grain.social/grain-ios

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import Foundation
import NaturalLanguage

/// Detects whether a caption is in a language the reader probably doesn't read,
/// which drives the "translate" affordance under a gallery description.
///
/// This exists as an actor because `NLLanguageRecognizer` is far too slow to run
/// inline during a scroll: profiling measured 2.6ms average and 13.4ms worst
/// case per call, against a 16.7ms frame budget — a single bad call could drop a
/// frame on its own. Running it off the main actor keeps it out of the way, and
/// memoizing by text means the repeated work disappears too, since the same
/// captions come back every time a card leaves and re-enters the lazy stack.
actor LanguageDetector {
    static let shared = LanguageDetector()

    private var cache: [String: Bool] = [:]

    /// Cap chosen to comfortably cover a long scrolling session without letting
    /// the cache grow unbounded. Cleared wholesale rather than evicting LRU —
    /// a rebuild is cheap now that it's off the main thread.
    private static let cacheLimit = 500

    /// Fewer letters than this and the recogniser is guessing. It will still
    /// name a language confidently for a caption that is only "#35mm" or an
    /// emoji, which is how an offer to translate ends up under a caption that
    /// has no prose in it at all.
    private static let minimumLetters = 8

    func isForeign(_ text: String) -> Bool {
        if let cached = cache[text] {
            return cached
        }

        let result = detect(text)

        if cache.count >= Self.cacheLimit {
            cache.removeAll(keepingCapacity: true)
        }
        cache[text] = result
        return result
    }

    private func detect(_ text: String) -> Bool {
        guard text.unicodeScalars.count(where: { CharacterSet.letters.contains($0) }) >= Self.minimumLetters else {
            return false
        }

        let recognizer = NLLanguageRecognizer()
        recognizer.processString(text)

        guard let detected = recognizer.dominantLanguage?.rawValue else { return false }
        let preferred = Locale.preferredLanguages.first ?? "en"
        return !preferred.hasPrefix(detected)
    }
}