//
//  TimelineViewController.swift
//  Tusker
//
//  Created by Shadowfacts on 9/20/22.
//  Copyright © 2022 Shadowfacts. All rights reserved.
//

import UIKit
import Pachyderm
import Combine

class TimelineViewController: UIViewController, TimelineLikeCollectionViewController, CollectionViewController, RefreshableViewController {
    let timeline: Timeline
    weak var mastodonController: MastodonController!
    let filterer: Filterer
    
    private(set) var controller: TimelineLikeController<TimelineItem>!
    let confirmLoadMore = PassthroughSubject<Void, Never>()
    // stored separately because i don't want to query the snapshot every time the user scrolls
    private var isShowingTimelineDescription = false
    
    private(set) var collectionView: UICollectionView!
    private(set) var dataSource: UICollectionViewDiffableDataSource<Section, Item>!
    
    private var contentOffsetObservation: NSKeyValueObservation?
    private var activityToRestore: NSUserActivity?
    
    init(for timeline: Timeline, mastodonController: MastodonController!) {
        self.timeline = timeline
        self.mastodonController = mastodonController
        let filterContext: FilterV1.Context
        switch timeline {
        case .home, .list(id: _):
            filterContext = .home
        default:
            filterContext = .public
        }
        self.filterer = Filterer(mastodonController: mastodonController, context: filterContext)
        self.filterer.htmlConverter.font = TimelineStatusCollectionViewCell.contentFont
        self.filterer.htmlConverter.paragraphStyle = TimelineStatusCollectionViewCell.contentParagraphStyle
        
        super.init(nibName: nil, bundle: nil)
        
        self.controller = TimelineLikeController(delegate: self)
                
        self.navigationItem.title = timeline.title
        addKeyCommand(MenuController.refreshCommand(discoverabilityTitle: "Refresh Timeline"))
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        var config = UICollectionLayoutListConfiguration(appearance: .plain)
        config.leadingSwipeActionsConfigurationProvider = { [unowned self] in
            (collectionView.cellForItem(at: $0) as? TimelineStatusCollectionViewCell)?.leadingSwipeActions()
        }
        config.trailingSwipeActionsConfigurationProvider = { [unowned self] in
            (collectionView.cellForItem(at: $0) as? TimelineStatusCollectionViewCell)?.trailingSwipeActions()
        }
        config.itemSeparatorHandler = { [unowned self] indexPath, sectionSeparatorConfiguration in
            guard let item = self.dataSource.itemIdentifier(for: indexPath) else {
                return sectionSeparatorConfiguration
            }
            var config = sectionSeparatorConfiguration
            if item.hideSeparators {
                config.topSeparatorVisibility = .hidden
                config.bottomSeparatorVisibility = .hidden
            } else if case .status(id: let id, collapseState: _, filterState: let filterState) = item,
                      case (.hide, _) = filterResult(state: filterState, statusID: id) {
                // this runs after the cell is setup, so the filter state is already known and this check is cheap
                config.topSeparatorVisibility = .hidden
                config.bottomSeparatorVisibility = .hidden
            } else {
                config.topSeparatorInsets = TimelineStatusCollectionViewCell.separatorInsets
                config.bottomSeparatorInsets = TimelineStatusCollectionViewCell.separatorInsets
            }
            return config
        }
        let layout = UICollectionViewCompositionalLayout.list(using: config)
        collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout)
        collectionView.delegate = self
        collectionView.dragDelegate = self
        collectionView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(collectionView)
        NSLayoutConstraint.activate([
            collectionView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            collectionView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
            collectionView.topAnchor.constraint(equalTo: view.topAnchor),
            collectionView.bottomAnchor.constraint(equalTo: view.bottomAnchor),
        ])
        
        registerTimelineLikeCells()
        collectionView.register(PublicTimelineDescriptionCollectionViewCell.self, forCellWithReuseIdentifier: "publicTimelineDescription")
        dataSource = createDataSource()
        applyInitialSnapshot()
        
#if !targetEnvironment(macCatalyst)
        collectionView.refreshControl = UIRefreshControl()
        collectionView.refreshControl!.addTarget(self, action: #selector(refresh), for: .valueChanged)
#endif
        
        contentOffsetObservation = collectionView.observe(\.contentOffset) { [weak self] _, _ in
            if let indexPath = self?.dataSource.indexPath(for: .gap),
              let cell = self?.collectionView.cellForItem(at: indexPath) as? TimelineGapCollectionViewCell {
               cell.update()
           }
        }
        
        filterer.filtersChanged = { [unowned self] actionsChanged in
            self.reapplyFilters(actionsChanged: actionsChanged)
        }
        
