import Foundation
/*
findNthSetBit32:
----------------
Given:
- x : a 32-bit unsigned integer (UInt32)
- 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 x.trailingZeroBitCount to locate the lowest set bit.
- Remove that bit using x &= (x &- 1).
- When we reach the Nth one, return 1 << index.
Notes:
- trailingZeroBitCount maps to a single CPU instruction.
- Swift's UInt32 is a true 32-bit unsigned integer.
*/
func findNthSetBit32(_ x0: UInt32, _ n0: UInt32) -> UInt32 {
var x = x0
var n = n0
while x != 0 {
// Index (0–31) of the lowest set bit
let index = x.trailingZeroBitCount
n -= 1
if n == 0 {
return 1 << UInt32(index)
}
// Remove the lowest set bit
x &= (x &- 1)
}
// Fewer than n set bits
return 0
}
/*
toBinary32:
-----------
Convert UInt32 to a padded 32-bit binary string.
*/
func toBinary32(_ x: UInt32) -> String {
let s = String(x, radix: 2)
return String(repeating: "0", count: 32 - s.count) + s
}
//
// --------------------
// Main
// --------------------
//
let value: UInt32 = 0b00001101001101101100100010100000
let n: UInt32 = 4
let result = findNthSetBit32(value, n)
print("Input value: \(toBinary32(value))")
print("N = \(n)")
print("Result mask: \(toBinary32(result))")
/*
run:
Input value: 00001101001101101100100010100000
N = 4
Result mask: 00000000000000000100000000000000
*/
