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# Use the Luhn algorithm to validate a numeric identifier, such as credit card
# numbers, national insurance numbers, etc.
# See: http://en.wikipedia.org/wiki/Luhn_algorithm
is_valid_identifier = (identifier) ->
sum = 0
alt = false
for c in identifier by -1
# Get the next digit.
num = parseInt c, 10
# If it's not a valid number, abort.
return false if isNaN num
# If it's 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's valid.
sum % 10 is 0
# Tests.
console.log is_valid_identifier("49927398716") is true
console.log is_valid_identifier("4408041234567893") is true
console.log is_valid_identifier("4408041234567890") is false
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