Sieve of Erastosthene

Prime.swift

func sieveOfErastosthene(max: Int) -> [Bool] {

    // we assume every number is prime
    var flags = Array(count: max + 1, repeatedValue: true)
    flags[0] = false
    flags[1] = false

    let squareRoot = Int(sqrt(Double(max)))
    for i in 2...squareRoot {
        if flags[i] {
            crossOfMultiples(max, prime: i, flags: &flags)
        }
    }
    return flags
}

func crossOfMultiples(max: Int, prime: Int, inout flags: [Bool]) {

    for i in (prime * prime).stride(to: max, by: prime) {
        flags[i] = false
    }

}