오일러 프로젝트 27번 풀이

E027.swift

import Foundation

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

func isPrime(_ n: Int) -> Bool {
	if n < 2 { return false }
	if (2...3) ~= n { return true }
	if n % 2 == 0 || n % 3 == 0 { return false }
	if n < 10 { return true }
	var k = 5
	let l = Int(sqrt(Double(n)) + 0.5)
	while k <= l {
		if n % k == 0 || n % (k + 2) == 0 { return false }
		k += 6
	}
	return true
}


func test(_ a: Int, _ b: Int) -> Int {
	let eq: (Int) -> Bool = { n -> Bool in 
		let k = n * n + a * n + b 
		return isPrime(k)
	}
	var n = 0

	while true {
		if eq(n) {
			n += 1
		} else {
			return n
		}
	}
}

func solve() {
	var m = 0
	var result = 0
	for a in stride(from: -1, through:-500, by: -1) {
		for b in ((-2*a)...1000) {
			let n = test(a, b)
			if n > m { 
				m = n 
				result = a * b
			}
		}
	}
	print(result)
}

timeit(solve)