//
//  HomeViewController.swift
//  Reader
//
//  Created by Shadowfacts on 11/25/21.
//

import UIKit
import CoreData

class HomeViewController: UIViewController, UICollectionViewDelegate {
    
    let fervorController: FervorController
    
    private var collectionView: UICollectionView!
    private var dataSource: UICollectionViewDiffableDataSource<Section, Item>!
    private var groupResultsController: NSFetchedResultsController<Group>!
    private var feedResultsController: NSFetchedResultsController<Feed>!
    
    init(fervorController: FervorController) {
        self.fervorController = fervorController
        
        super.init(nibName: nil, bundle: nil)
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        view.backgroundColor = .systemBackground
        
//        let label = UILabel()
//        label.translatesAutoresizingMaskIntoConstraints = false
//        label.text = "Logged in to \(fervorController.instanceURL.host!)"
//        view.addSubview(label)
//        NSLayoutConstraint.activate([
//            label.centerXAnchor.constraint(equalTo: view.centerXAnchor),
//            label.centerYAnchor.constraint(equalTo: view.centerYAnchor),
//        ])
        
        let configuration = UICollectionLayoutListConfiguration(appearance: .insetGrouped)
        let layout = UICollectionViewCompositionalLayout.list(using: configuration)
        collectionView = UICollectionView(frame: view.bounds, collectionViewLayout: layout)
        collectionView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        collectionView.delegate = self
        view.addSubview(collectionView)
        
        dataSource = createDataSource()
        
        // todo: maybe NSFetchedResultsController to automatically update when the database changes?
//        applyInitialSnapshot()
        var snapshot = NSDiffableDataSourceSnapshot<Section, Item>()
        snapshot.appendSections([.groups, .feeds])
        dataSource.apply(snapshot, animatingDifferences: false)
        
        let groupReq = Group.fetchRequest()
        groupReq.sortDescriptors = [NSSortDescriptor(key: "title", ascending: true)]
        groupResultsController = NSFetchedResultsController(fetchRequest: groupReq, managedObjectContext: fervorController.persistentContainer.viewContext, sectionNameKeyPath: nil, cacheName: nil)
        groupResultsController.delegate = self
        try! groupResultsController.performFetch()
        
        let feedReq = Feed.fetchRequest()
        feedReq.sortDescriptors = [NSSortDescriptor(key: "title", ascending: true)]
        feedResultsController = NSFetchedResultsController(fetchRequest: feedReq, managedObjectContext: fervorController.persistentContainer.viewContext, sectionNameKeyPath: nil, cacheName: nil)
        feedResultsController.delegate = self
        try! feedResultsController.performFetch()
    }
    
    private func createDataSource() -> UICollectionViewDiffableDataSource<Section, Item> {
        let listCell = UICollectionView.CellRegistration<UICollectionViewListCell, Item> { cell, indexPath, item in
            var config = cell.defaultContentConfiguration()
            config.text = item.title
            cell.contentConfiguration = config
            
            cell.accessories = [.disclosureIndicator()]
        }
        let dataSource = UICollectionViewDiffableDataSource<Section, Item>(collectionView: collectionView) { collectionView, indexPath, item in
            return collectionView.dequeueConfiguredReusableCell(using: listCell, for: indexPath, item: item)
        }
        return dataSource
    }
    
//    private func applyInitialSnapshot() {
//        let groups = try! fervorController.persistentContainer.viewContext.fetch(Group.fetchRequest())
//        let feeds = try! fervorController.persistentContainer.viewContext.fetch(Feed.fetchRequest())
//
//        var snapshot = NSDiffableDataSourceSnapshot<Section, Item>()
//        snapshot.appendSections([.groups, .feeds])
//        snapshot.appendItems(groups.map { .group($0) }, toSection: .groups)
//        snapshot.appendItems(feeds.map { .feed($0) }, toSection: .feeds)
//        dataSource.apply(snapshot, animatingDifferences: false)
//    }
    
}

extension HomeViewController {
    enum Section: Hashable {
        case groups
        case feeds
    }
    enum Item: Hashable {
        case group(Group)
        case feed(Feed)
        
        var title: String {
            switch self {
            case let .group(group):
                // todo: manual codegen for models so this force unwrap isn't necessary
                return group.title!
            case let .feed(feed):
                return feed.title!
            }
        }
    }
}

extension HomeViewController: NSFetchedResultsControllerDelegate {
    func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChangeContentWith snapshot: NSDiffableDataSourceSnapshotReference) {
        var snapshot = dataSource.snapshot()
        if controller == groupResultsController {
            if snapshot.sectionIdentifiers.contains(.groups) {
                snapshot.deleteItems(snapshot.itemIdentifiers(inSection: .groups))
            }
            snapshot.appendItems(controller.fetchedObjects!.map { .group($0 as! Group) }, toSection: .groups)
        } else if controller == feedResultsController {
            if snapshot.sectionIdentifiers.contains(.feeds) {
                snapshot.deleteItems(snapshot.itemIdentifiers(inSection: .feeds))
            }
            snapshot.appendItems(controller.fetchedObjects!.map { .feed($0 as! Feed) }, toSection: .feeds)
        }
        dataSource.apply(snapshot, animatingDifferences: false)
    }
}