//
//  ReadViewController.swift
//  Reader
//
//  Created by Shadowfacts on 1/9/22.
//

import UIKit
import WebKit
import HTMLEntities
import SafariServices
import Combine
import Persistence

class ReadViewController: UIViewController {
    
    private static let publishedFormatter: DateFormatter = {
        let f = DateFormatter()
        f.dateStyle = .medium
        f.timeStyle = .medium
        return f
    }()
    
    let fervorController: FervorController
    let item: Item
    
    private let scrollPositionChangedSubject = PassthroughSubject<Void, Never>()
    private var cancellables = Set<AnyCancellable>()
    private var scrollTargetSelector: String?
    
    private var webView: WKWebView!
    
    #if targetEnvironment(macCatalyst)
    private var itemReadObservation: NSKeyValueObservation?
    #endif
    
    override var prefersStatusBarHidden: Bool {
        navigationController?.isNavigationBarHidden ?? false
    }
    
    override var preferredStatusBarUpdateAnimation: UIStatusBarAnimation {
        .slide
    }
    
    init(item: Item, fervorController: FervorController) {
        self.fervorController = fervorController
        self.item = item
        
        super.init(nibName: nil, bundle: nil)
        
        self.userActivity = .readItem(item, account: fervorController.account!)
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        navigationItem.largeTitleDisplayMode = .never
        
        view.backgroundColor = .appBackground
        view.addInteraction(StretchyMenuInteraction(delegate: self))
        
        webView = WKWebView()
        webView.translatesAutoresizingMaskIntoConstraints = false
        webView.navigationDelegate = self
        webView.uiDelegate = self
        webView.scrollView.delegate = self
        // transparent background required to prevent white flash in dark mode, just using .appBackground doesn't work
        webView.isOpaque = false
        webView.backgroundColor = .clear
        if #available(iOS 16.0, *) {
            webView.isFindInteractionEnabled = true
        }
        if let content = itemContentHTML() {
            webView.loadHTMLString(content, baseURL: item.url)
        }
        webView.scrollView.alwaysBounceVertical = true
        webView.scrollView.alwaysBounceHorizontal = false
        view.addSubview(webView)
        
        // subtract 0.5, because otherwise, on ipad, the web view's scroll content view ends up being wider than the scroll view itself, causing the content to bounce horizontally
        let webViewWidthFix = UIDevice.current.userInterfaceIdiom == .pad ? -0.5 : 0
        
        NSLayoutConstraint.activate([
            webView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            webView.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: webViewWidthFix),
            webView.topAnchor.constraint(equalTo: view.topAnchor),
            webView.bottomAnchor.constraint(equalTo: view.bottomAnchor),
        ])
        
        if let url = item.url {
            activityItemsConfiguration = UIActivityItemsConfiguration(objects: [url as NSURL])
        }
        
        scrollPositionChangedSubject
            .debounce(for: .milliseconds(500), scheduler: RunLoop.main)
            .sink { [unowned self] in
                self.updateUserActivityScrollPosition()
            }
            .store(in: &cancellables)
        
        #if targetEnvironment(macCatalyst)
        itemReadObservation = item.observe(\.read) { [unowned self] _, _ in
            self.updateToggleReadToolbarImage()
        }
        #endif
    }
    
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        
        updateScrollIndicatorStyle()
    }
    
    override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
        super.traitCollectionDidChange(previousTraitCollection)
        
        updateScrollIndicatorStyle()
    }
    
    override func restoreUserActivityState(_ activity: NSUserActivity) {
        if let selector = activity.topElementSelector {
            scrollTargetSelector = selector
            if isViewLoaded {
                scrollToTargetSelector()
            }
        }
    }
    
    private func scrollToTargetSelector() {
        guard let selector = scrollTargetSelector else {
            return
        }
        
        let js = """
        const doScroll = () => {
            const el = document.querySelector(sel);
            if (!el) {
                throw new Error("not ready yet");
            }
            el.scrollIntoView();
        };
        if (document.readyState !== "complete") {
            window.addEventListener("load", doScroll);
        } else {
            doScroll();
        }
        """
        webView.callAsyncJavaScript(js, arguments: ["sel": selector], in: nil, in: .defaultClient) { result in
            switch result {
            case .failure(let error):
                print(error)
            case .success(_):
                self.scrollTargetSelector = nil
            }
        }
    }
    
