gmp_popcount

(PHP 4 >= 4.0.4, PHP 5, PHP 7, PHP 8)

gmp_popcount — Population count

说明

gmp_popcount(GMP|int|string $num): int

Get the population count.

参数

num: GMP 对象或 int ，或数字string。

返回值

The population count of num, as an int.

范例

示例 #1 gmp_popcount() example


<?php
$pop1 = gmp_init("10000101", 2); // 3 1's
echo gmp_popcount($pop1) . "\n";
$pop2 = gmp_init("11111110", 2); // 7 1's
echo gmp_popcount($pop2) . "\n";
?>

以上例程会输出：

3
7

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User Contributed Notes 2 notes

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ketrab2004 ¶

7 months ago


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

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phpmanual at headbank dot co dot uk ¶

3 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.

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