(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
num にセットされているビットの数を返します。
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
Another way to get the population count when you don't have the gmp extension is using bitwise operations:<?php$int = 133; // 10000101for($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 $count; // 3?>This is Kernighan's population count.https://youtu.be/ZRNO-ewsNcQ?t=510 has a nice explanation on how it worksIf you don't have gmp extension enabled (or don't want to use it for any reason), you can get popcount of an int using decbin() and substr_count().<?php$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.