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Inter plurima admiranda Naturæ Phœnomena aquarum congelationem non minoris momenti esse semper judicavi; hinc sæpe experiundi cupidus fui, quinam effectus frigoris futuri essent, si aqua in spatio ab aere vacuo clauderetur. This text was harvested from ...

VIII. Experimenta & observationes de congelatione aquæ in vacuo factæ a D. G. Fahrenheit, R. S. S

Plurima

plurima

Sylvie and Bruno by Lewis Carroll, is part of the HackerNoon Books Series. You can jump to any chapter in this book . THE MAGIC LOCKET here CHAPTER VI. THE MAGIC LOCKET “Where are we, father?” Sylvie whispered, with her arms twined closely around the old man's neck, and with her rosy cheek lovingly pressed to his. “In Elfland, darling. It's one of the provinces of Fairyland.” “But I thought Elfland was ever so far from Outland: and we've come such a tiny little way!” “You came by the Royal Road, sweet one. Only those of royal blood can travel along it: but you've been royal ever since I was made King of Elfland that's nearly a month ago. They sent two ambassadors, to make sure that their invitation to me, to be their new King, should reach me. One was a Prince; so he was able to come by the Royal Road, and to come invisibly to all but me: the other was a Baron; so he had to come by the common road, and I dare say he hasn't even arrived yet.” “Then how far have we come?” Sylvie enquired. “Just a thousand miles, sweet one, since the Gardener unlocked that door for you.” “A thousand miles!” Bruno repeated. “And may I eat one?” “Eat a mile, little rogue?” “No,” said Bruno. “I mean may I eat one of that fruits?” “Yes, child,” said his father: “and then you'll find out what Pleasure is like—the Pleasure we all seek so madly, and enjoy so mournfully!” Bruno ran eagerly to the wall, and picked a fruit that was shaped something like a banana, but had the colour of a strawberry. He ate it with beaming looks, that became gradually more gloomy, and were very blank indeed by the time he had finished. “It hasn't got no taste at all!” he complained. “I couldn't feel nuffin in my mouf! It's a—what's that hard word, Sylvie?” “It was a Phlizz,” Sylvie gravely replied. “Are they all like that, father?” “They're all like that to you, darling, because you don't belong to Elfland—yet. But to me they are real.” Bruno looked puzzled. “I'll try anuvver kind of fruits!” he said, and jumped down off the King's knee. “There's some lovely striped ones, just like a rainbow!” And off he ran. Meanwhile the Fairy-King and Sylvie were talking together, but in such low tones that I could not catch the words: so I followed Bruno, who was picking and eating other kinds of fruit, in the vain hope of finding some that had a taste. I tried to pick so me myself—but it was like grasping air, and I soon gave up the attempt and returned to Sylvie. “Look well at it, my darling,” the old man was saying, “and tell me how you like it.” “'It's just lovely,” cried Sylvie, delightedly. “Bruno, come and look!” And she held up, so that he might see the light through it, a heart-shaped Locket, apparently cut out of a single jewel, of a rich blue colour, with a slender gold chain attached to it. “It are welly pretty,” Bruno more soberly remarked: and he began spelling out some words inscribed on it. “All—will—love—Sylvie,” he made them out at last. “And so they doos!” he cried, clasping his arms round her neck. “Everybody loves Sylvie!” “But we love her best, don't we, Bruno?” said the old King, as he took possession of the Locket. “Now, Sylvie, look at this.” And he showed her, lying on the palm of his hand, a Locket of a deep crimson colour, the same shape as the blue one and, like it, attached to a slender golden chain. “Lovelier and lovelier!” exclaimed Sylvie, clasping her hands in ecstasy. “Look, Bruno!” “And there's words on this one, too,” said Bruno. “Sylvie—will—love—all.” “Now you see the difference,” said the old man: “different colours and different words.” “Choose one of them, darling. I'll give you which ever you like best.” {Image...The crimson locket} Sylvie whispered the words, several times over, with a thoughtful smile, and then made her decision. “It's very nice to be loved,” she said: “but it's nicer to love other people! May I have the red one, Father?” The old man said nothing: but I could see his eyes fill with tears, as he bent his head and pressed his lips to her forehead in a long loving kiss. Then he undid the chain, and showed her how to fasten it round her neck, and to hide it away under the edge of her frock. “It's for you to keep you know he said in a low voice, not for other people to see. You'll remember how to use it?” “Yes, I'll remember,” said Sylvie. “And now darlings it's time for you to go back or they'll be missing you and then that poor Gardener will get into trouble!” Once more a feeling of wonder rose in my mind as to how in the world we were to get back again—since I took it for granted that wherever the children went I was to go—but no shadow of doubt seemed to cross their minds as they hugged and kissed him murmuring over and over again “Good-bye darling Father!” And then suddenly and swiftly the darkness of midnight seemed to close in upon us and through the darkness harshly rang a strange wild song:— He thought he saw a Buffalo
    Upon the chimney-piece:
    He looked again, and found it was
    His Sister's Husband's Niece.
    'Unless you leave this house,' he said,
