Maths problem in PHP

[toastyman]

Greetings to all!

I'm currently working on a permissions system for a CMS i'm building, but have hit a slight problem... maths

Take the following list:
1
2
4
8
16
32
etc...

Yep, they're all just doubling the previous value. Easy enough!


Now what I want to do is supply a number, let's say 11. I then need to find all the numbers in that list which when added together, produce that number. So in the case of 11, this would return 1, 2 and 8 (1 + 2 + 8 = 11).

Via a bit of googling, i've got the following code:

$mask = 5;
$return = array();
while ($mask > 0) {
	for($i = 0, $n = 0; $i <= $mask; $i = 1 * pow(2, $n), $n++) {
		$end = $i;
		$return[] = $end;
	}        
	$mask = $mask - $end;
}
sort($return);

However, it doesn't work very well... it returns
[0] => 0
[1] => 0
[2] => 1
[3] => 1
[4] => 2
[5] => 4

when it should return
[0] => 1
[1] => 4

Does anyone have any ideas on how to go about solving this problem? Any ideas, attempts etc would be appreciated!

 

[toastyman]

Haha fantastic! I didn't realise it was so simple!

I adapted your code to give the array of numbers:

<?php
$mask = 11;
$permissions = array();
$perm = 1;
while ($mask > 0)
{
	if(($mask & 1) == 1)
		$permissions[] = $perm;
	$perm = $perm * 2;
	$mask >>= 1;
	
}
print_r($permissions);
?>

Thanks very much

 

[toastyman]

Ah ha, very useful information! I did wonder what the << and the >> did, and was just looking them up when I noticed there was a reply on this thread.

Binary never was my strong point, but I see it and it's mathamatical techniques are definetely worth learning for situtations like this!

Again, thanks very much