Swift port of the ID generator used in Firebase Firestore

AutoId.swift

import Foundation

class AutoId {
    private init() {}

    static func newId() -> String {
        // Alphanumeric characters
        let chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"
        // The largest byte value that is a multiple of `char.length`.
        let maxMultiple = Int(256 / chars.count) * chars.count
        assert(
            0 < maxMultiple && maxMultiple < 256,
            "Expect maxMultiple to be (0, 256), but got \(maxMultiple)"
        )

        var autoId = ""
        let targetLength = 20
        while autoId.count < targetLength {
            let bytes = Self.randomBytes(40)
            for i in 0..<bytes.count {
                // Only accept values that are [0, maxMultiple), this ensures they can
                // be evenly mapped to indices of `chars` via a modulo operation.
                if autoId.count < targetLength && bytes[i] < maxMultiple {
                    autoId += String(chars[chars.index(chars.startIndex, offsetBy: Int(bytes[i]) % chars.count)])
                }
            }
        }
        assert(autoId.count == targetLength, "Invalid auto ID: \(autoId)")

        return autoId
    }

    private static func randomBytes(_ count: Int) -> [UInt8] {
        var bytes = [UInt8](repeating: 0, count: count)
        let _ = SecRandomCopyBytes(kSecRandomDefault, count, &bytes)
        return bytes
    }
}