luhn number generation problem

[ris-leigh]

A niggling problem for any PHP meisters out there...

I am using an algorithm lifted from...
http://www.webreference.com/program.../chap7/3/4.html
... to create luhn numbers, however it does not always generate a valid number when I test it using the same algorithm.
I have been looking at my test code on and off for the past two days and can not see what my fundamental mistake is!

I was hoping some one can point out my mistake.

If the following code is run repeatedly some times it will generate a valid number, other times it won't! Most peculiar.

$luhnseed = time();
$sum=0;$digit=0;$first=0;$second=0;
$luhnnumber = false;
while (!$luhnnumber){
for($c=0;$c<strlen($luhnseed);$c++){
$digit=substr($luhnseed,$c,1);
if($c%2==1){
$digit=2;
if($digit>9){
$first=substr($digit,0,1);
$second=substr($digit,1,1);
$digit=$first+$second;
}
$sum+=$digit;
}
}
if($sum%10!=0){
$luhnseed++;
}
else{
$luhnnumber=true;
}
}
echo "luhn number ($luhnseed) generated<br />sum = $sum<br />";
// test generated
$sum=0;$digit=0;$first=0;$second=0;
for($c=0;$c<strlen($luhnseed);$c++){
$digit=substr($luhnseed,$c,1);
if($c%2==1){
$digit=2;
if($digit>9){
$first=substr($digit,0,1);
$second=substr($digit,1,1);
$digit=$first+$second;
}
$sum+=$digit;
}
}
if($sum%10!=0){
echo "test number $luhnseed is not luhn<br />sum = $sum<br />";
}
else{
echo "test number $luhnseed is luhn !!!!!!!!!!!!!!!<br />sum = $sum<br />";
}

--
Regards

Leigh

[HalfaBee]

Your validation code is incorrect.

if($c%2==1) // checks the second number not the first.
This is a bit of a problem for different length CC numbers.

For standard 16 digit numbers the first character has to be doubled.

Halfabee

[ris-leigh]

I have changed the code to $c%2==0 but what I am after is still not happening!

I am not validating (or creating !) credit card numbers.
Using time() as my starting point I am currently getting numbers that are 10 digits long.

What does intrigue me is that if the script is run repeatedly mostly the generated number does not pass the validation test that uses the same algorithm that was used to generate it! Some thing like every tenth attempt a number is generated that does pass the second test!

I can not figure out where this unpredictability is coming from!

To be honest with this particular application I am not concerned whether the numbers are strictly Luhn, so long as they predictably satisfy a test using the same algortihm that was used to generate them !!

These are to be used as part of a licencing number system so I need to be certain that the numbers emailed out will validate correctly by the scripts using them.

[ris-leigh]

SORTED !!!

One of my problems was in having the variable resetting string prior to the generation loop rather than at the beginning of each cycle within the loop. Doh!

While I was looking at this problem I came across a number of PHP Luhn number validation scripts which all seemed to share a similar mistake. Curious!

All of them only added the doubled (and added if appropriate) digits to the sum for the alternating "odd" digits, rather than then adding the "even" digits.

I am assuming the following definition is correct...
(http://www.webopedia.com/TERM/L/Luhn_formula.html)

1) Starting with the second to last digit and moving left, double the value of all the alternating digits.
2) Starting from the left, take all the unaffected digits and add them to the results of all the individual digits from step 1. If the results from any of the numbers from step 1 are double digits, make sure to add the two numbers first (i.e. 18 would yield 1+8).
3) The total from step 2 must end in zero for the credit-card number to be valid.

For referrence the final working script for my purposes is as follows...

$luhnseed = time();
if (strlen($luhnseed)%2 == 1){
$luhnseed = $luhnseed10;
}
echo "luhnseed = $luhnseed<br />";
$luhnnumber = 0;
while ($luhnnumber == 0){
$sum=0;$digit=0;$first=0;$second=0;$x=0;
for ($x=0; $x<strlen($luhnseed); $x++) {
$digit = substr($luhnseed,$x,1);
if ($x%2 == 0){
$digit = 2;
if (strlen($digit) == 2){
$digit = substr($digit,0,1) + substr($digit,1,1);
}
}
$sum += $digit;
}
if($sum%10!=0){
$luhnseed = $luhnseed+1;
}
else{
$luhnnumber = 1;
}
}
echo "luhn number ($luhnseed) generated<br />sum = $sum<br />";

// test generated
$sum=0;$digit=0;$first=0;$second=0;$x=0;
for ($x=0; $x<strlen($luhnseed); $x++) {
$digit = substr($luhnseed,$x,1);
if ($x%2 == 0){
$digit *= 2;
if (strlen($digit) == 2){
$digit = substr($digit,0,1) + substr($digit,1,1);
}
}
$sum += $digit;
}
if($sum%10!=0){
echo "test number $luhnseed is not luhn<br />sum = $sum<br />";
}
else{
echo "test number $luhnseed is luhn !!!!!!!!!!!!!!!<br />sum = $sum<br />";
}