Data.swift

import Foundation

extension Data {
  public init?(hexString: String) {
    let utf16 = hexString.utf16
    var data = Data(capacity: utf16.count / 2)
    var hiIndex = utf16.startIndex

    while hiIndex != utf16.endIndex {
      guard let hi = Data.decodeNibble(utf16[hiIndex]),
            let loIndex = utf16.index(hiIndex, offsetBy: 1, limitedBy: utf16.endIndex),
            let lo = Data.decodeNibble(utf16[loIndex]) else {
        return nil
      }
      
      var value = (hi << 4) + lo

      data.append(&value, count: 1)
      hiIndex = utf16.index(hiIndex, offsetBy: 2, limitedBy: utf16.endIndex) ?? utf16.endIndex
    }
    
    self = data
  }
  
  public func hexString() -> String {
    return self.withUnsafeBytes() { (bytes: UnsafePointer<UInt8>) in
      var string = ""
      
      for i in 0..<self.count {
        string += String(format: "%02x", bytes[i])
      }
      
      return string
    }
  }
  
  fileprivate static func decodeNibble(_ u: UInt16) -> UInt8? {
    switch u {
      case 0x30...0x39:
        return UInt8(u - 0x30)
      case 0x41...0x46:
        return UInt8(u - 0x41 + 0x0a)
      case 0x61...0x66:
        return UInt8(u - 0x61 + 0x0a)
      default:
        return nil
    }
  }
}