Fast Luhn credit card check in Scala

luhn.scala

final object Luhn {
  /** An O(1) Map[Int, Int] of precomputed double values. */
  private[this] final val doubles = Array(0, 2, 4, 6, 8, 1, 3, 5, 7, 9)

  /**
   * Validate a card using the Luhn algorithm with a single pass through the string,
   * no other validation needed prior.
   *
   * Pure speed here (only 1 function call to get the byte array).
   */
  final def check(card: String): Boolean = {
    val bytes = card.getBytes("UTF-8")
    val max = bytes.length
    if (max < 10 || max > 19) false
    else {
      var sum = 0
      var i = max - 1
      var even = true
      while (i >= 0) {
        val n = bytes(i) - 0x30           // ASCII/UTF numeric digit range = 0x30 - 0x39
        if (n < 0 || n > 9) return false  // bail on non-numeric character, also range validation
        else {
          sum += (if (even) n else doubles(n))
          even = !even
          i -= 1
        }
      }
      sum % 10 == 0
    }
  }

  /**
   * Validate a card using the Luhn algorithm with a single pass through the string,
   * no other validation needed prior.
   *
   * Idiomatic code.
   */
  final def checkIdiomatic(card: String): Boolean = {
    if (card.length < 10 || card.length > 19) false
    else {
      val sum = card.reverseIterator.zipWithIndex.foldLeft(0) {
        case (sum, (ch, i)) =>
          val n = ch - 0x30                 // ASCII/UTF numeric digit range = 0x30 - 0x39
          if (n < 0 || n > 9) return false  // bail on non-numeric character, also range validation
          else sum + (if (i % 2 == 0) n else doubles(n))
      }
      sum % 10 == 0
    }
  }
}

On my Macbook Pro, JVM and code warmed-up:

time(Luhn.check("valid-card-number"), 1000)
1,000 iterations: 236,643 ns total, 236.64 avg, 178 ns min, 10,742 ns max, stddev = 384.5881

time(Luhn.check("invalid-card-number"), 1000)
1,000 iterations: 232,024 ns total, 232.02 avg, 173 ns min, 9,846 ns max, stddev = 355.2237

time(Luhn.checkIdiomatic("valid-card-number"), 1000)
1,000 iterations: 505,607 ns total, 505.61 avg, 407 ns min, 15,313 ns max, stddev = 531.7108

time(Luhn.checkIdiomatic("invalid-card-number"), 1000)
1,000 iterations: 465,138 ns total, 465.14 avg, 385 ns min, 29,142 ns max, stddev = 928.1494