    'I'll send for the Police!' {Image...'He thought he saw a buffalo'} “That was me!” he added, looking out at us, through the half-opened door, as we stood waiting in the road.' “And that's what I'd have done—as sure as potatoes aren't radishes—if she hadn't have tooken herself off! But I always loves my pay-rints like anything.” “Who are oor pay-rints?” said Bruno. “Them as pay rint for me, a course!” the Gardener replied. “You can come in now, if you like.” He flung the door open as he spoke, and we got out, a little dazzled and stupefied (at least I felt so) at the sudden transition from the half-darkness of the railway-carriage to the brilliantly-lighted platform of Elveston Station. A footman, in a handsome livery, came forwards and respectfully touched his hat. “The carriage is here, my Lady,” he said, taking from her the wraps and small articles she was carrying: and Lady Muriel, after shaking hands and bidding me “Good-night!” with a pleasant smile, followed him. It was with a somewhat blank and lonely feeling that I betook myself to the van from which the luggage was being taken out: and, after giving directions to have my boxes sent after me, I made my way on foot to Arthur's lodgings, and soon lost my lonely feeling in the hearty welcome my old friend gave me, and the cozy warmth and cheerful light of the little sitting-room into which he led me. “Little, as you see, but quite enough for us two. Now, take the easy-chair, old fellow, and let's have another look at you! Well, you do look a bit pulled down!” and he put on a solemn professional air. “I prescribe Ozone, quant. suff. Social dissipation, fiant pilulae quam plurimae: to be taken, feasting, three times a day!” “But, Doctor!” I remonstrated. “Society doesn't 'receive' three times a day!” “That's all you know about it!” the young Doctor gaily replied. “At home, lawn-tennis, 3 P.M. At home, kettledrum, 5 P.M. At home, music (Elveston doesn't give dinners), 8 P.M. Carriages at 10. There you are!” It sounded very pleasant, I was obliged to admit. “And I know some of the lady-society already,” I added. “One of them came in the same carriage with me.” “What was she like? Then perhaps I can identify her.” “The name was Lady Muriel Orme. As to what she was like—well, I thought her very beautiful. Do you know her?” “Yes—I do know her.” And the grave Doctor coloured slightly as he added “Yes, I agree with you. She is beautiful.” “I quite lost my heart to her!” I went on mischievously. “We talked—” “Have some supper!” Arthur interrupted with an air of relief, as the maid entered with the tray. And he steadily resisted all my attempts to return to the subject of Lady Muriel until the evening had almost worn itself away. Then, as we sat gazing into the fire, and conversation was lapsing into silence, he made a hurried confession. “I hadn't meant to tell you anything about her,” he said (naming no names, as if there were only one 'she' in the world!) “till you had seen more of her, and formed your own judgment of her: but somehow you surprised it out of me. And I've not breathed a word of it to any one else. But I can trust you with a secret, old friend! Yes! It's true of me, what I suppose you said in jest. “In the merest jest, believe me!” I said earnestly. “Why, man, I'm three times her age! But if she's your choice, then I'm sure she's all that is good and—” “—and sweet,” Arthur went on, “and pure, and self-denying, and true-hearted, and—” he broke off hastily, as if he could not trust himself to say more on a subject so sacred and so precious. Silence followed: and I leaned back drowsily in my easy-chair, filled with bright and beautiful imaginings of Arthur and his lady-love, and of all the peace and happiness in store for them. I pictured them to myself walking together, lingeringly and lovingly, under arching trees, in a sweet garden of their own, and welcomed back by their faithful gardener, on their return from some brief excursion. It seemed natural enough that the gardener should be filled with exuberant delight at the return of so gracious a master and mistress and how strangely childlike they looked! I could have taken them for Sylvie and Bruno less natural that he should show it by such wild dances, such crazy songs! “He thought he saw a Rattlesnake
    That questioned him in Greek:
    He looked again, and found it was
    The Middle of Next Week.
    'The one thing I regret,' he said,
    'Is that it cannot speak!” —least natural of all that the Vice-Warden and 'my Lady' should be standing close beside me, discussing an open letter, which had just been handed to him by the Professor, who stood, meekly waiting, a few yards off. “If it were not for those two brats,” I heard him mutter, glancing savagely at Sylvie and Bruno, who were courteously listening to the Gardener's song, “there would be no difficulty whatever.” “Let's hear that bit of the letter again,” said my Lady. And the Vice-Warden read aloud:— “—and we therefore entreat you graciously to accept the Kingship, to which you have been unanimously elected by the Council of Elfland: and that you will allow your son Bruno of whose goodness, cleverness, and beauty, reports have reached us—to be regarded as Heir-Apparent.” “But what's the difficulty?” said my Lady. “Why, don't you see? The Ambassador, that brought this, is waiting in the house: and he's sure to see Sylvie and Bruno: and then, when he sees Uggug, and remembers all that about 'goodness, cleverness, and beauty,' why, he's sure to—” “And where will you find a better boy than Uggug?” my Lady indignantly interrupted. “Or a wittier, or a lovelier?” To all of which the Vice-Warden simply replied “Don't you be a great blethering goose! Our only chance is to keep those two brats out of sight. If you can manage that, you may leave the rest to me. I'll make him believe Uggug to be a model of cleverness and all that.” “We must change his name to Bruno, of course?” said my Lady. The Vice-Warden rubbed his chin. “Humph! No!” he said musingly. “Wouldn't do. The boy's such an utter idiot, he'd never learn to answer to it.” “Idiot, indeed!” cried my Lady. “He's no more an idiot than I am!” “You're right, my dear,” the Vice-Warden soothingly I replied. “He isn't, indeed!” My Lady was appeased. “Let's go in and receive the Ambassador,” she said, and beckoned to the Professor. “Which room is he waiting in?” she inquired. “In the Library, Madam.” “And what did you say his name was?” said the Vice-Warden. The Professor referred to a card he held in his hand. “His Adiposity the Baron Doppelgeist.” “Why does he come with such a funny name?” said my Lady. “He couldn't well change it on the journey,” the Professor meekly replied, “because of the luggage.” “You go and receive him,” my Lady said to the Vice-Warden, “and I'll attend to the children.” About HackerNoon Book Series: We bring you the most important technical, scientific, and insightful public domain books. This book is part of the public domain. Lewis Carroll (1996). Sylvie and Bruno. Urbana, Illinois: Project Gutenberg. Retrieved October 2022 https://www.gutenberg.org/cache/epub/620/pg620-images.html This eBook is for the use of anyone anywhere at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this eBook or online at , located at . www.gutenberg.org https://www.gutenberg.org/policy/license.html