    private func updateScrollIndicatorStyle() {
        guard #available(iOS 15.4, *) else {
            // different workaround pre-iOS 15.4
            return
        }
        // can't use .default because that causes the WKScrollView to think the indicator style has not been set by the client (us):
        // https://github.com/WebKit/WebKit/blob/1dbd34cf01d8b5aedcb8820b13cb6553ed60e8ed/Source/WebKit/UIProcess/ios/WKScrollView.mm#L247
        // if that happens, it goes back to trying to set it based on background color which doesn't work because we give it a clear background
        // https://github.com/WebKit/WebKit/blob/d085008d57f784d0913a8c37351f60b4a0eb8a3a/Source/WebKit/UIProcess/API/ios/WKWebViewIOS.mm#L562
        // https://github.com/WebKit/WebKit/blob/d085008d57f784d0913a8c37351f60b4a0eb8a3a/Source/WebKit/UIProcess/API/ios/WKWebViewIOS.mm#L520
        // so, we set it ourselves based on the user interface style
        if traitCollection.userInterfaceStyle == .dark {
            webView.scrollView.indicatorStyle = .white
        } else {
            webView.scrollView.indicatorStyle = .black
        }
    }
    
    private static let css = try! String(contentsOf: Bundle.main.url(forResource: "read", withExtension: "css")!)
    private static let js = try! String(contentsOf: Bundle.main.url(forResource: "read", withExtension: "js")!)
    
    private func itemContentHTML() -> String? {
        guard let content = item.content else {
            return nil
        }
        var info = ""
        if let title = item.title, !title.isEmpty {
            info += "<h1 id=\"item-title\">"
            if let url = item.url {
                info += "<a href=\"\(url.absoluteString)\">"
            }
            info += title.htmlEscape()
            if item.url != nil {
                info += "</a>"
            }
            info += "</h1>"
        }
        if let feedTitle = item.feed!.title, !feedTitle.isEmpty {
            info += "<h2 id=\"item-feed-title\">\(feedTitle.htmlEscape())</h2>"
        }
        if let author = item.author, !author.isEmpty {
            info += "<h3 id=\"item-author\">\(author)</h3>"
        }
        if let published = item.published {
            let formatted = ReadViewController.publishedFormatter.string(from: published)
            info += "<h3 id=\"item-published\">\(formatted)</h3>"
        }
        return """
        <!DOCTYPE html>
        <html>
        <head>
            <meta charset="UTF-8" />
            <meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0, user-scalable=no" />
            <style>\(ReadViewController.css)</style>
            <script async>\(ReadViewController.js)</script>
        </head>
        <body>
            <div id="item-info">
                \(info)
            </div>
            <div id="item-content">
        \(content)
            </div>
        </body>
        </html>
        """
    }
    
    private func createSafariVC(url: URL) -> SFSafariViewController {
        let vc = SFSafariViewController(url: url)
        vc.preferredControlTintColor = .appTintColor
        return vc
    }
    
    func updateUserActivityScrollPosition() {
        // can't use needsSave and updateUserActivityState(_:) because running JS is asynchronous
        let js = """
        (() => {
            function topVisibleChild(element) {
                if (element.children.length <= 0 || element.tagName === "svg") {
                    return element;
                }
                let min, minDistanceToTop;
                for (const c of element.children) {
                    const rect = c.getBoundingClientRect();
                    if (!min || Math.abs(rect.top) < minDistanceToTop) {
                        min = c;
                        minDistanceToTop = Math.abs(rect.top);
                    }
                }
                return topVisibleChild(min);
            }
            function uniqueSelector(self) {
                if (!self) {
                    return null;
                } else if (self.tagName === "BODY") {
                    return "body";
                } else {
                    let selfSel;
                    if (self.id) {
                        return `#${self.id}`;
                    } else if (Array.from(self.parentNode.children).filter((e) => e.tagName === self.tagName).length === 1) {
                        selfSel = ` > ${self.tagName.toLowerCase()}`;
                    } else {
                        const index = Array.from(self.parentNode.children).indexOf(self);
                        selfSel = ` > ${self.tagName.toLowerCase()}:nth-child(${index + 1})`;
                    }
                    const parentSel = uniqueSelector(self.parentNode);
                    return parentSel + selfSel;
                }
            }
            return uniqueSelector(topVisibleChild(document.getElementById("item-content")));
        })()
        """
        // todo: check if <source> can be scrolled into view
        webView.evaluateJavaScript(js) { result, error in
            guard let result = result as? String else {
                return
            }
            self.userActivity?.topElementSelector = result
        }
    }
    
