Another way to get the population count when you don't have the gmp extension is using bitwise operations:
<?php
$int = 133; // 10000101
for($count = 0; $int != 0; $count++) // repeat until $int is 0 (and count the amount of steps it takes in $count)
{
$int = $int & $int-1; // remove the right most 1 from $int using the bitwise and operator
}
echo $count; // 3
?>
This is Kernighan's population count.
https://youtu.be/ZRNO-ewsNcQ?t=510 has a nice explanation on how it works
gmp_popcount
(PHP 4 >= 4.0.4, PHP 5, PHP 7, PHP 8)
gmp_popcount — セットされているビットの数
戻り値
num にセットされているビットの数を返します。
例
例1 gmp_popcount() の例
<?php
$pop1 = gmp_init("10000101", 2); // 1 が 3 つ
echo gmp_popcount($pop1) . "\n";
$pop2 = gmp_init("11111110", 2); // 1 が 7 つ
echo gmp_popcount($pop2) . "\n";
?>上の例の出力は以下となります。
3 7
add a note
User Contributed Notes 2 notes
ketrab2004 ¶
3 years ago
phpmanual at headbank dot co dot uk ¶
6 years ago
If you don't have gmp extension enabled (or don't want to use it for any reason), you can get popcount of an int using decbin() and substr_count().
<?php
$int1 = 133;
$bin1 = decbin($int1); // "10000101"
echo substr_count($bin1, "1");
// Result: 3
?>
Being a string-comparison this is far less efficient than gmp_popcount() (for which there is a dedicated instruction on most if not all modern processors), but may be handy if gmp is unavailable, or in non-performance-critical code that doesn't otherwise need it.