THE MAGIC LOCKET

The Idea of a University Defined and Illustrated: Lecture VI - University Preaching

Second Treatise of Government: Chapter XIX

Physical Geography

Of the eleventh horn of Daniel's fourth Beast

I am Boris Dobretsov, and this is the sixth part of a series Understanding Parallel Programming: A Guide for Beginners. If you haven’t read the first five parts, have a look at Understanding Parallel Programming: A Guide for Beginners, Understanding Parallel Programming: A Guide for Beginners, Part II, Understanding Threads to Better Manage Threading in iOS, Understanding Parallel Programming: Thread Management for Beginners, How Grand Central Dispatch Library Helps Organise Threads. Executing multiple code blocks in parallel may seem straightforward. However, writing concurrent code is often regarded as one of the most challenging aspects of programming. The complexity appears not only from running tasks in separate threads but also from ensuring they interact correctly with shared data. In this article, we’ll explore real-world scenarios and their solutions. Offloading Intensive UI Tasks to the Background Thread A common problem in mobile development is offloading heavy computations from the main thread to prevent UI freezes. Typically, these tasks don't require synchronization with others but must reflect their results back in the main thread. Consider this example: rendering a table view with images, applying a blur effect to each image. override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell(withIdentifier: "SomeCell", for: indexPath)
    
    let inputImage = UIImage(named: "treeSmall")!
    let inputCIImage = CIImage(image: inputImage)!
    
