UTF16 encode and decode

main.swift

import Foundation

func encodeString(str: String) -> String {
    return Array(str.utf16).map {
        String(format: "%04x", $0)
    }.reduce("") {
        return $0 + $1
    }
}

func decodeString(encoded: String) -> String {
    var utf16Array = [UInt16]()
    0.stride(to: encoded.characters.count, by: 4).forEach {
        let startIndex = encoded.characters.startIndex.advancedBy($0)
        let endIndex = encoded.characters.startIndex.advancedBy($0 + 3)
        let hex4 = encoded.substringWithRange(startIndex...endIndex)
        
        utf16Array.append(UInt16(hex4, radix: 16)!)
    }
    
    return String(utf16CodeUnits: utf16Array, count: utf16Array.count)
}

let str = "Hello, Playground"

let encoded = encodeString(str)
let decoded = decodeString(encoded)

print(encoded)
print(decoded)