Firebase push algorithm in Swift (ref: https://www.firebase.com/blog/2015-02-11-firebase-unique-identifiers.html)

generate-pushid.swift

/// custom unique identifier
/// @see https://www.firebase.com/blog/2015-02-11-firebase-unique-identifiers.html
private let ASC_CHARS = Array("-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz")
private let DESC_CHARS = ASC_CHARS.reverse()
private var lastPushTime: UInt64 = 0
private var lastRandChars = Array<Int>(count: 12, repeatedValue: 0)

func generatePushID(ascending: Bool = true) -> String {
  let PUSH_CHARS = ascending ? ASC_CHARS: DESC_CHARS
  var timeStampChars = Array<Character>(count: 8, repeatedValue: PUSH_CHARS.first!)
  var now = UInt64(NSDate().timeIntervalSince1970 * 1000)
  let duplicateTime = (now == lastPushTime)
  lastPushTime = now

  for var i = 7; i >= 0; i-- {
    timeStampChars[i] = PUSH_CHARS[Int(now % 64)]
    now >>= 6
  }

  assert(now == 0, "We should have converted the entire timestamp.")
  var id: String = String(timeStampChars)

  if !duplicateTime {
    for i in 0..<12 { lastRandChars[i] = Int(64 *  Double(rand()) / Double(RAND_MAX)) }
  } else {
    var i: Int
    for i = 11; i >= 0 && lastRandChars[i] == 63; i-- {
      lastRandChars[i] = 0
    }

    lastRandChars[i]++
  }

  for i in 0..<12 { id.append(PUSH_CHARS[lastRandChars[i]]) }
  assert(count(id) == 20, "Length should be 20.")
  return id
}