    let blurFilter = CIFilter(name: "CIGaussianBlur", parameters: [kCIInputImageKey: inputCIImage])!
    let outputImage = blurFilter.outputImage!
    let context = CIContext()
    let cgiImage = context.createCGImage(outputImage, from: outputImage.extent)
    
    cell.imageView?.image = UIImage(cgImage: cgiImage!)
    return cell
} Running this will result in slow loading and laggy scrolling, even with just 10 rows and small images. Why? Applying a blur effect is computationally expensive. Let's refactor this to offload the task to a background thread: func blur(image: UIImage, for imageView: UIImageView) {
    DispatchQueue.global(qos: .userInteractive).async {
        let inputCIImage = CIImage(image: image)!
        let blurFilter = CIFilter(name: "CIGaussianBlur", parameters: [kCIInputImageKey: inputCIImage])!
        let outputImage = blurFilter.outputImage!
        let context = CIContext()
        let cgiImage = context.createCGImage(outputImage, from: outputImage.extent)
        let blurredImage = UIImage(cgImage: cgiImage!)

        DispatchQueue.main.async {
            imageView.image = blurredImage
        }
    }
} This eliminates lag and ensures the UI updates are performed in the main thread, adhering to threading rules. Waiting for Parallel Tasks to Complete Sometimes, we need to perform an action only after several parallel tasks have finished. Using our earlier example, the blur operation could trigger repeatedly during scrolling. To fix this, we'll blur all images in parallel and update the table view once all tasks are complete. var images = Array(repeating: UIImage(named: "treeSmall")!, count: 10)

func blurImages() {
    let dispatchGroup = DispatchGroup()
    var blurredImages = images
    
    for (index, image) in images.enumerated() {
        dispatchGroup.enter()
        DispatchQueue.global().async {
            let inputCIImage = CIImage(image: image)!
            let blurFilter = CIFilter(name: "CIGaussianBlur", parameters: [kCIInputImageKey: inputCIImage])!
            let outputImage = blurFilter.outputImage!
            let context = CIContext()
            let cgiImage = context.createCGImage(outputImage, from: outputImage.extent)
            blurredImages[index] = UIImage(cgImage: cgiImage!)
            dispatchGroup.leave()
        }
    }
    
    dispatchGroup.notify(queue: .main) {
        self.images = blurredImages
        self.tableView.reloadData()
    }
} Race Conditions: The Subtle Bug Lurking in Parallel Code Race conditions occur when multiple threads access shared data without proper synchronization, leading to unpredictable behavior. Let’s explore a simple example of assigning unique IDs to documents. Here’s a straightforward implementation: struct Document: CustomStringConvertible {
    let id: Int
    let name: String
    var description: String { "\(id) - \(name)" }
}

for charCode in UnicodeScalar("A").value...UnicodeScalar("Z").value {
    let docName = String(UnicodeScalar(charCode)!)
    let lastId = documents.last?.id ?? 0
    documents.append(Document(id: lastId + 1, name: docName))
} In a multithreaded setup, the above approach fails as multiple threads simultaneously read the same lastId. To fix this, we’ll use a thread-safe document store. Implementing a Thread-Safe Data Structure We’ll create a thread-safe document store using DispatchQueue. class DocumentStore: CustomStringConvertible {
    private var documents = [Document]()
    private let syncQueue = DispatchQueue(label: "syncQueue", attributes: .concurrent)

    func append(document: Document) {
        syncQueue.async(flags: .barrier) {
            self.documents.append(document)
        }
    }

