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
  }
}

@josephlord
I like your idea about Iterator, it looks more... erm, swift-ish.
IMHO, Iterator here gives another piece of complexity (under the hood) and, supposedly, takes more memory than having int (startIndex) stored in local var.
Another concern is that you're explicitly unwrapping the result of .next call so there might be crash.
Of course, that's my concerns only, I don't pretend to judge, just interested in promising conversation here.

@fallback I don't think the iterator makes a copy, it is really very similar under the hood with it just managing the index.

Regarding the force unwraps I think it preserves the original behaviour (indexing also crashes when out of bounds) but in both cases the count being hexString.count / 2 means that it is safe.

The even Swiftier approach would probably map the string into pairs of letters (itself a function taking an iterator) and then zip that with the data indices and forEach on it. That does involve multiple passes though which I was avoiding (although it can probably be done lazily to get back to the same fundamental operations).

embrace your inner Substring

import Foundation

extension Data {
    init?<Hex: StringProtocol>(hexString: Hex) {
        var hex = hexString[...]

        self.init(capacity: hex.count / 2)
        while !hex.isEmpty {
            guard
                let hi = hex.popFirst()?.hexDigitValue,
                let lo = hex.popFirst()?.hexDigitValue
            else { return nil }
            append(UInt8(hi << 4 | lo))
        }
    }
}

That works and shows me hexDigitValue which I should have used in the iterator approach. Is there any advantage to the Substring over the iterator? I imagine it just has an additional end index though I haven’t checked.

Also if we use iterator it can be made generic over collections of Characters so it should work on Strings, Substrings and arrays of characters. It could work on Sequences if we didn’t need the count upfront.

extension Data {
    init?<S>(hexString: S) where S : Collection, S.Element == Character {
        let count = hexString.count / 2
        var data = Data(capacity: count)
        var itr = hexString.makeIterator()
        for _ in 0 ..< count {
            guard let hi = itr.next()?.hexDigitValue,
                  let lo = itr.next()?.hexDigitValue else { return nil }
            data.append(UInt8(hi << 4 | lo))
        }
        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.