/*
countTrailingZeros:
-------------------
Returns the number of trailing zeros in a 32‑bit integer.
Equivalent to C++ std::countr_zero or GCC __builtin_ctz.
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(lowest); // index of that bit
}
/*
findNthSetBit32:
----------------
Given:
- x : a 32-bit integer
- n : which set bit to find (1-based index)
Returns:
A 32-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 findNthSetBit32(x, n) {
while (x !== 0) {
// Index (0–31) of the lowest set bit
const index = countTrailingZeros(x);
// If this is the Nth set bit, return mask
if (--n === 0)
return 1 << index;
// Remove the lowest set bit
x &= (x - 1);
}
// Fewer than n set bits
return 0;
}
/*
toBinary32:
-----------
Convert integer to a padded 32-bit binary string.
*/
function toBinary32(x) {
const s = (x >>> 0).toString(2); // >>> 0 forces unsigned 32-bit
return s.padStart(32, "0");
}
// --------------------
// Main
// --------------------
const value = 0b00001101001101101100100010100000;
const n = 4;
const result = findNthSetBit32(value, n);
console.log("Input value: ", toBinary32(value));
console.log("N =", n);
console.log("Result mask: ", toBinary32(result));
/*
run:
Input value: 00001101001101101100100010100000
N = 4
Result mask: 00000000000000000100000000000000
*/