    var description: String {
        var result = ""
        syncQueue.sync {
            result = self.documents.map { $0.description }.joined(separator: ", ")
        }
        return result
    }
} The .barrier flag ensures write operations are isolated, while reads can occur in parallel. Using this, our document generation works reliably. By combining these techniques, you can tackle synchronization challenges, optimize performance, and prevent vague bugs like race conditions. Mastering concurrency takes time and effort, but practice and experimentation are your strongest allies on this journey! I am Boris Dobretsov, and this is the sixth part of a series Understanding Parallel Programming: A Guide for Beginners. If you haven’t read the first five parts, have a look at Understanding Parallel Programming: A Guide for Beginners, Understanding Parallel Programming: A Guide for Beginners, Part II , Understanding Threads to Better Manage Threading in iOS , Understanding Parallel Programming: Thread Management for Beginners, How Grand Central Dispatch Library Helps Organise Threads. Understanding Parallel Programming: A Guide for Beginners , Understanding Parallel Programming: A Guide for Beginners Understanding Parallel Programming: A Guide for Beginners, Part II Understanding Parallel Programming: A Guide for Beginners, Part II Understanding Threads to Better Manage Threading in iOS Understanding Threads to Better Manage Threading in iOS Understanding Parallel Programming: Thread Management for Beginners , Understanding Parallel Programming: Thread Management for Beginners How Grand Central Dispatch Library Helps Organise Threads . How Grand Central Dispatch Library Helps Organise Threads Executing multiple code blocks in parallel may seem straightforward. However, writing concurrent code is often regarded as one of the most challenging aspects of programming. The complexity appears not only from running tasks in separate threads but also from ensuring they interact correctly with shared data. In this article, we’ll explore real-world scenarios and their solutions . real-world scenarios and their solutions Offloading Intensive UI Tasks to the Background Thread A common problem in mobile development is offloading heavy computations from the main thread to prevent UI freezes. Typically, these tasks don't require synchronization with others but must reflect their results back in the main thread. Consider this example: rendering a table view with images, applying a blur effect to each image. override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell(withIdentifier: "SomeCell", for: indexPath)
    
    let inputImage = UIImage(named: "treeSmall")!
    let inputCIImage = CIImage(image: inputImage)!
    
    let blurFilter = CIFilter(name: "CIGaussianBlur", parameters: [kCIInputImageKey: inputCIImage])!
    let outputImage = blurFilter.outputImage!
    let context = CIContext()
    let cgiImage = context.createCGImage(outputImage, from: outputImage.extent)
    
    cell.imageView?.image = UIImage(cgImage: cgiImage!)
    return cell
} override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell(withIdentifier: "SomeCell", for: indexPath)
    
    let inputImage = UIImage(named: "treeSmall")!
    let inputCIImage = CIImage(image: inputImage)!
    
    let blurFilter = CIFilter(name: "CIGaussianBlur", parameters: [kCIInputImageKey: inputCIImage])!
    let outputImage = blurFilter.outputImage!
    let context = CIContext()
    let cgiImage = context.createCGImage(outputImage, from: outputImage.extent)
    
    cell.imageView?.image = UIImage(cgImage: cgiImage!)
    return cell
} Running this will result in slow loading and laggy scrolling, even with just 10 rows and small images. Why? Applying a blur effect is computationally expensive. Let's refactor this to offload the task to a background thread: func blur(image: UIImage, for imageView: UIImageView) {
    DispatchQueue.global(qos: .userInteractive).async {
        let inputCIImage = CIImage(image: image)!
        let blurFilter = CIFilter(name: "CIGaussianBlur", parameters: [kCIInputImageKey: inputCIImage])!
        let outputImage = blurFilter.outputImage!
        let context = CIContext()
        let cgiImage = context.createCGImage(outputImage, from: outputImage.extent)
        let blurredImage = UIImage(cgImage: cgiImage!)

        DispatchQueue.main.async {
            imageView.image = blurredImage
        }
    }
} func blur(image: UIImage, for imageView: UIImageView) {
    DispatchQueue.global(qos: .userInteractive).async {
        let inputCIImage = CIImage(image: image)!
        let blurFilter = CIFilter(name: "CIGaussianBlur", parameters: [kCIInputImageKey: inputCIImage])!
        let outputImage = blurFilter.outputImage!
        let context = CIContext()
        let cgiImage = context.createCGImage(outputImage, from: outputImage.extent)
        let blurredImage = UIImage(cgImage: cgiImage!)

