singleFolderScanCompleteCompletionHandler: { metadatas, error in

                    guard error == nil else {
                        Logger.enumeration.error("There was an error during recursive item enumeration of working set for user: \(self.ncAccount.ncKitAccount, privacy: OSLogPrivacy.auto(mask: .hash)) with error: \(error!.errorDescription, privacy: .public)")
                        observer.finishEnumeratingWithError(error!.toFileProviderError())
                        return;
                    }

                    guard let metadatas = metadatas else {
                        Logger.enumeration.warning("Received nil metadatas during recursive item enumeration of working set for user: \(self.ncAccount.ncKitAccount, privacy: OSLogPrivacy.auto(mask: .hash)) with error: \(error!.errorDescription, privacy: .public)")
                        return
                    }

                    let items = FileProviderEnumerator.metadatasToFileProviderItems(metadatas, ncKit: self.ncKit)
                    observer.didEnumerate(items)

                }) { metadatas, _, _, _, error in

                    if self.isInvalidated {
                        Logger.enumeration.info("Enumerator invalidated during working set item enumeration. For user: \(self.ncAccount.ncKitAccount, privacy: OSLogPrivacy.auto(mask: .hash))")
                        observer.finishEnumeratingWithError(NSFileProviderError(.cannotSynchronize))
                        return
                    }

                    guard error == nil else {
                        Logger.enumeration.info("Finished recursive iteme enumeration of working set for user: \(self.ncAccount.ncKitAccount, privacy: OSLogPrivacy.auto(mask: .hash)) with error: \(error!.errorDescription, privacy: .public)")
                        observer.finishEnumeratingWithError(error!.toFileProviderError())
                        return;
                    }

                    Logger.enumeration.info("Finished recursive item enumeration of working set for user: \(self.ncAccount.ncKitAccount, privacy: OSLogPrivacy.auto(mask: .hash)). Enumerating items.")

                    observer.finishEnumerating(upTo: FileProviderEnumerator.fileProviderPageforNumPage(1))
                }

                return
            } else {
                Logger.enumeration.debug("Enumerating page \(page.rawValue) of working set for user: \(self.ncAccount.ncKitAccount, privacy: OSLogPrivacy.auto(mask: .hash))")
                // TODO!

                observer.finishEnumerating(upTo: nil)
            }

            return
        } else if enumeratedItemIdentifier == .trashContainer {
            Logger.enumeration.debug("Enumerating trash set for user: \(self.ncAccount.ncKitAccount, privacy: OSLogPrivacy.auto(mask: .hash)) with serverUrl: \(self.serverUrl, privacy: OSLogPrivacy.auto(mask: .hash))")
            // TODO!

            observer.finishEnumerating(upTo: nil)
            return
        }

        guard serverUrl != "" else {
            Logger.enumeration.error("Enumerator has empty serverUrl -- can't enumerate that! For identifier: \(self.enumeratedItemIdentifier.rawValue, privacy: .public)")
            observer.finishEnumeratingWithError(NSFileProviderError(.noSuchItem))
            return
        }

        // TODO: Make better use of pagination and andle paging properly
        if page == NSFileProviderPage.initialPageSortedByDate as NSFileProviderPage ||
            page == NSFileProviderPage.initialPageSortedByName as NSFileProviderPage {

            Logger.enumeration.debug("Enumerating initial page for user: \(self.ncAccount.ncKitAccount, privacy: OSLogPrivacy.auto(mask: .hash)) with serverUrl: \(self.serverUrl, privacy: OSLogPrivacy.auto(mask: .hash))")

            NextcloudSyncEngine.readServerUrl(serverUrl, ncAccount: ncAccount, ncKit: ncKit) { _, _, _, _, readError in

                guard readError == nil else {
                    Logger.enumeration.error("Finishing enumeration for user: \(self.ncAccount.ncKitAccount, privacy: OSLogPrivacy.auto(mask: .hash)) with serverUrl: \(self.serverUrl, privacy: OSLogPrivacy.auto(mask: .hash)) with error \(readError!.localizedDescription, privacy: .public)")

                    let nkReadError = NKError(error: readError!)
                    observer.finishEnumeratingWithError(nkReadError.toFileProviderError())
                    return
                }

                let ncKitAccount = self.ncAccount.ncKitAccount

                // Return all now known metadatas
                var metadatas: [NextcloudItemMetadataTable]

