Question

How to find the position of the first set bit (1-bit) of a number in PHP

Answer 1

// Function to find the position of the first set bit
function findFirstSetBitPosition(int $num): int {
    // Edge case: If the number is 0, there are no set bits
    if ($num === 0) {
        return 0; // Return 0 to indicate no set bit
    }

    /*
     * $num & -$num isolates the lowest set bit.
     * log(..., 2) gives the zero-based index of that bit.
     */
    return (int)log($num & -$num, 2);
}

$num = 4224; // Binary: 0001 0000 1000 0000

// Find and display the position of the first set bit
$position = findFirstSetBitPosition($num);

if ($position === 0) {
    echo "The number has no set bits (it's 0)." . PHP_EOL;
} else {
    echo "The position of the first set bit is: " . $position . PHP_EOL;
}



/* 
run:

The position of the first set bit is: 7

*/

 

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