Efficient way to apply bitwise logic in a php array

Question

I have an array of integers, where each integer value is a 'group membership' bitmask. For example, array(1, 4, 5) is interpreted as:

The first element (1) is a member of Group 0 (i.e. 2**0)
The second element (4) is a member of Group 2 (i.e. 2**2)
The third element (5) is a member of Groups 0 and 2 (i.e. 2**0 + 2**2)

So, my question is, how can I efficiently count the number of members in each of the groups?

My starting point would be to do something like this:

$count = array();

for ($i=0; $i<64; $i++) {
  $count[$i] = 0;
  $comparator = 2**$i;
  foreach ($array as $value) {
    if ($comparator & $value) $count[$i]++;
  }
}

That seems like a long winded way to go about things. If I have 1,000 elements in the array, I am doing 64 X 1000 iterations.

Is there an easier way to go about this? Using an advanced array technique, for example, or a php extension for bitwise operations?

olegarch · Accepted Answer · 2014-08-27 12:58:40Z

1

If you do not really need to keep 64 groups, and 62 is enough for you (or 30 on 32-bit platform), following code will help you:

  $count = array();
  $data  = array(1, 4, 5, 7);

  foreach ($data as $x)
    for ($i=0, $j = 1; $j <= $x; $i++, $j <<= 1)
        if($x & $j)
            $count[$i]++;

  print "count=";
  print_r($count);

answered Aug 27, 2014 at 12:58

olegarch

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3 Comments

DatsunBing Over a year ago

thanks maxihatop, this is brilliant. One question though, why do I need to reduce the number of groups by two? I can understand reducing by one (for the sign of the integer), but why two?

DatsunBing Over a year ago

further to my comment above, I have now tested it and you are right - I can have 62 bit integers fine but as soon as I go to 63 bit, it seems to go into a continuous loop. But why?

olegarch Over a year ago

Because of max value of single-bit positive is 263. Therefore, all sum of another bits must be less than 2*63. So, highest bit is 262. Please note: my algorithm do not iterate all 62 bits. If input balue is 5, it perform only 3 iterations, and stops on 4th, when $j == 8.

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