                if self.enumeratingSystemIdentifier || (self.enumeratedItemMetadata != nil && self.enumeratedItemMetadata!.directory) {
                    metadatas = NextcloudFilesDatabaseManager.shared.itemMetadatas(account: ncKitAccount, serverUrl: self.serverUrl)
                } else if (self.enumeratedItemMetadata != nil) {
                    guard let updatedEnumeratedItemMetadata = NextcloudFilesDatabaseManager.shared.itemMetadataFromOcId(self.enumeratedItemMetadata!.ocId) else {
                        Logger.enumeration.error("Could not finish enumeration for user: \(ncKitAccount, privacy: OSLogPrivacy.auto(mask: .hash)) with serverUrl: \(self.serverUrl, privacy: OSLogPrivacy.auto(mask: .hash)) as the enumerated item could not be fetched from database. \(self.enumeratedItemIdentifier.rawValue, privacy: .public)")
                        observer.finishEnumeratingWithError(NSFileProviderError(.noSuchItem))
                        return
                    }

                    metadatas = [updatedEnumeratedItemMetadata]
                } else { // We need to have an enumeratedItemMetadata to have a non empty serverUrl
                    Logger.enumeration.error("Cannot finish enumeration for user: \(ncKitAccount, privacy: OSLogPrivacy.auto(mask: .hash)) as we do not have a valid server URL. NOTE: this error should not be possible and indicates something is going wrong before.")
                    observer.finishEnumeratingWithError(NSFileProviderError(.noSuchItem))
                    return
                }

                Logger.enumeration.info("Finished reading serverUrl: \(self.serverUrl, privacy: OSLogPrivacy.auto(mask: .hash)) for user: \(ncKitAccount, privacy: OSLogPrivacy.auto(mask: .hash)). Processed \(metadatas.count) metadatas")

                FileProviderEnumerator.completeEnumerationObserver(observer, ncKit: self.ncKit, numPage: 1, itemMetadatas: metadatas)
            }

            return;
        }

        let numPage = Int(String(data: page.rawValue, encoding: .utf8)!)!
        Logger.enumeration.debug("Enumerating page \(numPage, privacy: .public) for user: \(self.ncAccount.ncKitAccount, privacy: OSLogPrivacy.auto(mask: .hash)) with serverUrl: \(self.serverUrl, privacy: OSLogPrivacy.auto(mask: .hash))")
        // TODO: Handle paging properly
        // FileProviderEnumerator.completeObserver(observer, ncKit: ncKit, numPage: numPage, itemMetadatas: nil)
        observer.finishEnumerating(upTo: nil)
    }
    
    func enumerateChanges(for observer: NSFileProviderChangeObserver, from anchor: NSFileProviderSyncAnchor) {
        Logger.enumeration.debug("Received enumerate changes request for enumerator for user: \(self.ncAccount.ncKitAccount, privacy: OSLogPrivacy.auto(mask: .hash)) with serverUrl: \(self.serverUrl, privacy: OSLogPrivacy.auto(mask: .hash))")
        /*
         - query the server for updates since the passed-in sync anchor (TODO)
         
         If this is an enumerator for the working set:
         - note the changes in your local database
         
         - inform the observer about item deletions and updates (modifications + insertions)
         - inform the observer when you have finished enumerating up to a subsequent sync anchor
         */

        if enumeratedItemIdentifier == .workingSet {
            Logger.enumeration.debug("Enumerating changes in working set for user: \(self.ncAccount.ncKitAccount, privacy: OSLogPrivacy.auto(mask: .hash))")

            // Unlike when enumerating items we can't progressively enumerate items as we need to wait to resolve which items are truly deleted and which
            // have just been moved elsewhere.
            syncEngine.fullRecursiveScan(ncAccount: self.ncAccount,
                              ncKit: self.ncKit,
                              scanChangesOnly: true,
                              singleFolderScanCompleteCompletionHandler: { _, _ in }) { _, newMetadatas, updatedMetadatas, deletedMetadatas, error in

                if self.isInvalidated {
                    Logger.enumeration.info("Enumerator invalidated during working set change scan. For user: \(self.ncAccount.ncKitAccount, privacy: OSLogPrivacy.auto(mask: .hash))")
                    observer.finishEnumeratingWithError(NSFileProviderError(.cannotSynchronize))
                    return
                }

                guard error == nil else {
                    Logger.enumeration.info("Finished recursive change enumeration of working set for user: \(self.ncAccount.ncKitAccount, privacy: OSLogPrivacy.auto(mask: .hash)) with error: \(error!.errorDescription, privacy: .public)")
                    observer.finishEnumeratingWithError(error!.toFileProviderError())
                    return
                }

                Logger.enumeration.info("Finished recursive change enumeration of working set for user: \(self.ncAccount.ncKitAccount, privacy: OSLogPrivacy.auto(mask: .hash)). Enumerating items.")

