Question

How to get the highest power of 2 that is less than or equal to N in PHP

Answer 1

function highest_power_of_2_less_or_equal_to_n($n) { 
    $power = 0; 
    for ($i = $n; $i >= 1; $i--) { 
        // $i == power of 2 ?
        if (($i & ($i - 1)) == 0) { // 10 (1010) 9 (1001) 8 (1000) 7 (0111)
            $power = $i; 
            break; 
        } 
    } 
    return $power; 
} 

$n = 17;
echo highest_power_of_2_less_or_equal_to_n($n) . "<br >"; 
      
$n = 10;
echo highest_power_of_2_less_or_equal_to_n($n) . "<br >"; 
      
$n = 64;
echo highest_power_of_2_less_or_equal_to_n($n) . "<br >"; 
 
 
 
/*
run:
      
16
8
64
     
*/

 

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