73 lines
2.1 KiB
Factor
73 lines
2.1 KiB
Factor
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! Copyright (c) 2012 Anonymous
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! See http://factorcode.org/license.txt for BSD license.
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USING: kernel math math.parser math.order math.ranges sequences ;
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IN: rosetta-code.luhn-test
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! http://rosettacode.org/wiki/Luhn_test_of_credit_card_numbers
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! The Luhn test is used by some credit card companies to
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! distinguish valid credit card numbers from what could be a
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! random selection of digits.
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! Those companies using credit card numbers that can be
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! validated by the Luhn test have numbers that pass the following
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! test:
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! 1. Reverse the order of the digits in the number.
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! 2. Take the first, third, ... and every other odd digit in the
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! reversed digits and sum them to form the partial sum s1
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! 3. Taking the second, fourth ... and every other even digit in
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! the reversed digits:
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! a. Multiply each digit by two and sum the digits if the
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! answer is greater than nine to form partial sums for the
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! even digits
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! b. Sum the partial sums of the even digits to form s2
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! 4. If s1 + s2 ends in zero then the original number is in the
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! form of a valid credit card number as verified by the Luhn test.
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! For example, if the trial number is 49927398716:
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! Reverse the digits:
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! 61789372994
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! Sum the odd digits:
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! 6 + 7 + 9 + 7 + 9 + 4 = 42 = s1
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! The even digits:
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! 1, 8, 3, 2, 9
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! Two times each even digit:
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! 2, 16, 6, 4, 18
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! Sum the digits of each multiplication:
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! 2, 7, 6, 4, 9
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! Sum the last:
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! 2 + 7 + 6 + 4 + 9 = 28 = s2
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! s1 + s2 = 70 which ends in zero which means that 49927398716
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! passes the Luhn test
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! The task is to write a function/method/procedure/subroutine
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! that will validate a number with the Luhn test, and use it to
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! validate the following numbers:
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! 49927398716
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! 49927398717
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! 1234567812345678
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! 1234567812345670
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: reversed-digits ( n -- list )
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{ } swap
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[ dup 0 > ]
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[ 10 /mod swapd suffix swap ]
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while drop ;
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: luhn-digit ( n -- n )
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reversed-digits dup length iota [
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2dup swap nth
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swap odd? [ 2 * 10 /mod + ] when
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] map sum 10 mod
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nip ;
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: luhn? ( n -- ? )
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luhn-digit 0 = ;
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