        DispatchQueue.main.async {
            imageView.image = blurredImage
        }
    }
} This eliminates lag and ensures the UI updates are performed in the main thread, adhering to threading rules. Waiting for Parallel Tasks to Complete Sometimes, we need to perform an action only after several parallel tasks have finished. Using our earlier example, the blur operation could trigger repeatedly during scrolling. To fix this, we'll blur all images in parallel and update the table view once all tasks are complete. var images = Array(repeating: UIImage(named: "treeSmall")!, count: 10)

func blurImages() {
    let dispatchGroup = DispatchGroup()
    var blurredImages = images
    
    for (index, image) in images.enumerated() {
        dispatchGroup.enter()
        DispatchQueue.global().async {
            let inputCIImage = CIImage(image: image)!
            let blurFilter = CIFilter(name: "CIGaussianBlur", parameters: [kCIInputImageKey: inputCIImage])!
            let outputImage = blurFilter.outputImage!
            let context = CIContext()
            let cgiImage = context.createCGImage(outputImage, from: outputImage.extent)
            blurredImages[index] = UIImage(cgImage: cgiImage!)
            dispatchGroup.leave()
        }
    }
    
    dispatchGroup.notify(queue: .main) {
        self.images = blurredImages
        self.tableView.reloadData()
    }
} var images = Array(repeating: UIImage(named: "treeSmall")!, count: 10)

func blurImages() {
    let dispatchGroup = DispatchGroup()
    var blurredImages = images
    
    for (index, image) in images.enumerated() {
        dispatchGroup.enter()
        DispatchQueue.global().async {
            let inputCIImage = CIImage(image: image)!
            let blurFilter = CIFilter(name: "CIGaussianBlur", parameters: [kCIInputImageKey: inputCIImage])!
            let outputImage = blurFilter.outputImage!
            let context = CIContext()
            let cgiImage = context.createCGImage(outputImage, from: outputImage.extent)
            blurredImages[index] = UIImage(cgImage: cgiImage!)
            dispatchGroup.leave()
        }
    }
    
    dispatchGroup.notify(queue: .main) {
        self.images = blurredImages
        self.tableView.reloadData()
    }
} Race Conditions: The Subtle Bug Lurking in Parallel Code Race conditions occur when multiple threads access shared data without proper synchronization, leading to unpredictable behavior. Let’s explore a simple example of assigning unique IDs to documents. Here’s a straightforward implementation: struct Document: CustomStringConvertible {
    let id: Int
    let name: String
    var description: String { "\(id) - \(name)" }
}

for charCode in UnicodeScalar("A").value...UnicodeScalar("Z").value {
    let docName = String(UnicodeScalar(charCode)!)
    let lastId = documents.last?.id ?? 0
    documents.append(Document(id: lastId + 1, name: docName))
} struct Document: CustomStringConvertible {
    let id: Int
    let name: String
    var description: String { "\(id) - \(name)" }
}

for charCode in UnicodeScalar("A").value...UnicodeScalar("Z").value {
    let docName = String(UnicodeScalar(charCode)!)
    let lastId = documents.last?.id ?? 0
    documents.append(Document(id: lastId + 1, name: docName))
} In a multithreaded setup, the above approach fails as multiple threads simultaneously read the same lastId . To fix this, we’ll use a thread-safe document store. lastId Implementing a Thread-Safe Data Structure We’ll create a thread-safe document store using DispatchQueue . DispatchQueue class DocumentStore: CustomStringConvertible {
    private var documents = [Document]()
    private let syncQueue = DispatchQueue(label: "syncQueue", attributes: .concurrent)

    func append(document: Document) {
        syncQueue.async(flags: .barrier) {
            self.documents.append(document)
        }
    }

    var description: String {
        var result = ""
        syncQueue.sync {
            result = self.documents.map { $0.description }.joined(separator: ", ")
        }
        return result
    }
} class DocumentStore: CustomStringConvertible {
    private var documents = [Document]()
    private let syncQueue = DispatchQueue(label: "syncQueue", attributes: .concurrent)

    func append(document: Document) {
        syncQueue.async(flags: .barrier) {
            self.documents.append(document)
        }
    }

    var description: String {
        var result = ""
        syncQueue.sync {
            result = self.documents.map { $0.description }.joined(separator: ", ")
        }
        return result
    }
} The .barrier flag ensures write operations are isolated, while reads can occur in parallel. Using this, our document generation works reliably. .barrier By combining these techniques, you can tackle synchronization challenges, optimize performance, and prevent vague bugs like race conditions. Mastering concurrency takes time and effort, but practice and experimentation are your strongest allies on this journey!