        NotificationCenter.default.addObserver(self, selector: #selector(sceneWillEnterForeground), name: UIScene.willEnterForegroundNotification, object: nil)
    }
    
    // separate method because InstanceTimelineViewController needs to be able to customize it
    func configureStatusCell(_ cell: TimelineStatusCollectionViewCell, id: String, state: CollapseState, filterResult: Filterer.Result, precomputedContent: NSAttributedString?) {
        cell.delegate = self
        if case .home = timeline {
            cell.showFollowedHashtags = true
        } else {
            cell.showFollowedHashtags = false
        }
        cell.updateUI(statusID: id, state: state, filterResult: filterResult, precomputedContent: precomputedContent)
    }
    
    private func createDataSource() -> UICollectionViewDiffableDataSource<Section, Item> {
        let statusCell = UICollectionView.CellRegistration<TimelineStatusCollectionViewCell, (String, CollapseState, Filterer.Result, NSAttributedString?)> { [unowned self] cell, indexPath, item in
            self.configureStatusCell(cell, id: item.0, state: item.1, filterResult: item.2, precomputedContent: item.3)
        }
        let zeroHeightCell = UICollectionView.CellRegistration<ZeroHeightCollectionViewCell, Void> { _, _, _ in
        }
        let gapCell = UICollectionView.CellRegistration<TimelineGapCollectionViewCell, Void> { cell, indexPath, _ in
            cell.showsIndicator = false
        }
        let timelineDescriptionCell = UICollectionView.CellRegistration<PublicTimelineDescriptionCollectionViewCell, Item> { [unowned self] cell, indexPath, item in
            guard case .public(let local) = timeline else {
                fatalError()
            }
            cell.mastodonController = self.mastodonController
            cell.local = local
            cell.didDismiss = { [unowned self] in
                self.removeTimelineDescriptionCell()
            }
        }
        return UICollectionViewDiffableDataSource(collectionView: collectionView) { [unowned self] collectionView, indexPath, itemIdentifier in
            switch itemIdentifier {
            case .status(id: let id, collapseState: let state, filterState: let filterState):
                let (result, attributedString) = filterResult(state: filterState, statusID: id)
                switch result {
                case .allow, .warn(_):
                    return collectionView.dequeueConfiguredReusableCell(using: statusCell, for: indexPath, item: (id, state, result, nil))
                case .hide:
                    return collectionView.dequeueConfiguredReusableCell(using: zeroHeightCell, for: indexPath, item: ())
                }
            case .gap:
                return collectionView.dequeueConfiguredReusableCell(using: gapCell, for: indexPath, item: ())
            case .loadingIndicator:
                return loadingIndicatorCell(for: indexPath)
            case .confirmLoadMore:
                return confirmLoadMoreCell(for: indexPath)
            case .publicTimelineDescription:
                self.isShowingTimelineDescription = true
                return collectionView.dequeueConfiguredReusableCell(using: timelineDescriptionCell, for: indexPath, item: itemIdentifier)
            }
        }
    }
    
    // non-private, because ListTimelineViewController needs to be able to reload it from scratch
    func applyInitialSnapshot() {
        var snapshot = NSDiffableDataSourceSnapshot<Section, Item>()
        if case .public(let local) = timeline,
           (local && !Preferences.shared.hasShownLocalTimelineDescription) ||
            (!local && !Preferences.shared.hasShownFederatedTimelineDescription) {
            snapshot.appendSections([.header])
            snapshot.appendItems([.publicTimelineDescription], toSection: .header)
        }
        dataSource.apply(snapshot, animatingDifferences: false)
    }
    
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        
        collectionView.indexPathsForSelectedItems?.forEach {
            collectionView.deselectItem(at: $0, animated: true)
        }
        
        if case .notLoadedInitial = controller.state {
            if doRestore() {
                Task {
                    await checkPresent()
                }
            } else {
                Task {
                    await controller.loadInitial()
                }
            }
        }
    }
    
