ccnum.scm: Credit Card Number Utilities in Scheme
ccnum.scm: Credit Card Number Utilities in Scheme
*************************************************
Version 0.2, 2005-03-29, `http://www.neilvandyke.org/ccnum-scm/'
by Neil W. Van Dyke <neil@neilvandyke.org>
Copyright (C) 2004 - 2005 Neil W. Van Dyke. This program is Free
Software; you can redistribute it and/or modify it under the terms
of the GNU Lesser General Public License as published by the Free
Software Foundation; either version 2.1 of the License, or (at
your option) any later version. This program is distributed in
the hope that it will be useful, but without any warranty; without
even the implied warranty of merchantability or fitness for a
particular purpose. See the GNU Lesser General Public License
[LGPL] for details. For other license options and consulting,
contact the author.
Introduction
************
This is a Scheme library of a few utilities for validating and
formatting credit card numbers. Credit card numbers are represented as
strings containing digits and arbitrary whitespace. The procedures are
based on information gleaned from dozens of written artifacts of credit
card number oral tradition, including [Bradbury], [Gilleland], and
[Hippy]. The author invites free copies of authoritative documentation.
This library should work with any R5RS-compliant Scheme
implementation that has an `error' procedure similar to that in
[SRFI-23].
Achtung! Do not use this library as anything other than a novelty
unless you understand the code thoroughly and can invest in validation
of it. (The same caution applies to all the other credit card number
checking routines the author has seen in other languages, most of which
are surprisingly inefficient and otherwise do not instill confidence.)
Validation
**********
The following procedures provide different ways of validating credit
card numbers. Most applications will use
`credit-card-number-check-digit-ok?' or `credit-card-number-seems-ok?'.
> (check-credit-card-number str)
Performs a partial validation of the credit card number in STR. If
the check digit is incorrect, then `#f' is yielded:
(check-credit-card-number "4408041234567890") => #f
If the check digit is correct, but the issuer cannot be
determined, then an integer representing the digit count is
yielded:
(check-credit-card-number "1234567890123452") => 16
If the check digit is correct and issuer can be determined, then a
list of three elements is returned. The first element is a
boolean value for whether or not the digit count matches what is
known about how many digits the issuer uses for this class of
cards. The second element is the digit count. The third element
is a symbol loosely identifying the issuer. For example:
(check-credit-card-number "5551 2121 9") => (#f 9 mastercard)
(check-credit-card-number "4408041234567893") => (#t 16 visa)
> (credit-card-number-check-digit-ok? str)
Predicate for whether or not the check digit of credit card number
STR is correct.
(credit-card-number-check-digit-ok? "4408 0412 3456 7893") => #t
(credit-card-number-check-digit-ok? "4408 0412 3456 7890") => #f
(credit-card-number-check-digit-ok? "trump") => #f
> (credit-card-number-seems-ok? str)
Predicate for whether or not the credit card number STR "seems" to
be valid. For a credit card number to "seem" valid, the check
digit must be correct, the issuer must be identified, and the
digit count must match what is known about issuer digit counts.
In the following example the check digit is correct, and the
issuer (MasterCard) has been identified, but the digit count is
too low for a MasterCard number:
(credit-card-number-check-digit-ok? "5551 2121 9") => #t
(credit-card-number-seems-ok? "5551 2121 9") => #f
Formatting
**********
Two procedures are provided for formatting credit card numbers.
> (write-formatted-credit-card-number str port)
Writes credit card number STR to output port PORT, using a format
similar to that used on many credit cards. In the current version
of this package, the format is always groups of four digits
separated by single space characters, although a future version
might mimic the format used by the issuer.
(write-formatted-credit-card-number " 1 23 456 7890 12345 6 "
(current-output-port))
-| 1234 5678 9012 3456
> (formatted-credit-card-number str)
Yields a formatted string representation of credit card number STR
like that written by `write-formatted-credit-card-number'.
(formatted-credit-card-number "1234567890123456")
=> "1234 5678 9012 3456"
(formatted-credit-card-number " 12 34 56 7890 1234 56")
=> "1234 5678 9012 3456"
(formatted-credit-card-number "123 abc") => #f
Note that `(write-formatted-credit-card-number N P)' is more
efficient than `(display (formatted-credit-card-number N) P)'.
History
*******
Version 0.2 -- 2005-03-29
Minus characters (`#\-') are now accepted as blanks in credit card
numbers.
Version 0.1 -- 2004-05-15
First release.
References
**********
[Bradbury]
Jeremy Scott Bradbury, "Credit Card Check Digit," Web page, viewed
2004-05-15.
`http://www.cs.queensu.ca/~bradbury/checkdigit/creditcardcheck.htm'
[Gilleland]
Michael Gilleland, "Anatomy of Credit Card Numbers," Web page,
viewed 2004-05-15.
`http://www.merriampark.com/anatomycc.htm'
[Hippy]
Happy Hippy, "Credit Card Magic," Web page, viewed 2004-05-15.
`http://www.hippy.freeserve.co.uk/credcard.htm'
[LGPL]
Free Software Foundation, "GNU Lesser General Public License,"
Version 2.1, 1999-02, 59 Temple Place, Suite 330, Boston, MA
02111-1307 USA.
`http://www.gnu.org/copyleft/lesser.html'
[SRFI-23]
Stephan Houben, "Error reporting mechanism," SRFI 23, 2001-04-26.
`http://srfi.schemers.org/srfi-23/srfi-23.html'
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