session-ios/Signal/src/util/DisplayableTextFilter.swift

//
//  Copyright (c) 2017 Open Whisper Systems. All rights reserved.
//

import Foundation

@objc class DisplayableTextFilter: NSObject {

    let TAG = "[DisplayableTextFilter]"

    @objc
    func displayableText(_ text: String?) -> String? {
        guard let text = text else {
            return nil
        }

        if (self.hasExcessiveDiacriticals(text: text)) {
            Logger.warn("\(TAG) filtering text for excessive diacriticals.")
            let filteredText = text.folding(options: .diacriticInsensitive, locale: .current)
            return filteredText
        }

        return text
    }

    private func hasExcessiveDiacriticals(text: String) -> Bool {
        // discard any zalgo style text, by detecting maximum number of glyphs per character
        for char in text.characters.enumerated() {
            let scalarCount = String(char.element).unicodeScalars.count
            if scalarCount > 4 {
                Logger.warn("\(TAG) detected excessive diacriticals at \(char.element) scalarCount: \(scalarCount)")
                return true
            }
        }

        return false
    }
}