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        
        if isShowingTimelineDescription,
           case .public(let local) = timeline {
            if local {
                Preferences.shared.hasShownLocalTimelineDescription = true
            } else {
                Preferences.shared.hasShownFederatedTimelineDescription = true
            }
        }
    }
    
    func stateRestorationActivity() -> NSUserActivity? {
        guard isViewLoaded else {
            return nil
        }
        let visible = collectionView.indexPathsForVisibleItems.sorted()
        let snapshot = dataSource.snapshot()
        let visibleRect = CGRect(origin: collectionView.contentOffset, size: collectionView.bounds.size)
        let midPoint = CGPoint(x: visibleRect.midX, y: visibleRect.midY)
        guard let currentAccountID = mastodonController.accountInfo?.id,
              !visible.isEmpty,
              let statusesSection = snapshot.sectionIdentifiers.firstIndex(of: .statuses),
              let rawCenterVisible = collectionView.indexPathForItem(at: midPoint),
              let centerVisible = visible.first(where: { $0.section == statusesSection && $0 >= rawCenterVisible }) else {
            return nil
        }
        let allItems = snapshot.itemIdentifiers(inSection: .statuses)
        
        let startIndex = max(0, centerVisible.row - 20)
        let endIndex = min(allItems.count - 1, centerVisible.row + 20)
        
        let centerVisibleItem: Item
        var items = allItems[startIndex...endIndex]
        if let gapIndex = items.firstIndex(of: .gap) {
            // if the gap is above the top visible item, we take everything below the gap
            // otherwise, we take everything above the gap
            if gapIndex <= centerVisible.row {
                items = allItems[(gapIndex + 1)...endIndex]
                if gapIndex == centerVisible.row {
                    centerVisibleItem = allItems.first!
                } else {
                    assert(items.indices.contains(centerVisible.row))
                    centerVisibleItem = allItems[centerVisible.row]
                }
            } else {
                items = allItems[startIndex..<gapIndex]
                centerVisibleItem = allItems[centerVisible.row]
            }
        } else {
            centerVisibleItem = allItems[centerVisible.row]
        }
        let ids = items.map {
            if case .status(id: let id, _, _) = $0 {
                return id
            } else {
                fatalError()
            }
        }
        let centerVisibleID: String
        if case .status(id: let id, _, _) = centerVisibleItem {
            centerVisibleID = id
        } else {
            fatalError()
        }
        stateRestorationLogger.debug("TimelineViewController: creating state restoration activity with topID \(centerVisibleID)")
        
        let activity = UserActivityManager.showTimelineActivity(timeline: timeline, accountID: currentAccountID)!
        activity.addUserInfoEntries(from: [
            "statusIDs": ids,
            "centerID": centerVisibleID,
        ])
        activity.isEligibleForPrediction = false
        return activity
    }
    
    func restoreActivity(_ activity: NSUserActivity) {
        self.activityToRestore = activity
    }
    
    private func doRestore() -> Bool {
        guard let activity = activityToRestore else {
            return false
        }
        guard let statusIDs = activity.userInfo?["statusIDs"] as? [String] else {
            stateRestorationLogger.fault("TimelineViewController: activity missing statusIDs")
            return false
        }
        activityToRestore = nil
        loadViewIfNeeded()
        controller.restoreInitial {
            var snapshot = dataSource.snapshot()
            snapshot.appendSections([.statuses])
            let items = statusIDs.map { Item.status(id: $0, collapseState: .unknown, filterState: .unknown) }
            snapshot.appendItems(items, toSection: .statuses)
            dataSource.apply(snapshot, animatingDifferences: false) {
                if let centerID = activity.userInfo?["centerID"] as? String ?? activity.userInfo?["topID"] as? String,
                   let index = statusIDs.firstIndex(of: centerID),
                   let indexPath = self.dataSource.indexPath(for: items[index]) {
                    // it sometimes takes multiple attempts to convert on the right scroll position
                    // since we're dealing with a bunch of unmeasured cells, so just try a few times in a loop
                    var count = 0
                    while count < 5 {
                        count += 1
                        let origOffset = self.collectionView.contentOffset
                        self.collectionView.layoutIfNeeded()
                        self.collectionView.scrollToItem(at: indexPath, at: .centeredVertically, animated: false)
                        let newOffset = self.collectionView.contentOffset
                        if abs(origOffset.y - newOffset.y) <= 1 {
                            break
                        }
                    }
                    stateRestorationLogger.fault("TimelineViewController: restored statuses with center ID \(centerID)")
                } else {
                    stateRestorationLogger.fault("TimelineViewController: restored statuses, but couldn't find center ID")
                }
            }
        }
        return true
    }
    
