Find prime factors of a number

primeFactorsOf.swift

import Foundation


/// Given a positive integer, find its prime factors.
///
/// - parameter number: The number to be factored.
/// 
/// - returns: Array of factors.
///
/// - TODO: Use a less-naive algorithm. Construct the result lazily.

func primeFactorsOf(number: Int) -> [Int] {
    if number < 4 {
        return [number]
    }
    let lim = Int(sqrt(Double(number)))
    for x in 2...lim {
        if number % x == 0 {
            var result = [x]
            result.appendContentsOf(primeFactorsOf(number / x))
            return result
        }
    }
    return [number]
}

let f_2 = primeFactorsOf(2)         // [2]
let f_4 = primeFactorsOf(4)         // [2, 2]
let f_6 = primeFactorsOf(6)         // [2, 3]
let f_12 = primeFactorsOf(12)       // [2, 2, 3]
let f_100 = primeFactorsOf(100)     // [2, 2, 5, 5]
let f_113 = primeFactorsOf(113)     // [113]
let f_1967 = primeFactorsOf(1967)   // [7, 281]