takes any positive integer as an argument and returns the next highest and next lowest integers having the same number of 1s as the passed argument's binary representation

sameNumberOfBinaryOnes.swift

import Foundation

extension FixedWidthInteger {
	func sameNumberOfBinaryOnes() -> (Self?, Self?) {
		guard self != 0 else { return (nil,nil) }

		let myCount = binaryOneCount()

		var nextHigher: Self?
		for maybe in (self + 1)...Self.max {
			if myCount == maybe.binaryOneCount() {
				nextHigher = Self(maybe)
				break
			}
		}
		// keeping just as an example of my first iteration
//		var counter = self + 1
//		while counter.binaryOneCount() != myCount {
//			counter += 1
//		}
//		nextHigher = counter

		var nextLower: Self?
		for maybe in (0..<self).reversed() {
			if myCount == maybe.binaryOneCount() {
				nextLower = Self(maybe)
				break
			}
		}

		return (nextHigher, nextLower)
	}

	func toBinaryString() -> String {
		return String(self, radix: 2)
	}

	private func binaryOneCount() -> Int {
		let str = toBinaryString()
		let before = str.count
		let after = str.replacingOccurrences(of: "1", with: "").count
		return before - after
	}
}


12.sameNumberOfBinaryOnes()
28.sameNumberOfBinaryOnes()
3.sameNumberOfBinaryOnes()
2.sameNumberOfBinaryOnes()
1.sameNumberOfBinaryOnes()
0.sameNumberOfBinaryOnes()

edited to make generic for all int variants