    #if targetEnvironment(macCatalyst)
    @objc func toggleItemRead(_ item: NSToolbarItem) {
        Task {
            await fervorController.markItem(self.item, read: !self.item.read)
            updateToggleReadToolbarImage()
        }
    }
    
    private func updateToggleReadToolbarImage() {
        guard let titlebar = view.window?.windowScene?.titlebar,
              let item = titlebar.toolbar?.items.first(where: { $0.itemIdentifier == .toggleItemRead }) else {
                  return
              }
        
        item.image = UIImage(systemName: self.item.read ? "checkmark.circle.fill" : "checkmark.circle")
    }
    #endif

}

extension ReadViewController: WKNavigationDelegate {
    func webView(_ webView: WKWebView, decidePolicyFor navigationAction: WKNavigationAction) async -> WKNavigationActionPolicy {
        if navigationAction.navigationType == .linkActivated {
            let url = navigationAction.request.url!
            if url.fragment != nil {
                var components = URLComponents(url: url, resolvingAgainstBaseURL: false)!
                components.fragment = nil
                if components.url == item.url {
                    return .allow
                }
            }
            present(createSafariVC(url: url), animated: true)
            return .cancel
        } else {
            return .allow
        }
    }
    
    func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
        // we try to scroll after the navigation triggered by the loadHTML, since that's asynchronous but doesn't have a callback
        scrollToTargetSelector()
    }
}

extension ReadViewController: WKUIDelegate {
    func webView(_ webView: WKWebView, contextMenuConfigurationFor elementInfo: WKContextMenuElementInfo) async -> UIContextMenuConfiguration? {
        guard let url = elementInfo.linkURL,
              ["http", "https"].contains(url.scheme?.lowercased()) else {
            return nil
        }
        return UIContextMenuConfiguration(identifier: nil) { [unowned self] in
            self.createSafariVC(url: url)
        } actionProvider: { _ in
            return UIMenu(children: [
                UIAction(title: "Open in Safari", image: UIImage(systemName: "safari"), handler: { [weak self] _ in
                    guard let self = self else { return }
                    self.present(self.createSafariVC(url: url), animated: true)
                }),
                UIAction(title: "Share", image: UIImage(systemName: "square.and.arrow.up"), handler: { [weak self] _ in
                    self?.present(UIActivityViewController(activityItems: [url], applicationActivities: nil), animated: true)
                }),
            ])
        }

    }
    
    func webView(_ webView: WKWebView, contextMenuForElement elementInfo: WKContextMenuElementInfo, willCommitWithAnimator animator: UIContextMenuInteractionCommitAnimating) {
        if let vc = animator.previewViewController as? SFSafariViewController {
            animator.preferredCommitStyle = .pop
            animator.addCompletion {
                self.present(vc, animated: true)
            }
        }
    }
}

extension ReadViewController: UIScrollViewDelegate {
    func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
        scrollPositionChangedSubject.send()
    }
    func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
        scrollPositionChangedSubject.send()
    }
}

extension ReadViewController: StretchyMenuInteractionDelegate {
    func stretchyMenuTitle() -> String? {
        return nil
    }
    
    func stretchyMenuItems() -> [StretchyMenuItem] {
        guard let url = item.url else {
            return []
        }
        var items = [
            StretchyMenuItem(title: "Open in Safari", subtitle: nil, action: { [unowned self] in
                self.present(createSafariVC(url: url), animated: true)
            }),
            StretchyMenuItem(title: item.read ? "Mark as Unread" : "Mark as Read", subtitle: nil, action: { [unowned self] in
                Task {
                    await self.fervorController.markItem(item, read: !item.read)
                }
            }),
        ]
        #if !targetEnvironment(macCatalyst)
        items.insert(StretchyMenuItem(title: "Share", subtitle: nil, action: { [unowned self] in
                self.present(UIActivityViewController(activityItems: [url], applicationActivities: nil), animated: true)
            }), at: 1)
        #endif
        return items
    }
}