Tusker/Tusker/Views/BaseEmojiLabel.swift

//
//  BaseEmojiLabel.swift
//  Tusker
//
//  Created by Shadowfacts on 10/18/20.
//  Copyright © 2020 Shadowfacts. All rights reserved.
//

import UIKit
import Pachyderm
import WebURLFoundationExtras

private let emojiRegex = try! NSRegularExpression(pattern: ":(\\w+):", options: [])

protocol BaseEmojiLabel: AnyObject {
    var emojiIdentifier: String? { get set }
    var emojiRequests: [ImageCache.Request] { get set }
    var emojiFont: UIFont { get }
    var emojiTextColor: UIColor { get }
}

extension BaseEmojiLabel {
    func replaceEmojis(in attributedString: NSAttributedString, emojis: [Emoji], identifier: String?, setAttributedString: @escaping (_ attributedString: NSAttributedString, _ didReplaceEmojis: Bool) -> Void) {
        // blergh
        precondition(Thread.isMainThread)
        
        emojiIdentifier = identifier
        emojiRequests.forEach { $0.cancel() }
        emojiRequests = []
        
        guard !emojis.isEmpty else {
            setAttributedString(attributedString, false)
            return
        }
        
        let matches = emojiRegex.matches(in: attributedString.string, options: [], range: NSRange(location: 0, length: attributedString.length))
        guard !matches.isEmpty else {
            setAttributedString(attributedString, false)
            return
        }
        
        let emojiImages = MultiThreadDictionary<String, UIImage>()
        var foundEmojis = false
        
        let group = DispatchGroup()
        
        for emoji in emojis {
            // only make requests for emojis that are present in the text to avoid making unnecessary network requests
            guard matches.contains(where: { (match) -> Bool in
                let matchShortcode = (attributedString.string as NSString).substring(with: match.range(at: 1))
                return emoji.shortcode == matchShortcode
            }) else {
                continue
            }
            
            foundEmojis = true
            
            if let image = ImageCache.emojis.get(URL(emoji.url)!)?.image {
                // if the image is cached, add it immediately
                emojiImages[emoji.shortcode] = image
            } else {
                // otherwise, perform the network request
                
                group.enter()
                // todo: ImageCache.emojis.get here will re-check the memory and disk caches, there should be another method to force-refetch
                let request = ImageCache.emojis.get(URL(emoji.url)!) { (_, image) in
                    guard let image = image,
                          let transformedImage = ImageGrayscalifier.convertIfNecessary(url: URL(emoji.url)!, image: image) else {
                              group.leave()
                              return
                    }
                    emojiImages[emoji.shortcode] = transformedImage
                    group.leave()
                }
                if let request = request {
                    emojiRequests.append(request)
                }
            }
        }
        
        guard foundEmojis else {
            setAttributedString(attributedString, false)
            return
        }
        
        func buildStringWithEmojisReplaced(usePlaceholders: Bool) -> NSAttributedString {
            let mutAttrString = NSMutableAttributedString(attributedString: attributedString)
            
            // lock once for the entire loop, rather than lock/unlocking for each iteration to do the lookup
            // OSAllocatedUnfairLock.withLock expects a @Sendable closure, so this warns about captures of non-sendable types (attribute dstrings, text checking results)
            // even though the closures is invoked on the same thread that withLock is called, so it's unclear why it needs to be @Sendable (FB11494878)
            // so, just ignore the warnings
            emojiImages.withLock { emojiImages in
                // replaces the emojis starting from the end of the string as to not alter the indices of preceeding emojis
                for match in matches.reversed() {
                    let shortcode = (attributedString.string as NSString).substring(with: match.range(at: 1))
                    let attachment: NSTextAttachment
                    
                    if let emojiImage = emojiImages[shortcode] {
                        attachment = NSTextAttachment(emojiImage: emojiImage, in: self.emojiFont, with: self.emojiTextColor)
                    } else if usePlaceholders {
                        attachment = NSTextAttachment(emojiPlaceholderIn: self.emojiFont)
                    } else {
                        continue
                    }
                    
                    let attachmentStr = NSAttributedString(attachment: attachment)
                    mutAttrString.replaceCharacters(in: match.range, with: attachmentStr)
                }
            }
            
            return mutAttrString
        }
        
        if emojiRequests.isEmpty {
            // waiting on the group when there are only cached emojis results in a bad first layout,
            // because it would still wait until the next runloop iteration
            // but, since there were no requests, we can generate the correct string immediately
            let replacedString = buildStringWithEmojisReplaced(usePlaceholders: false)
            setAttributedString(replacedString, true)
        } else {
            // if we did find emojis and there are pending network requests,
            // set an attributed strings with placeholders for non-loaded emojis to avoid layout shift
            setAttributedString(buildStringWithEmojisReplaced(usePlaceholders: true), true)
            
            group.notify(queue: .main) { [weak self] in
                // if e.g. the account changes before all emojis are loaded, don't bother trying to set them
                guard let self = self, self.emojiIdentifier == identifier else { return }
                
                let replacedString = buildStringWithEmojisReplaced(usePlaceholders: false)
                setAttributedString(replacedString, true)
            }
        }
    }
    
    func replaceEmojis(in string: String, emojis: [Emoji], identifier: String?, setAttributedString: @escaping (_ attributedString: NSAttributedString, _ didReplaceEmojis: Bool) -> Void) {
        replaceEmojis(in: NSAttributedString(string: string), emojis: emojis, identifier: identifier, setAttributedString: setAttributedString)
    }
}