                FileProviderEnumerator.completeChangesObserver(observer,
                                                               anchor: anchor,
                                                               ncKit: self.ncKit,
                                                               newMetadatas: newMetadatas,
                                                               updatedMetadatas: updatedMetadatas,
                                                               deletedMetadatas: deletedMetadatas)
            }
            return
        } else if enumeratedItemIdentifier == .trashContainer {
            Logger.enumeration.debug("Enumerating changes in trash set for user: \(self.ncAccount.ncKitAccount, privacy: OSLogPrivacy.auto(mask: .hash))")
            // TODO!

            observer.finishEnumeratingChanges(upTo: anchor, moreComing: false)
            return
        }

        Logger.enumeration.info("Enumerating changes for user: \(self.ncAccount.ncKitAccount, privacy: OSLogPrivacy.auto(mask: .hash)) with serverUrl: \(self.serverUrl, privacy: OSLogPrivacy.auto(mask: .hash))")

        // No matter what happens here we finish enumeration in some way, either from the error
        // handling below or from the completeChangesObserver
        NextcloudSyncEngine.readServerUrl(serverUrl, ncAccount: ncAccount, ncKit: ncKit, stopAtMatchingEtags: true) { _, newMetadatas, updatedMetadatas, deletedMetadatas, readError in

            // If we get a 404 we might add more deleted metadatas
            var currentDeletedMetadatas: [NextcloudItemMetadataTable] = []
            if let notNilDeletedMetadatas = deletedMetadatas {
                currentDeletedMetadatas = notNilDeletedMetadatas
            }

            guard readError == nil else {
                Logger.enumeration.error("Finishing enumeration of changes for user: \(self.ncAccount.ncKitAccount, privacy: OSLogPrivacy.auto(mask: .hash)) with serverUrl: \(self.serverUrl, privacy: OSLogPrivacy.auto(mask: .hash)) with error: \(readError!.localizedDescription, privacy: .public)")

                let nkReadError = NKError(error: readError!)
                let fpError = nkReadError.toFileProviderError()

                if nkReadError.isNotFoundError {
                    Logger.enumeration.info("404 error means item no longer exists. Deleting metadata and reporting \(self.serverUrl, privacy: OSLogPrivacy.auto(mask: .hash)) as deletion without error")

                    guard let itemMetadata = self.enumeratedItemMetadata else {
                        Logger.enumeration.error("Invalid enumeratedItemMetadata, could not delete metadata nor report deletion")
                        observer.finishEnumeratingWithError(fpError)
                        return
                    }

                    let dbManager = NextcloudFilesDatabaseManager.shared
                    if itemMetadata.directory {
                        if let deletedDirectoryMetadatas = dbManager.deleteDirectoryAndSubdirectoriesMetadata(ocId: itemMetadata.ocId) {
                            currentDeletedMetadatas += deletedDirectoryMetadatas
                        } else {
                            Logger.enumeration.error("Something went wrong when recursively deleting directory not found.")
                        }
                    } else {
                        dbManager.deleteItemMetadata(ocId: itemMetadata.ocId)
                    }

                    FileProviderEnumerator.completeChangesObserver(observer, anchor: anchor, ncKit: self.ncKit, newMetadatas: nil, updatedMetadatas: nil, deletedMetadatas: [itemMetadata])
                    return
                } else if nkReadError.isNoChangesError { // All is well, just no changed etags
                    Logger.enumeration.info("Error was to say no changed files -- not bad error. Finishing change enumeration.")
                    observer.finishEnumeratingChanges(upTo: anchor, moreComing: false)
                    return;
                }

                observer.finishEnumeratingWithError(fpError)
                return
            }

            Logger.enumeration.info("Finished reading serverUrl: \(self.serverUrl, privacy: OSLogPrivacy.auto(mask: .hash)) for user: \(self.ncAccount.ncKitAccount, privacy: OSLogPrivacy.auto(mask: .hash))")

            FileProviderEnumerator.completeChangesObserver(observer, anchor: anchor, ncKit: self.ncKit, newMetadatas: newMetadatas, updatedMetadatas: updatedMetadatas, deletedMetadatas: deletedMetadatas)
        }
    }

    func currentSyncAnchor(completionHandler: @escaping (NSFileProviderSyncAnchor?) -> Void) {
        completionHandler(anchor)
    }

    // MARK: - Helper methods

    private static func metadatasToFileProviderItems(_ itemMetadatas: [NextcloudItemMetadataTable], ncKit: NextcloudKit) -> [NSFileProviderItem] {
        var items: [NSFileProviderItem] = []

        for itemMetadata in itemMetadatas {
            if itemMetadata.e2eEncrypted {
                Logger.enumeration.info("Skipping encrypted metadata in enumeration: \(itemMetadata.ocId) \(itemMetadata.fileName, privacy: OSLogPrivacy.auto(mask: .hash))")
                continue
            }

