/*
countTrailingZeros:
-------------------
Returns the number of trailing zeros in a 64‑bit integer (BigInt).
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(x) {
const lowest = x & -x; // isolate lowest set bit
return Math.log2(Number(lowest)); // index (0–63)
}
/*
findNthSetBit64:
----------------
Given:
- x : a 64-bit integer (BigInt)
- n : which set bit to find (1-based index)
Returns:
A 64-bit mask (BigInt) with ONLY the Nth set bit of x turned on.
If n is larger than the number of set bits, returns 0n.
Algorithm:
- Use countTrailingZeros(x) to locate the lowest set bit.
- Remove that bit using x &= (x - 1n).
- When we reach the Nth one, return 1n << index.
*/
function findNthSetBit64(x, n) {
while (x !== 0n) {
const index = countTrailingZeros(x);
n -= 1;
if (n === 0) {
return 1n << BigInt(index);
}
// Remove the lowest set bit
x &= (x - 1n);
}
return 0n;
}
/*
toBinary64:
-----------
Convert BigInt to a padded 64-bit binary string.
*/
function toBinary64(x) {
const s = x.toString(2);
return s.padStart(64, "0");
}
// --------------------
// Main
// --------------------
const value =
0b0000000000000000000010000000000000001101001101101100100010100000n;
const n = 4;
const result = findNthSetBit64(value, n);
console.log("Input value: ", toBinary64(value));
console.log("N =", n);
console.log("Result mask: ", toBinary64(result));
/*
run:
Input value: 0000000000000000000010000000000000001101001101101100100010100000
N = 4
Result mask: 0000000000000000000000000000000000000000000000000100000000000000
*/
