credit card validation

cc.coffee

cardType = (cc) ->
  result = "unknown"
  result = "mastercard" if /^5[1-5]/.test(cc)
  result = "visa"       if /^4/.test(cc)
  result = "amex"       if /^3[47]/.test(cc)
  result

luhn = (cc) ->
  sum = 0
  alt = false
 
  for i in [cc.length - 1..0] by -1
    # Get the next digit.
    num = parseInt cc.charAt(i), 10
 
    # If it is not a valid number, abort.
    return false if isNaN(num)
 
    # If it is an alternate number...
    if alt
      num *= 2
      num = (num % 10) + 1 if num > 9
 
    # Flip the alternate bit.
    alt = !alt
 
    # Add to the rest of the sum.
    sum += num
 
  # Determine if it is valid.
  sum % 10 is 0

$ ->
  $('form.credit-card #number').change (ev) ->
    cc = $(@).val()
    type = cardType(cc)
    validity = if luhn(cc) then "Valid" else "Invalid"

    $('form.credit-card .credit-card-type').html(type)

    $('form.credit-card .valid-credit-card').html(validity)