            if let parentItemIdentifier = NextcloudFilesDatabaseManager.shared.parentItemIdentifierFromMetadata(itemMetadata) {
                let item = FileProviderItem(metadata: itemMetadata, parentItemIdentifier: parentItemIdentifier, ncKit: ncKit)
                Logger.enumeration.debug("Will enumerate item with ocId: \(itemMetadata.ocId) and name: \(itemMetadata.fileName, privacy: OSLogPrivacy.auto(mask: .hash))")
                items.append(item)
            } else {
                Logger.enumeration.error("Could not get valid parentItemIdentifier for item with ocId: \(itemMetadata.ocId, privacy: .public) and name: \(itemMetadata.fileName, privacy: OSLogPrivacy.auto(mask: .hash)), skipping enumeration")
            }
        }

        return items
    }

    private static func fileProviderPageforNumPage(_ numPage: Int) -> NSFileProviderPage {
        return NSFileProviderPage("\(numPage)".data(using: .utf8)!)
    }

    private static func completeEnumerationObserver(_ observer: NSFileProviderEnumerationObserver, ncKit: NextcloudKit, numPage: Int, itemMetadatas: [NextcloudItemMetadataTable]) {

        let items = FileProviderEnumerator.metadatasToFileProviderItems(itemMetadatas, ncKit: ncKit)
        observer.didEnumerate(items)
        Logger.enumeration.info("Did enumerate \(items.count) items")

        // TODO: Handle paging properly
        /*
        if items.count == maxItemsPerFileProviderPage {
            let nextPage = numPage + 1
            let providerPage = NSFileProviderPage("\(nextPage)".data(using: .utf8)!)
            observer.finishEnumerating(upTo: providerPage)
        } else {
            observer.finishEnumerating(upTo: nil)
        }
         */
        observer.finishEnumerating(upTo: fileProviderPageforNumPage(numPage))
    }

    private static func completeChangesObserver(_ observer: NSFileProviderChangeObserver, anchor: NSFileProviderSyncAnchor, ncKit: NextcloudKit, newMetadatas: [NextcloudItemMetadataTable]?, updatedMetadatas: [NextcloudItemMetadataTable]?, deletedMetadatas: [NextcloudItemMetadataTable]?) {

        guard newMetadatas != nil || updatedMetadatas != nil || deletedMetadatas != nil else {
            Logger.enumeration.error("Received invalid newMetadatas, updatedMetadatas or deletedMetadatas. Finished enumeration of changes with error.")
            observer.finishEnumeratingWithError(NSFileProviderError(.noSuchItem))
            return
        }

        // Observer does not care about new vs updated, so join
        var allUpdatedMetadatas: [NextcloudItemMetadataTable] = []
        var allDeletedMetadatas: [NextcloudItemMetadataTable] = []

        if let newMetadatas = newMetadatas {
            allUpdatedMetadatas += newMetadatas
        }

        if let updatedMetadatas = updatedMetadatas {
            allUpdatedMetadatas += updatedMetadatas
        }

        if let deletedMetadatas = deletedMetadatas {
            allDeletedMetadatas = deletedMetadatas
        }

        var allFpItemUpdates: [FileProviderItem] = []
        var allFpItemDeletionsIdentifiers = Array(allDeletedMetadatas.map { NSFileProviderItemIdentifier($0.ocId) })

        for updMetadata in allUpdatedMetadatas {
            guard let parentItemIdentifier = NextcloudFilesDatabaseManager.shared.parentItemIdentifierFromMetadata(updMetadata) else {
                Logger.enumeration.warning("Not enumerating change for metadata: \(updMetadata.ocId) \(updMetadata.fileName, privacy: OSLogPrivacy.auto(mask: .hash)) as could not get parent item metadata.")
                continue
            }

            guard !updMetadata.e2eEncrypted else {
                // Precaution, if all goes well in NKFile conversion then this should not happen
                // TODO: Remove when E2EE supported
                Logger.enumeration.info("Encrypted metadata in changes enumeration \(updMetadata.ocId) \(updMetadata.fileName, privacy: OSLogPrivacy.auto(mask: .hash)), adding to deletions")
                allFpItemDeletionsIdentifiers.append(NSFileProviderItemIdentifier(updMetadata.ocId))
                continue
            }

            let fpItem = FileProviderItem(metadata: updMetadata, parentItemIdentifier: parentItemIdentifier, ncKit: ncKit)
            allFpItemUpdates.append(fpItem)
        }

        if !allFpItemUpdates.isEmpty {
            observer.didUpdate(allFpItemUpdates)
        }

        if !allFpItemDeletionsIdentifiers.isEmpty {
            observer.didDeleteItems(withIdentifiers: allFpItemDeletionsIdentifiers)
        }

        Logger.enumeration.info("Processed \(allUpdatedMetadatas.count) new or updated metadatas, \(allDeletedMetadatas.count) deleted metadatas.")
        observer.finishEnumeratingChanges(upTo: anchor, moreComing: false)
    }
}