Synchronization Challenges in Multithreading

If you are building a social app where users can upload images, then content moderation should be the most important task for your developers. If a user uploads any disturbing image on your platform, it may hurt others' sentiments or in the extreme case, it can become a legal issue. Therefore your best bet should be to remove or blur the NSFW content that violates your content policy. In this tutorial, we are going to look at how to detect NSFW images using machine learning algorithms and then automatically blur the image. The Problem Statement If you have thousands or millions of users, then it is not possible to review every image manually and blur them by hand. It needs a lot of manpower and a lot more money. Even watching hundreds of disturbing images for a human every day can lead to mental illness after some time. Our Approach In the era of artificial intelligence and machine learning, we can use AI technologies for our benefit. By using artificial intelligence, we can automatically detect NSFW images and give them an NSFW score. Then we can programmatically blur them as per their NSFW score. It means developers have full control over the NSFW content. Now the natural question that comes to your mind is, How can we make an AI model ourselves? The obvious answer is you don't need to. We can utilize APIs from any SaaS (software as a service) provided that provides artificial intelligence-based solutions. In this article, we are using to automate NSFW detection and blur the image when it goes above a certain NSFW limit. Pixlab APIs Agenda of The Article In this article, we are going to make a web project. With the help of this project, I demonstrate to you, How to detect the NSFW score of an online image. How to detect the NSFW score of a local image. How to blur an image depending on its NSFW score. This is a long-form article and I am trying to give you as much hands-on practice as I can through this article. I believe that you can make your own NSFW detection system after reading this article. So please grab a cup of coffee and let's start our tutorial. As I am using in this tutorial, make sure to get an API key for the same. Pixlab API endpoints Need For a Proxy Server The Pixlab API is not CORS (Cross-origin resource sharing) enabled. To avoid security issues, our browser doesn't allow us to connect with cross-origin websites. Therefore we need a proxy server to bypass the CORS restriction. Servers don't have CORS restrictions. So make a Nodejs and Expressjs server and do the basic setup. we write all our server code in file. server.js var express = require("express");
var cors = require("cors");
var fileUpload = require("express-fileupload");

var app = express();

// To parse application/json
app.use(express.json());

// Enable cors on all request
app.use(cors());

// File upload middleware
app.use(fileUpload());

app.listen(5000, () => {
  console.log("Server is running in port 5000 and cors enable");
}); Get NSFW Score of Online Images To get the NSFW score of an online image, we add an express route to . We use from pexels website for this comparison. GET /nsfw this NSFW image Inside the route, we call Pixlab's . This endpoint requires 2 parameters. The image link and the API key. When the API responds back, we send the response directly to the client. /nsfw NSFW API endpoint app.get("/nsfw", (req, res) => {
  axios
    .get("https://api.pixlab.io/nsfw", {
      params: {
        img: "https://images.pexels.com/photos/2495905/pexels-photo-2495905.jpeg",
        key: process.env.PIXLAB_KEY,
      },
    })
    .then((resp) => res.json(resp.data))
    .catch((err) => console.log(err));
}); For the front-end, create a new web project by making a file along with a javascript file called . Link the javascript file inside the HTML file. The content inside the HTML file looks like this. index.html index.js <!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Pixlab NSFW</title>
  </head>
  <body>
    <script src="./index.js"></script>
  </body>
</html> Now we need to add a button to invoke our NSFW API call. <button id="getscore">Get NSFW Score</button> Now inside the file, we make a function . This function lets us make calls to endpoint of our proxy server. index.js getnsfwscore() /nsfw const getScore= document.getElementById("getscore");

async function getnsfwscore() {
  try {
    let response =await fetch("http://localhost:5000/nsfw");
    console.log(await response.json());
  } catch (error) {
    throw error;
  }
} In response, the Pixlab NSFW API returns a NSFW score. The NSFW score ranges from 0 to 1. The typical response from the API looks like this. {
  "score": 0.7187498807907104,
  "status": 200
} Now you can use this score to filter the uploaded images. NSFW Score of Local Image You can also upload your local images to determine the NSFW score of that image. This is a 2 step process. At first, we upload this image to Pixlab's online storage and get the online link of that image. Then we can follow the previous step with that online image URL provided by Pixlab. To upload our local image, we use Pixlab's store API. We made a route in our express proxy server. To support image uploads, we use the plugin. /upload express-fileupload To upload an image to the store API, we make a request with the image and the API key. We are using FormData object to construct our query. POST app.post("/upload", (req, res) => {
  try {
    if (!req.files || Object.keys(req.files).length === 0) {
      return res.status(400).send("No files were uploaded.");
    }
    let image = req.files.image;