    private func removeTimelineDescriptionCell() {
        var snapshot = dataSource.snapshot()
        snapshot.deleteSections([.header])
        dataSource.apply(snapshot, animatingDifferences: true)
        isShowingTimelineDescription = false
    }
    
    private func filterResult(state: FilterState, statusID: String) -> (Filterer.Result, NSAttributedString?) {
        let status = {
            let status = self.mastodonController.persistentContainer.status(for: statusID)!
            // if the status is a reblog of another one, filter based on that one
            return status.reblog ?? status
        }
        return filterer.resolve(state: state, status: status)
    }
    
    private func reapplyFilters(actionsChanged: Bool) {
        let visible = collectionView.indexPathsForVisibleItems
        let items = visible
            .compactMap { dataSource.itemIdentifier(for: $0) }
            .filter {
                if case .status(_, _, _) = $0 {
                    return true
                } else {
                    return false
                }
            }
        guard !items.isEmpty else {
            return
        }
        var snapshot = dataSource.snapshot()
        if actionsChanged {
            // need to reload not just reconfigure because hidden posts use a separate cell type
            snapshot.reloadItems(items)
        } else {
            // reconfigure when possible to avoid the content offset jumping around
            snapshot.reconfigureItems(items)
        }
        dataSource.apply(snapshot)
    }
    
    @objc private func sceneWillEnterForeground(_ notification: Foundation.Notification) {
        guard let scene = notification.object as? UIScene,
              // view.window is nil when the VC is not on screen
              view.window?.windowScene == scene else {
            return
        }
        Task {
            await checkPresent()
        }
    }
    
    @objc func refresh() {
        Task {
            if case .notLoadedInitial = controller.state {
                await controller.loadInitial()
                #if !targetEnvironment(macCatalyst)
                collectionView.refreshControl?.endRefreshing()
                #endif
            } else {
                @MainActor
                func loadNewerAndEndRefreshing() async {
                    await controller.loadNewer()
                    #if !targetEnvironment(macCatalyst)
                    collectionView.refreshControl?.endRefreshing()
                    #endif
                }
                
                // I'm not sure whether this should move into TimelineLikeController/TimelineLikeCollectionViewController
                let (_, presentItems) = await (loadNewerAndEndRefreshing(), try? loadInitial())
                if let presentItems, !presentItems.isEmpty {
                    insertPresentItemsIfNecessary(presentItems)
                }
            }
        }
    }
    
    private func checkPresent() async {
        if case .idle = controller.state,
           let presentItems = try? await loadInitial() {
            insertPresentItemsIfNecessary(presentItems)
        }
    }
    
    private func insertPresentItemsIfNecessary(_ presentItems: [String]) {
        let snapshot = dataSource.snapshot()
        guard snapshot.indexOfSection(.statuses) != nil else {
            return
        }
        let currentItems = snapshot.itemIdentifiers(inSection: .statuses)
        if case .status(id: let firstID, _, _) = currentItems.first,
           // if there's no overlap between presentItems and the existing items in the data source, prompt the user
           !presentItems.contains(firstID) {
            
            // create a new snapshot to reset the timeline to the "present" state
            var snapshot = NSDiffableDataSourceSnapshot<Section, Item>()
            snapshot.appendSections([.statuses])
            snapshot.appendItems(presentItems.map { .status(id: $0, collapseState: .unknown, filterState: .unknown) }, toSection: .statuses)
            
            var config = ToastConfiguration(title: "Jump to present")
            config.edge = .top
            config.systemImageName = "arrow.up"
            config.dismissAutomaticallyAfter = 4
            config.action = { [unowned self] toast in
                toast.dismissToast(animated: true)
                
                let origSnapshot = self.dataSource.snapshot()
                let origItemAtTop: (Item, CGFloat)?
                if let statusesSection = origSnapshot.indexOfSection(.statuses),
                   let indexPath = self.collectionView.indexPathsForVisibleItems.sorted().first(where: { $0.section == statusesSection }),
                   let cell = self.collectionView.cellForItem(at: indexPath),
                   let item = self.dataSource.itemIdentifier(for: indexPath) {
                    origItemAtTop = (item, cell.convert(.zero, to: self.view).y - self.view.safeAreaInsets.top)
                } else {
                    origItemAtTop = nil
                }
                
                self.dataSource.apply(snapshot, animatingDifferences: true) {
                    self.collectionView.scrollToItem(at: IndexPath(row: 0, section: 0), at: .top, animated: true)
                    
