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# # ## ### ##### ######## ############# ######################
## Luhn test of numbers
## From Rosetta Code
## http://rosettacode.org/wiki/Luhn_test#Tcl
## Author Donal K. Fellows
## See also
## http://en.wikipedia.org/wiki/Luhn_algorithm
## ISO/IEC 7812-1
## http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=39698
## US Patent 2,950,048 (Aug 23, 1960): expired.
## http://www.google.com/patents?q=2950048
## Public Domain.
#
# The Luhn test is used by some credit card companies to distinguish
# valid credit card numbers from what could be a random selection of
# digits.
#
# Those companies using credit card numbers that can be validated by
# the Luhn test have numbers that pass the following test:
##
# 1. Reverse the order of the digits in the number.
#
# 2. Take the first, third, ... and every other odd digit in the
# reversed digits and sum them to form the partial sum s1
#
# 3. Taking the second, fourth ... and every other even digit in the
# reversed digits:
#
# a. Multiply each digit by two and sum the digits if the answer
# is greater than nine to form partial sums for the even digits
# b. Sum the partial sums of the even digits to form s2
#
# Note that the steps above induce a simple permutation on digits
# 0-9 which can be handled through a lookup table instead of doing
# the doubling and summing explicitly.
#
# 4. If s1 + s2 ends in zero then the original number is in the form
# of a valid credit card number as verified by the Luhn test.
# 3.a/3.b lookup table
# i|0 1 2 3 4 5 6 7 8 9
# *2|0 2 4 6 8 10 12 14 16 18
# sum|0 2 4 6 8 1 3 5 7 9 (butterfly)
# # ## ### ##### ######## ############# ######################
# The code below implements the interface of a snit validation type,
# making it directly usable with snit's -type option in option
# specifications.
# # ## ### ##### ######## ############# ######################
## Requisites
package require Tcl 8.5 9
package require snit
package require valtype::common
# # ## ### ##### ######## ############# ######################
## Implementation
namespace eval ::valtype::luhn {
namespace import ::valtype::common::*
}
snit::type ::valtype::luhn {
#-------------------------------------------------------------------
# Type Methods
typemethod validate {value {code LUHN}} {
if {[regexp {[^0-9]} $value]} {
badchar $code "$code number, expected only digits"
}
# Luhn test. 8.5 required for lreverse.
set sum [Sum $value 1]
if {($sum % 10) != 0} {
badcheck $code "$code number"
}
return $value
}
typemethod checkdigit {value {code LUHN}} {
if {[regexp {[^0-9]} $value]} {
badchar LUHN "$code number, expected only digits"
}
set c [expr {10 - ([Sum $value 0] % 10)}]
if {$c == 10} { set c 0 }
return $c
}
proc Sum {value flip} {
# 8.5 required for lreverse.
# Check digit computation starts with flip == 0!
#
# In the validation (see above) the check-digit is the last
# digit, and flip initialized to 1. The next-to-last digit is
# our last here and processed with the bit flipped. Hence our
# different, pre-flipped, starting point.
set sum 0
foreach ch [lreverse [split $value {}]] {
incr sum [lindex {
{0 1 2 3 4 5 6 7 8 9}
{0 2 4 6 8 1 3 5 7 9}
} [expr {[incr flip] & 1}] $ch]
}
return $sum
}
#-------------------------------------------------------------------
# Constructor
# None needed; no options
#-------------------------------------------------------------------
# Public Methods
method validate {value} {
$type validate $value
}
}
# # ## ### ##### ######## ############# ######################
## Ready
package provide valtype::luhn 1.1
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