Data+Hex.swift

extension Data {
  init?(hexString: String) {
    let count = hexString.count / 2
    var data = Data(capacity: count)
    var i = hexString.startIndex
    for _ in 0 ..< count {
      let j = hexString.index(after: i)
      if var byte = UInt8(hexString[i ... j], radix: 16) {
        data.append(&byte, count: 1)
      } else {
        return nil
      }
      i = hexString.index(after: j)
    }
    self = data
  }
}

From my own timings, popping a substring is slower than using index.after(). Not sure how it compares to iterator: https://twitter.com/nicklockwood/status/1382142248247308292

Try using hexString.utf8.withContiguousStorageIfAvailable. You'll have to write your own hexDigitValue though.