                    var config = ToastConfiguration(title: "Go back")
                    config.edge = .top
                    config.systemImageName = "arrow.down"
                    config.dismissAutomaticallyAfter = 4
                    config.action = { [unowned self] toast in
                        toast.dismissToast(animated: true)
                        // todo: it would be nice if we could animate this, but that doesn't work with the screen-position-maintaining stuff
                        if let (item, offset) = origItemAtTop {
                            self.applySnapshot(snapshot, maintainingScreenPosition: offset, ofItem: item)
                        } else {
                            self.dataSource.apply(origSnapshot, animatingDifferences: false)
                        }
                    }
                    self.showToast(configuration: config, animated: true)
                }
            }
            self.showToast(configuration: config, animated: true)
        }
    }
    
    // NOTE: this only works when items are being inserted ABOVE the item to maintain
    private func applySnapshot(_ snapshot: NSDiffableDataSourceSnapshot<Section, Item>, maintainingBottomRelativeScrollPositionOf itemToMaintain: Item) {
        var firstItemAfterOriginalGapOffsetFromTop: CGFloat = 0
        if let indexPath = dataSource.indexPath(for: itemToMaintain),
           let cell = collectionView.cellForItem(at: indexPath) {
            // subtract top safe area inset b/c scrollToItem at .top aligns the top of the cell to the top of the safe area
            firstItemAfterOriginalGapOffsetFromTop = cell.convert(.zero, to: view).y - view.safeAreaInsets.top
        }
        applySnapshot(snapshot, maintainingScreenPosition: firstItemAfterOriginalGapOffsetFromTop, ofItem: itemToMaintain)
    }
    
    private func applySnapshot(_ snapshot: NSDiffableDataSourceSnapshot<Section, Item>, maintainingScreenPosition offsetFromTop: CGFloat, ofItem itemToMaintain: Item) {
        // use a snapshot of the collection view to hide the flicker as the content offset changes and then changes back
        let snapshotView = collectionView.snapshotView(afterScreenUpdates: false)!
        snapshotView.layer.zPosition = 1000
        snapshotView.frame = view.bounds
        view.addSubview(snapshotView)
        
        dataSource.apply(snapshot, animatingDifferences: false) {
            if let indexPathOfItemToMaintain = self.dataSource.indexPath(for: itemToMaintain) {
                // scroll up until we've accumulated enough MEASURED height that we can put the
                // firstItemAfterOriginalGapCell at the top of the screen and then scroll down by
                // firstItemAfterOriginalGapOffsetFromTop without intruding into unmeasured area
                var cur = indexPathOfItemToMaintain
                var amountScrolledUp: CGFloat = 0
                while true {
                    if cur.row <= 0 {
                        break
                    }
                    if let cell = self.collectionView.cellForItem(at: indexPathOfItemToMaintain),
                       cell.convert(.zero, to: self.view).y - self.view.safeAreaInsets.top > offsetFromTop {
                        break
                    }
                    cur = IndexPath(row: cur.row - 1, section: cur.section)
                    self.collectionView.scrollToItem(at: cur, at: .top, animated: false)
                    self.collectionView.layoutIfNeeded()
                    let attrs = self.collectionView.layoutAttributesForItem(at: cur)!
                    amountScrolledUp += attrs.size.height
                }
                self.collectionView.contentOffset.y += amountScrolledUp
                self.collectionView.contentOffset.y -= offsetFromTop
            }
            
            snapshotView.removeFromSuperview()
        }
    }
    
}

extension TimelineViewController {
    enum Section: TimelineLikeCollectionViewSection {
        case header
        case statuses
        case footer
        
        static var entries: Self { .statuses }
    }
    enum Item: TimelineLikeCollectionViewItem {
        typealias TimelineItem = String // status ID
        
        case status(id: String, collapseState: CollapseState, filterState: FilterState)
        case gap
        case loadingIndicator
        case confirmLoadMore
        case publicTimelineDescription
        
        static func fromTimelineItem(_ id: String) -> Self {
            return .status(id: id, collapseState: .unknown, filterState: .unknown)
        }
        
        static func ==(lhs: Item, rhs: Item) -> Bool {
            switch (lhs, rhs) {
            case let (.status(id: a, _, _), .status(id: b, _, _)):
                return a == b
            case (.gap, .gap):
                return true
            case (.loadingIndicator, .loadingIndicator):
                return true
            case (.confirmLoadMore, .confirmLoadMore):
                return true
            case (.publicTimelineDescription, .publicTimelineDescription):
                return true
            default:
                return false
            }
        }
        
