converting string to hex and back

HexUidConverter.swift

import Foundation

enum HexUidConverter {
    static func hexString(from uid: String) -> String {
        Data(uid.utf8).hexEncodedString()
    }
    
    static func uid(from hexString: String) -> String? {
        guard let data = Data(hexString: hexString) else {
            return nil
        }
        return String(data: data, encoding: .utf8)
    }
}

private extension Data {
    init?(hexString: String) {
        let length = hexString.count / 2
        var data = Data(capacity: length)
        for i in 0 ..< length {
            let j = hexString.index(hexString.startIndex, offsetBy: i * 2)
            let k = hexString.index(j, offsetBy: 2)
            let bytes = hexString[j..<k]
            if var byte = UInt8(bytes, radix: 16) {
                data.append(&byte, count: 1)
            } else {
                return nil
            }
        }
        self = data
    }
    
    func hexEncodedString() -> String {
        return map { String(format: "%02hhx", $0) }.joined()
    }
}