/*
countTrailingZeros:
-------------------
Returns the number of trailing zeros in a 64‑bit integer.
Equivalent to C++ std::countr_zero or GCC __builtin_ctzll.
Algorithm:
- Isolate lowest set bit: x & -x
- Its index is log2(lowest_bit)
*/
function countTrailingZeros(int $x): int {
$lowest = $x & (-$x); // isolate lowest set bit
return (int) log($lowest, 2); // index (0–63)
}
/*
findNthSetBit64:
----------------
Given:
- x : a 64-bit integer
- n : which set bit to find (1-based index)
Returns:
A 64-bit mask with ONLY the Nth set bit of x turned on.
If n is larger than the number of set bits, returns 0.
Algorithm:
- Use countTrailingZeros(x) to locate the lowest set bit.
- Remove that bit using x &= (x - 1).
- When we reach the Nth one, return 1 << index.
*/
function findNthSetBit64(int $x, int $n): int {
while ($x !== 0) {
$index = countTrailingZeros($x);
$n--;
if ($n === 0) {
return 1 << $index;
}
// Remove the lowest set bit
$x &= ($x - 1);
}
return 0;
}
/*
toBinary64:
-----------
Convert integer to a padded 64-bit binary string.
*/
function toBinary64(int $x): string {
$s = decbin($x);
return str_pad($s, 64, '0', STR_PAD_LEFT);
}
// --------------------
// Main
// --------------------
$value =
0b0000000000000000000010000000000000001101001101101100100010100000;
$n = 4;
$result = findNthSetBit64($value, $n);
echo "Input value: " . toBinary64($value) . PHP_EOL;
echo "N = $n" . PHP_EOL;
echo "Result mask: " . toBinary64($result) . PHP_EOL;
/*
run:
Input value: 0000000000000000000010000000000000001101001101101100100010100000
N = 4
Result mask: 0000000000000000000000000000000000000000000000000100000000000000
*/