    const form = new FormData();
    form.append("file", image.data, image.name);
    form.append("key", process.env.PIXLAB_KEY);

    form.submit(
      {
        protocol: "https:",
        host: "api.pixlab.io",
        path: "/store",
        method: "POST",
        headers: {
          "Content-Type": `multipart/form-data; boundary=${form.getBoundary()}`,
        },
      },
      (err, resp) => {
        if (err) {
          res.status(503).send("File server is currently unavailable");
        }
        resp.pipe(res);
      }
    );
  } catch (error) {
    res.status(500).send(error);
  }
}); The above code block makes the request and forwards the response to the front end. In the front-end, we use a file input to get the user's local image. <input type="file" id="localImage"> Inside our file, we made function to upload the image to our proxy server. index.js uploadToStore async function uploadToStore(image) {
  const formData = new FormData();
  formData.append("image", image);
  try {
    let response = await fetch("http://localhost:5000/upload", {
      method: "POST",
      body: formData,
    });
    return await response.json();
  } catch (error) {
    throw "Fetch request give some error";
  }
} Now we listen for any change in the file input. If the user picks any image, we call this function. uploadToStore const localImage=document.getElementById("localImage");

localImage.addEventListener("change",async(e)=>{
const  files=e.target.files;
  if (files.length >0){
    const file=files[0];
    let response= await uploadToStore(file);
    console.log(response);
  }
}) In response, we get the online image URL of our local image. You can check the image in your browser by using these URLs. { ssl_link: "https://s3.amazonaws.com/media.pixlab.xyz/24p62f73c9583ec6.jpeg", 
  link: "https://s3.amazonaws.com/media.pixlab.xyz/24p62f73c9583ec6.jpeg", 
  id: "24p62f73c9583ec6.jpeg", 
  status: 200 
} Now to calculate the NSFW score of this image, you can follow the previous section on getting the NSFW score for an online image. Blur Images Programmaticaly After filtering the images as per their NSFW scores, we blur the inappropriate images. To blur an image, we use the of Pixlab. blur API endpoint We create a GET route in our proxy server to call Pixlab's blur API. The API endpoint requires the image URL, radius of the blur, sigma parameter, and your pixlab API key. /blur app.get("/blur", (req, res) => {
  axios
    .get("https://api.pixlab.io/blur", {
      params: {
        img: "https://images.pexels.com/photos/2495905/pexels-photo-2495905.jpeg",
        radius: 50,
        sigma: 30,
        key: process.env.PIXLAB_KEY,
      },
    })
    .then((resp) => res.json(resp.data))
    .catch((err) => console.log(err));
}); You can modify the blurring process by varying the radius and sigma parameters. In the front-end, we make a function. This function makes a GET request to the endpoint of our proxy server. blurImage /blur async function blurImage(){
  try {
    let response =await fetch("http://localhost:5000/blur");
    console.log(await response.json());
  } catch (error) {
    throw error;
  }
} In response, the Pixlab server returns a blurred version of your uploaded image. You can use the blur image URL to display in your front end. { ssl_link: "https://s3.amazonaws.com/media.pixlab.xyz/24p62f74006c5fa1.jpeg", 
  link: "https://s3.amazonaws.com/media.pixlab.xyz/24p62f74006c5fa1.jpeg", 
  id: "24p62f74006c5fa1.jpeg", 
  status: 200 
} Conclusion You can find the code samples for this tutorial in my GitHub gist. If you like this tutorial, you can read more interesting . If you have any feedback, you can find me on Twitter as . tutorials on my blog @hrishikeshpathak Frequently Asked Question Can we blur our image without using these APIs? Yes, You can use CSS blur filter to blur any image on your website. Select your HTML image element by using a unique class, id, or using javascript, and then apply the blur filter. filter: blur(4px); Learn more about CSS blur filters on the . MDN website Is using a proxy server mandatory? No, I am using this proxy server just for demonstration. You can use your own production server for this purpose. I am using a proxy server just to bypass the browser CORS restriction.

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