Sieve of Eratosthenes to get primes below a specific positive integer.

Sieve.swift

/// All the primes number below & including 10e7 using Sieve algorithm O(n log log n).
/// https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
func sieve() -> [Int] {
    let limit = 10000000 // 10e7
    
    // Create an array assumping all numbers are primes
    var isPrime = Array(repeating: true, count: limit)
    
    // Mark trivial cases as not primes
    isPrime[0] = false
    isPrime[1] = false
    
    // Looping till the square root of the limit is enough (explaintion is provided in the link above)
    let root = Int(sqrt(Double(limit)))
    for current in 2...root where isPrime[current] {
        // Starting from the square and going up marking them as non-primes (explaing why square is in the link)
        var multiple = current * current
        while multiple < limit {
            isPrime[multiple] = false
            multiple += current
        }
    }
    
    // loop over isPrime array to filter prime numbers
    var primes = [Int]()
    for (index, isPrime) in isPrime.enumerated() where isPrime {
        primes.append(index)
    }
    return primes
}