37 lines
863 B
CoffeeScript
37 lines
863 B
CoffeeScript
# Use the Luhn algorithm to validate a numeric identifier, such as credit card
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# numbers, national insurance numbers, etc.
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# See: http://en.wikipedia.org/wiki/Luhn_algorithm
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is_valid_identifier = (identifier) ->
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sum = 0
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alt = false
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for i in [identifier.length - 1..0] by -1
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# Get the next digit.
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num = parseInt identifier.charAt(i), 10
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# If it's not a valid number, abort.
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return false if isNaN(num)
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# If it's an alternate number...
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if alt
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num *= 2
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num = (num % 10) + 1 if num > 9
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# Flip the alternate bit.
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alt = !alt
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# Add to the rest of the sum.
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sum += num
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# Determine if it's valid.
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sum % 10 is 0
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# Tests.
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console.log is_valid_identifier("49927398716") is true
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console.log is_valid_identifier("4408041234567893") is true
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console.log is_valid_identifier("4408041234567890") is false
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