@swift-5.1.5
func primes(upTo number: Int) -> [Int] { precondition(number >= 0) if number < 2 { return [] } var sieve: [Bool] = .init(repeating: false, count: number + 1) for m in stride(from: 3, through: Int(Double(number).squareRoot() + 1.5), by: 2) { if sieve[m] { continue } let maxK = number / m if maxK < 2 { continue } for k in 2 ... maxK { sieve[k * m] = true } } var result: [Int] = [2] for m in stride(from: 3, through: number, by: 2) { if sieve[m] { continue } result.append(m) } return result } print(primes(upTo: 100))