Decompose Hangul string.

public class HangulDecomposer {
    
    private static let hangulSet = CharacterSet(charactersIn: "가".unicodeScalars.first! ... "힣".unicodeScalars.first!)
    private static let firstIndex = "가".unicodeScalars.first!.value
    private static let lastIndex = "힣".unicodeScalars.first!.value
    
    private static let chosungCaseCount = UInt32(jungsungSet.count * jongsungSet.count)
    private static let jungsungCaseCount = UInt32(jongsungSet.count)
    
    private static let chosungSet = [
        "ㄱ","ㄲ","ㄴ","ㄷ","ㄸ","ㄹ","ㅁ","ㅂ","ㅃ","ㅅ",
        "ㅆ","ㅇ","ㅈ","ㅉ","ㅊ","ㅋ","ㅌ","ㅍ","ㅎ"
    ]
    private static let jungsungSet = [
        "ㅏ", "ㅐ", "ㅑ", "ㅒ", "ㅓ", "ㅔ","ㅕ", "ㅖ", "ㅗ", "ㅘ",
        "ㅙ", "ㅚ","ㅛ", "ㅜ", "ㅝ", "ㅞ", "ㅟ", "ㅠ", "ㅡ", "ㅢ",
        "ㅣ"
    ]
    private static let jongsungSet = [
        "","ㄱ","ㄲ","ㄳ","ㄴ","ㄵ","ㄶ","ㄷ","ㄹ","ㄺ",
        "ㄻ","ㄼ","ㄽ","ㄾ","ㄿ","ㅀ","ㅁ","ㅂ","ㅄ","ㅅ",
        "ㅆ","ㅇ","ㅈ","ㅊ","ㅋ","ㅌ","ㅍ","ㅎ"
    ]
    private static let doubleJongsungSet = [
        "ㄳ":"ㄱㅅ","ㄵ":"ㄴㅈ","ㄶ":"ㄴㅎ","ㄺ":"ㄹㄱ","ㄻ":"ㄹㅁ",
        "ㄼ":"ㄹㅂ","ㄽ":"ㄹㅅ","ㄾ":"ㄹㅌ","ㄿ":"ㄹㅍ","ㅀ":"ㄹㅎ",
        "ㅄ":"ㅂㅅ"
    ]
    
    static func get(from: String) -> String {
        from.unicodeScalars.map { decompose(from: $0) ?? "" }.joined()
    }
    
    private static func decompose(from unicode: UnicodeScalar) -> String? {
        if hangulSet.contains(unicode) {
            let index = unicode.value - firstIndex
            let chosung = chosungSet[Int(index / chosungCaseCount)]
            let jungsong = jungsungSet[Int((index % chosungCaseCount) / jungsungCaseCount)]
            var jongsung = jongsungSet[Int(index % jungsungCaseCount)]
            if let doubleJongsung = doubleJongsungSet[jongsung] {
                jongsung = doubleJongsung
            }
            return chosung + jungsong + jongsung
        }
        return String(unicode)
    }
    
}