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The Luhn Mod-10 Method (ISO 2894/ANSI 4.13) involves a check digit in the
one's position.  The check digit is calculated starting at the right with
the digit immediately preceding the check digit (ten's digit) and moving
toward the left, doubling every other digit.  If a doubled digit is greater
than nine, the two digits are added to together to obtain a single-digit
result.  The sum of all the resulting digits (including those skipped) is then
taken modulo with 10, to obtain the check digit.
 
Below, is untested code.
 
/************************************************************************
*  LuhnMod10 - self-checking scheme for validating card account numbers
*  according to ISO 2894/ANSI 4.13.
*
*    Ex:  cardNumber = "795102879015546"
*         strlen(cardNumber) == 15
*         LuhnMod10(cardNumber, 14) == 6 == cardNumber[14]
*
************************************************************************/
int LuhnMod10(char* cardNumber, int size)
{
  static int table[2][10] = { {0,1,2,3,4,5,6,7,8,9}, {0,2,4,6,8,1,3,5,7,9} };
 
  for (int i=size-1, odd=0, sum=0; i>=0; i--)
    if (isdigit(cardNumber[i]))
      sum += table[(odd=1-odd)][cardNumber[i]-'0'];
  sum %= 10;
  return (sum ? 10-sum : 0);               /* return the check digit */
}

#import 
 
@interface CheckCreditCard : NSObject {
	IBOutlet NSTextField *ccNum;
	IBOutlet NSTextField *ccTrue;
 
}
- (IBAction)checkIt:(id)sender;
@end

//  CheckCreditCard.m
//  Verify Credit Card
//
//  Created by cyher on 11-3-8.
//  Copyright 2011 __MyCompanyName__. All rights reserved.
//
 
#import "CheckCreditCard.h"
 
@implementation CheckCreditCard
 
- (id)init
{
	[super init];
	NSLog(@"Init!");
	return self;
}
 
- (IBAction)checkIt:(id)sender
{
	int sumTable[10][10] = {{0,1,2,3,4,5,6,7,8,9},{0,2,4,6,8,1,3,5,7,9}};
	int sum = 0, flip = 0;
 
	NSString *string = [ccNum stringValue];
	NSLog(@"ccNum = %@", string);
	if ([string length] == 0)
		return;
	for (int i = [string length] - 1; i >= 0; i--)
		sum +=
		sumTable[flip++ & 0x1][[string characterAtIndex:i] - '0'];
	NSLog(@"sum = %d", sum);
	if (0 == (sum % 10)) {
		[ccTrue setStringValue:[NSString stringWithFormat
				       :@"This is a real Credit Card!",
				       [string length]]];
	} else {
		[ccTrue setStringValue:[NSString stringWithFormat
					:@"This is fake Credit Card!",
					[string length]]];
	}
 
	return;
}
 
@end