        func hash(into hasher: inout Hasher) {
            switch self {
            case .status(id: let id, _, _):
                hasher.combine(0)
                hasher.combine(id)
            case .gap:
                hasher.combine(1)
            case .loadingIndicator:
                hasher.combine(2)
            case .confirmLoadMore:
                hasher.combine(3)
            case .publicTimelineDescription:
                hasher.combine(4)
            }
        }
        
        var hideSeparators: Bool {
            switch self {
            case .loadingIndicator, .publicTimelineDescription, .confirmLoadMore:
                return true
            default:
                return false
            }
        }
        
        var isSelectable: Bool {
            switch self {
            case .publicTimelineDescription, .gap, .status(_, _, _):
                return true
            default:
                return false
            }
        }
    }
}

// MARK: TimelineLikeControllerDelegate
extension TimelineViewController {
    typealias TimelineItem = String // status ID
    
    func loadInitial() async throws -> [TimelineItem] {
        try await Task.sleep(nanoseconds: 1 * NSEC_PER_SEC)
        
        let request = Client.getStatuses(timeline: timeline)
        let (statuses, _) = try await mastodonController.run(request)
        
        await withCheckedContinuation { continuation in
            mastodonController.persistentContainer.addAll(statuses: statuses) {
                continuation.resume()
            }
        }
        
        return statuses.map(\.id)
    }
    
    func loadNewer() async throws -> [TimelineItem] {
        let statusesSection = dataSource.snapshot().indexOfSection(.statuses)!
        guard case .status(id: let id, _, _) = dataSource.itemIdentifier(for: IndexPath(row: 0, section: statusesSection)) else {
            throw Error.noNewer
        }
        let newer = RequestRange.after(id: id, count: nil)
        
        let request = Client.getStatuses(timeline: timeline, range: newer)
        let (statuses, _) = try await mastodonController.run(request)
        
        guard !statuses.isEmpty else {
            throw TimelineViewController.Error.allCaughtUp
        }
        
        await withCheckedContinuation { continuation in
            mastodonController.persistentContainer.addAll(statuses: statuses) {
                continuation.resume()
            }
        }
        
        return statuses.map(\.id)
    }
    
    func loadOlder() async throws -> [TimelineItem] {
        let snapshot = dataSource.snapshot()
        let statusesSection = snapshot.indexOfSection(.statuses)!
        guard case .status(id: let id, _, _) = dataSource.itemIdentifier(for: IndexPath(row: snapshot.numberOfItems(inSection: .statuses) - 1, section: statusesSection)) else {
            throw Error.noNewer
        }
        let older = RequestRange.before(id: id, count: nil)
        
        let request = Client.getStatuses(timeline: timeline, range: older)
        let (statuses, _) = try await mastodonController.run(request)
        
        guard !statuses.isEmpty else {
            return []
        }
        
        await withCheckedContinuation { continuation in
            mastodonController.persistentContainer.addAll(statuses: statuses) {
                continuation.resume()
            }
        }
        
        return statuses.map(\.id)
    }
    
    func loadGap(in direction: TimelineGapDirection) async throws -> [TimelineItem] {
        guard let gapIndexPath = dataSource.indexPath(for: .gap) else {
            throw Error.noGap
        }
        let statusItemsCount = collectionView.numberOfItems(inSection: gapIndexPath.section)
        let range: RequestRange
        switch direction {
        case .above:
            guard gapIndexPath.row > 0,
                  case .status(id: let id, _, _) = dataSource.itemIdentifier(for: IndexPath(row: gapIndexPath.row - 1, section: gapIndexPath.section)) else {
                // not really the right error but w/e
                throw Error.noGap
            }
            range = .before(id: id, count: nil)
        case .below:
            guard gapIndexPath.row < statusItemsCount - 1,
                  case .status(id: let id, _, _) = dataSource.itemIdentifier(for: IndexPath(row: gapIndexPath.row + 1, section: gapIndexPath.section)) else {
                throw Error.noGap
            }
            range = .after(id: id, count: nil)
        }
        
        let request = Client.getStatuses(timeline: timeline, range: range)
        let (statuses, _) = try await mastodonController.run(request)
        
        guard !statuses.isEmpty else {
            return []
        }
        
        // NOTE: closing the gap (if necessary) happens in handleFillGap
        
        await withCheckedContinuation { continuation in
            mastodonController.persistentContainer.addAll(statuses: statuses) {
                continuation.resume()
            }
        }
        
