프로젝트 오일러 47번 풀이

E047.swift

//
//  main.swift
//  Euler-47
//
//  Created by sooop on 2017. 8. 25..
//  Copyright © 2017년 sooop. All rights reserved.
//

import Foundation

func factors(_ m:Int) -> Set<Int> {
    
    
    var a = 1
    var p = Set<Int>()
    var n = m

    func divisor(over a: Int) -> Int {
        switch a {
        case (1...2):
            if n % (a + 1) == 0 { return a + 1}
            return divisor(over: a + 1)
        case 3:
            if n % 5 == 0 { return 5 }
            return divisor(over: 5)
        default:
            var k = a
            var i = 0
            let ds = (k % 3 == 1) ? [4, 2] : [2, 4]
            let l = Int(sqrt(Double(n)) + 0.5)
            while k <= l {
                if n % k == 0 { return k }
                k += ds[i]
                i = (i + 1) % 2
            }
            return n
        }
    }
    while n > 1 {
        a = divisor(over: a)
        p.insert(a)
        while n % a == 0 {
            n /= a
        }
    }
    return p
}

func solve(){
    var s = 2*3*5*7 + 1
    var f0: Set<Int> = [2,3,5,7]
    var c = 0
    while true {
        let f1 = factors(s)
        if f1.count == 4 && f1.intersection(f0).isEmpty {
            c += 1
            if c > 3 {
                print(s - 3)
                return
            }
        } else {
            c = 0
        }
        f0 = f1
        s += 1
    }
}


func timeit(_ f: () -> ()) {
    let s = Date()
    f()
    print(s.timeIntervalSinceNow  * -1000)
}
timeit(solve)