checks to see if a number is a twin prime

Is Twin Prime.swift

// a twin prime is where a prime number +/- 2 is another prime number. For example, 3 is prime, and so is 5. They are twin primes. (Since 7 is 2 away from 5 as well, maybe that makes them triplets! But I'm not a mathematician. I'll let them decide.)

extension FixedWidthInteger {

	func isPrime() -> Bool {
		// negatives, 0, and 1 are all not prime and will break the range created below
		guard self >= 2 else { return false }
		// no even numbers are prime, except 2
		guard !self.isMultiple(of: 2) else {
			return self == 2 ? true : false
		}

		// ANY value above n / 2 is NEVER going to divide into n evenly. +1 is simply for the edge case of 3 (3/2 = 1 (Int, btw) and then trying to do a range of 2..<1 crashes)
		let max = self / 2 + 1

		// only need to compare against odd numbers - even numbers were eliminated in the second guard
		for divisor in 2..<max where !divisor.isMultiple(of: 2) {
			// if a number is divisible by anything other and itself and 1, it's not prime
			if self.isMultiple(of: divisor) {
				return false
			}
		}
		// exhausted all possibilities that could make self not prime
		return true
	}

	func isTwinPrime() -> Bool {
		// if self isn't prime, it definitely isnt a twin prime
		guard self.isPrime() else { return false }
		// but if it is, check to see if self - 2 is
		if (self - 2).isPrime() {
			return true
		} else if (self + 2).isPrime() { // maybe self + 2 is?!
			return true
		} else { // oh well, we tried
			return false
		}
	}
}

0.isPrime()
1.isPrime()
2.isPrime()
3.isPrime()
4.isPrime()
5.isPrime()


for testNum in 0...10000 {
	if testNum.isTwinPrime() {
		print(testNum)
	}
}