        return statuses.map(\.id)
    }
    
    func handleLoadGapError(_ error: Swift.Error, direction: TimelineGapDirection) async {
        // TODO: better title, involving direction?
        let config = ToastConfiguration(from: error, with: "Error Loading", in: self) { [weak self] toast in
            toast.dismissToast(animated: true)
            Task {
                await self?.controller.fillGap(in: direction)
            }
        }
        self.showToast(configuration: config, animated: true)
    }
    
    func handleFillGap(_ timelineItems: [String], direction: TimelineGapDirection) async {
        var snapshot = dataSource.snapshot()
        let addedItems: Bool
        
        let statusItems = snapshot.itemIdentifiers(inSection: .statuses)
        let gapIndex = statusItems.firstIndex(of: .gap)!
        
        switch direction {
        case .above:
            // dropFirst to remove .gap item
            let afterGap = statusItems[gapIndex...].dropFirst().prefix(20)
            precondition(!afterGap.contains(.gap))
            
            // if there is any overlap, the first overlapping item will be the first item below the gap
            var indexOfFirstTimelineItemExistingBelowGap: Int?
            if case .status(id: let id, _, _) = afterGap.first {
                indexOfFirstTimelineItemExistingBelowGap = timelineItems.firstIndex(of: id)
            }
            
            // the end index of the range of timelineItems that don't yet exist in the data source
            let endIndex = indexOfFirstTimelineItemExistingBelowGap ?? timelineItems.endIndex
            let toInsert = timelineItems[..<endIndex].map { Item.status(id: $0, collapseState: .unknown, filterState: .unknown) }
            if toInsert.isEmpty {
                addedItems = false
            } else {
                snapshot.insertItems(toInsert, beforeItem: .gap)
                addedItems = true
            }
            
            // if there's any overlap between the items we loaded to insert above the gap
            // and the items that already exist below the gap, we've completely filled the gap
            if indexOfFirstTimelineItemExistingBelowGap != nil {
                snapshot.deleteItems([.gap])
            }
        
            await apply(snapshot, animatingDifferences: !addedItems)
            
        case .below:
            let beforeGap = statusItems[..<gapIndex].suffix(20)
            precondition(!beforeGap.contains(.gap))
            
            // if there's any overlap, last overlapping item will be the last item below the gap
            var indexOfLastTimelineItemExistingAboveGap: Int?
            if case .status(id: let id, _, _) = beforeGap.last {
                indexOfLastTimelineItemExistingAboveGap = timelineItems.lastIndex(of: id)
            }
            
            // the start index of the reange of timeline items that don't yet exist in the data source
            let startIndex: Int
            if let indexOfLastTimelineItemExistingAboveGap {
                // index(after:) because the beginning of the range is inclusive, but we don't want the item at indexOfLastTimelineItemExistingAboveGap
                startIndex = timelineItems.index(after: indexOfLastTimelineItemExistingAboveGap)
            } else {
                startIndex = timelineItems.startIndex
            }
            let toInsert = timelineItems[startIndex...].map { Item.status(id: $0, collapseState: .unknown, filterState: .unknown) }
            if toInsert.isEmpty {
                addedItems = false
            } else {
                snapshot.insertItems(toInsert, afterItem: .gap)
                addedItems = true
            }
            
            // if there's any overlap between the items we loaded to insert below the gap
            // and the items that already exist above the gap, we've completely filled the gap
            if indexOfLastTimelineItemExistingAboveGap != nil {
                snapshot.deleteItems([.gap])
            }
            
            if addedItems {
                let firstItemAfterOriginalGap = statusItems[gapIndex + 1]
                applySnapshot(snapshot, maintainingBottomRelativeScrollPositionOf: firstItemAfterOriginalGap)
            } else {
                dataSource.apply(snapshot, animatingDifferences: true) {}
            }
        }
        
