Swift code to find and count all Circular Primes below 1 million (Project Euler problem 35)

main.swift

// Circular primes
// Problem 35
// The number, 197, is called a circular prime because all rotations of the digits: 197, 971, and 719, are themselves prime.
//
// There are thirteen such primes below 100: 2, 3, 5, 7, 11, 13, 17, 31, 37, 71, 73, 79, and 97.
//
// How many circular primes are there below one million?

import Foundation

struct ESieve {
    let primes: Set<Int>
    
    init(size: Int) {
        let sieve = ESieve.populateSieve(ofSize: size)
        primes = Set(ESieve.primes(fromSieve: sieve))
    }
    
    static func populateSieve(ofSize size: Int) -> [Bool] {
        var sieve = Array<Bool>(count: size, repeatedValue: true)
        let endIndex = Int(ceil(sqrt(Double(size))))
        
        for i in 0...1 {
            sieve[i] = false
        }
        
        for i in 2...endIndex {
            if sieve[i] {
                let startIndex = Int(pow(Double(i), 2.0))
                startIndex.stride(to: size, by: i).forEach {
                    sieve[$0] = false
                }
            }
        }
        
        return sieve
    }
    
    static func primes(fromSieve sieve: [Bool]) -> [Int] {
        var primes = [Int?]()
        for n in sieve.indices {
            primes.append(sieve[n] ? n : nil)
        }
        
        return primes.flatMap { $0 }
    }
    
    func isPrime(n: Int) -> Bool {
        return primes.contains(n)
    }
}

func getRotations(ofNumber number: Int) -> [Int] {
    var rotations = [Int]()
    
    let exponent = Int(floor(log10(Double(number))))
    guard exponent > 0 else {
        return rotations
    }
    
    let multiplier = Int(pow(10, Double(exponent)))
    
    var rotation = number
    for _ in 1...exponent {
        let units = rotation % 10
        let shifted = rotation / 10
        
        rotation = units * multiplier + shifted
        rotations.append(rotation)
    }
    
    return rotations
}

func circularPrimes(below size: Int) -> [Int] {
    let sieve = ESieve(size: size)
    
    var circulars = Set<Int>()
    
    for prime in sieve.primes where !circulars.contains(prime) {
        let rotations = Set(getRotations(ofNumber: prime))
        
        if rotations.count == 0 {
            circulars.insert(prime)
            continue
        }
        
        if rotations.isSubsetOf(sieve.primes) {
            circulars.unionInPlace(rotations)
            circulars.insert(prime)
        }
    }
    
    return circulars.sort()
}

let size = 1000000
let foo = circularPrimes(below: size)

print("Number of circular primes below \(size): \(foo.count)")