[String+disjointedHangul] Extends `disjointedHangul` method to `String` #extension #iOS

String+disjointedHangul.swift

//
//  String+disjointedHangul.swift
//
//  Created by Jay Lee on 08/03/2019.
//  Copyright © 2019 Jay Lee <jaeho.lee@snu.ac.kr>
//  This work is free. You can redistribute it and/or modify it under the
//  terms of the Do What The Fuck You Want To Public License, Version 2,
//  as published by Sam Hocevar. See http://www.wtfpl.net/ for more details.
//

import Foundation

private let chosung = ["ㄱ","ㄲ","ㄴ","ㄷ","ㄸ","ㄹ","ㅁ","ㅂ","ㅃ","ㅅ","ㅆ","ㅇ","ㅈ","ㅉ","ㅊ","ㅋ","ㅌ","ㅍ","ㅎ"]
private let jungsung = ["ㅏ","ㅐ","ㅑ","ㅒ","ㅓ","ㅔ","ㅕ","ㅖ","ㅗ","ㅘ","ㅙ","ㅚ","ㅛ","ㅜ","ㅝ","ㅞ","ㅟ","ㅠ","ㅡ","ㅢ","ㅣ"]
private let jonsung = ["","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"]

extension String {
    /// The string with Korean alphabet *jamo 자모* disjointed.
    ///
    /// If the string contains non-Korean alphabets,
    ///   ```
    ///   let text = "Hello, 세상!"
    ///   print(text.disjointedHangul)
    ///   // prints "Hello, ㅅㅔㅅㅏㅇ!"
    ///   ```
    ///
    /// 0xAC00 corresponds to 가, and 0xD7A3 corresonds to 힣.
    /// See [The Unicode Standard, Hangul Syllables](http://www.unicode.org/charts/PDF/UAC00.pdf)
    /// for more on the unicode range of Korean letters.
    var disjointedHangul: String {
        reduce(into: "") { partialResult, character in
            // Checks if `character` is a Korean alphabet, hangul. If not, continue to the next character.
            guard let unicode32 = character.unicodeScalars.first?.value,
                  (0xAC00...0xD7A3).contains(unicode32)
            else {
                partialResult += String(character)
                return
            }

            let unicode = Int(unicode32 - 0xAC00)

            let chosungIndex = unicode / 21 / 28
            let jungsungIndex = unicode % ( 21 * 28 ) / 28
            let jongsungIndex = unicode % 28

            partialResult += chosung[chosungIndex] + jungsung[jungsungIndex] + jonsung[jongsungIndex]
        }
    }
}