Swift String adding random skin tone modifier

Swift+SkinToneModifier.swift

import Foundation

public extension String {
    private static let skinToneModifiers = ["🏻", "🏼", "🏽", "🏾", "🏿"]
    private static var lastUsedModifier: String?
    
    static var randomSkinToneModifier: String {
        var randomModifier: String
        repeat {
            randomModifier = skinToneModifiers.randomElement()!
        } while randomModifier == lastUsedModifier
        lastUsedModifier = randomModifier
        return randomModifier
    }
    
    private var endsInModifiableEmoji: Bool {
        unicodeScalars.last?.properties.isEmojiModifierBase == true
    }
    
    /// Adds a skintone modifier to the last character if it’s an emoji and can be modified
    ///
    /// Use this method with any string ending in an emoji and not already modified
    /// ```
    /// "👋".addingRandomSkintoneModifier()
    /// ```
    func addingRandomSkintoneModifier() -> Self {
        guard endsInModifiableEmoji == true else {
            return self
        }
        return appending(Self.randomSkinToneModifier)
    }
}