gmp_popcount
(PHP 4 >= 4.0.4, PHP 5)
gmp_popcount — Population count
Description
int gmp_popcount
( resource
$a
)Get the population count.
Parameters
-
a -
It can be either a GMP number resource, or a numeric string given that it is possible to convert the latter to a number.
Return Values
The population count of a, as an integer.
Examples
Example #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";
?>
The above example will output:
3 7
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- gmp_abs
- gmp_add
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- gmp_clrbit
- gmp_cmp
- gmp_com
- gmp_div_q
- gmp_div_qr
- gmp_div_r
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- gmp_import
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- gmp_intval
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- gmp_legendre
- gmp_mod
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- gmp_or
- gmp_perfect_square
- gmp_popcount
- gmp_pow
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- gmp_random_range
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- gmp_setbit
- gmp_sign
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- gmp_sub
- gmp_testbit
- gmp_xor
Коментарии
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.
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