Implementation of the Luhn checksum algorithm, see http://en.wikipedia.org/wiki/Luhn_algorithm
First argument should be a number or a string composed of digits only. Second argument is optional. If second argument evaluates to true, the return value is the concatenation of the first argument and its Luhn checksum. Otherwise the return value is a boolean indicating if the first arguments checksum digit is valid.
| /* | |
| * @param b Number to validate or add checksum digit to, as Number or as String | |
| * @param y optional; if true, add checksum digit to b, otherwise validate | |
| * @param t placeholder | |
| * @param e placeholder | |
| * @param s placeholder | |
| * @param u placeholder | |
| */ | |
| function(b, y, t, e, s, u) { | |
| // initialize sum s=0 and multiplicator | |
| s = 0; u = y ? 1 : 2; | |
| // ensure b is a string and iterate over digits in b from right to left | |
| for (t = ( b = b + '').length; t--;) { | |
| e = b[t] * (u^=3); // if we are at an "even" position, double the digit | |
| s += e-(e>9?9:0); // reduce to one digit and add up | |
| } | |
| t = 10 - (s % 10 || 10); // calculate the difference to modulo10 | |
| return y ? b + t : !t; // if calculating checksum, concat; if validating, 0 is a pass | |
| } |
| function(b,y,t,e,s,u){s=0;u=y?1:2;for(t=(b=b+'').length;t--;){e=b[t]*(u^=3);s+=e-(e>9?9:0)}t=10-(s%10||10);return y?b+t:!t} |
| DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE | |
| Version 2, December 2004 | |
| Copyright (C) 2011 Timo Kissing http://kissing.name | |
| Everyone is permitted to copy and distribute verbatim or modified | |
| copies of this license document, and changing it is allowed as long | |
| as the name is changed. | |
| DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE | |
| TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION | |
| 0. You just DO WHAT THE FUCK YOU WANT TO. |
| { | |
| "name": "jsLuhn", | |
| "description": "Implementation of the Luhn checksum algorithm", | |
| "keywords": [ | |
| "140bytes", | |
| "number", | |
| "luhn", | |
| "checksum" | |
| ] | |
| } |
| <!DOCTYPE html> | |
| <head> | |
| <title>140byt.es Luhn Checksum</title> | |
| </head> | |
| <body> | |
| <div>Expected value: <b>true,49927398716, true</b></div> | |
| <div>Actual value: <b id="ret"></b></div> | |
| <script> | |
| // write a small example that shows off the API for your example | |
| // and tests it in one fell swoop. | |
| var luhn = function(b,y,t,e,s,u){s=0;u=y?1:2;for(t=(b=b+'').length;t--;){e=b[t]*(u^=3);s+=e-(e>9?9:0)}t=10-(s%10||10);return y?b+t:!t}; | |
| document.getElementById( "ret" ).innerHTML = [luhn(49927398716), luhn('4992739871', true), luhn('4111111111111111')]; | |
| </script> | |
| </body> | |
| </html> |
I just golfed the validate version, which is with maths so it should be fast.
function(a,b,c,d){for(a*=b=c=10;a/=10;c+=b++%2?d*2-(d>4)*9:d)a-=d=a%10;return!(c%10)}
Maybe you can make it suit you;)
However, I found that your version has some problems.
For example, testing 4111111111111111 should be true, you version says false.
Doh, you are right @tsaniel.
Have a fix, but need to update annotated version so will just put it in the comment here for now:
function(b,y,t,e,s,u){s=0;u=y?1:2;for(t=(b=b+'').length;t--;){e=b[t]*(u^=3);s+=e-(e>9?9:0)}t=10-(s%10||10);return y?b+t:!t}
And yes, that is 1 byte shorter than the broken version
And fixed.
e-(e>9?9:0) => e-9*(e>9)
y?1:2 => 2-!y
b=b+'' => b+=''
;){X;Y} => ;Y)X;
@madrobby beat you to it: https://gist.github.com/976805