        // if we didn't add any items, that implies the gap was removed, and we want to to make clear what's happening
        if !addedItems {
            var config = ToastConfiguration(title: "There's nothing in between!")
            config.dismissAutomaticallyAfter = 2
            showToast(configuration: config, animated: true)
        }
    }
    
    enum Error: TimelineLikeCollectionViewError {
        case noClient
        case noNewer
        case noOlder
        case allCaughtUp
        case noGap
    }
}

extension TimelineViewController: UICollectionViewDelegate {
    func collectionView(_ collectionView: UICollectionView, willDisplay cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
        guard case .statuses = dataSource.sectionIdentifier(for: indexPath.section) else {
            return
        }
        
        let itemsInSection = collectionView.numberOfItems(inSection: indexPath.section)
        if indexPath.row == itemsInSection - 1 {
            Task {
                await controller.loadOlder()
            }
        }
    }
    
    func collectionView(_ collectionView: UICollectionView, shouldSelectItemAt indexPath: IndexPath) -> Bool {
        return dataSource.itemIdentifier(for: indexPath)?.isSelectable ?? false
    }
    
    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        guard let item = dataSource.itemIdentifier(for: indexPath) else {
            return
        }
        switch item {
        case .publicTimelineDescription:
            removeTimelineDescriptionCell()
        case .status(id: let id, collapseState: let collapseState, filterState: let filterState):
            if filterState.isWarning {
                filterer.setResult(.allow, for: filterState)
                collectionView.deselectItem(at: indexPath, animated: true)
                var snapshot = dataSource.snapshot()
                snapshot.reconfigureItems([item])
                dataSource.apply(snapshot, animatingDifferences: true)
            } else {
                let status = mastodonController.persistentContainer.status(for: id)!
                // if the status in the timeline is a reblog, show the status that it is a reblog of
                selected(status: status.reblog?.id ?? id, state: collapseState.copy())
            }
        case .gap:
            let cell = collectionView.cellForItem(at: indexPath) as! TimelineGapCollectionViewCell
            cell.showsIndicator = true
            Task {
                await controller.fillGap(in: cell.direction)
                cell.showsIndicator = false
            }
        case .loadingIndicator, .confirmLoadMore:
            fatalError("unreachable")
        }
    }
    
    func collectionView(_ collectionView: UICollectionView, contextMenuConfigurationForItemAt indexPath: IndexPath, point: CGPoint) -> UIContextMenuConfiguration? {
        return (collectionView.cellForItem(at: indexPath) as? TimelineStatusCollectionViewCell)?.contextMenuConfiguration()
    }
    
    func collectionView(_ collectionView: UICollectionView, willPerformPreviewActionForMenuWith configuration: UIContextMenuConfiguration, animator: UIContextMenuInteractionCommitAnimating) {
        MenuPreviewHelper.willPerformPreviewAction(animator: animator, presenter: self)
    }
    
    func scrollViewWillBeginDragging(_ scrollView: UIScrollView) {
        if isShowingTimelineDescription {
            removeTimelineDescriptionCell()
        }
    }
}

extension TimelineViewController: UICollectionViewDragDelegate {
    func collectionView(_ collectionView: UICollectionView, itemsForBeginning session: UIDragSession, at indexPath: IndexPath) -> [UIDragItem] {
        (collectionView.cellForItem(at: indexPath) as? TimelineStatusCollectionViewCell)?.dragItemsForBeginning(session: session) ?? []
    }
}

extension TimelineViewController: TuskerNavigationDelegate {
    var apiController: MastodonController! { mastodonController }
}

extension TimelineViewController: MenuActionProvider {
}

extension TimelineViewController: StatusCollectionViewCellDelegate {
    func statusCellNeedsReconfigure(_ cell: StatusCollectionViewCell, animated: Bool, completion: (() -> Void)?) {
        if let indexPath = collectionView.indexPath(for: cell) {
            var snapshot = dataSource.snapshot()
            snapshot.reconfigureItems([dataSource.itemIdentifier(for: indexPath)!])
            dataSource.apply(snapshot, animatingDifferences: animated, completion: completion)
        }
    }
    
    func statusCellShowFiltered(_ cell: StatusCollectionViewCell) {
        if let indexPath = collectionView.indexPath(for: cell),
           let item = dataSource.itemIdentifier(for: indexPath),
           case .status(id: _, collapseState: _, filterState: let filterState) = item {
            filterer.setResult(.allow, for: filterState)
            var snapshot = dataSource.snapshot()
            snapshot.reconfigureItems([item])
            dataSource.apply(snapshot, animatingDifferences: true)
        }
    }
}

extension TimelineViewController: TabBarScrollableViewController {
    func tabBarScrollToTop() {
        collectionView.scrollToTop()
    }
}

extension TimelineViewController: StatusBarTappableViewController {
    func handleStatusBarTapped(xPosition: CGFloat) -> StatusBarTapActionResult {
        collectionView.scrollToTop()
        return .stop
    }
}