(Warning: Extreme nerdiness follows. You are not expected to understand this if you are a family member whose last name is not "Sambol". Feel free to move along.)

So instead of doing anything useful, I spent all of my free time today designing and implementing and measuring and optimizing and refining an algorithm for determining the prime factors of an integer. Here's the final result (written in Ruby):

def factor(n)
    def primes_up_to (n)
        r = (2..n).to_a
        primes = []
        while r.first**2 < n
            primes.push(r.first)
            r.reject! { |i| i % primes.last == 0 }
        end
        primes + r
    end
    factors = {}
    for p in primes_up_to((n**0.5).ceil)
        while n % p == 0
            factors[p] ? factors[p] += 1 : factors[p] = 1
            n /= p
        end
    end
    factors[n] = 1 if n != 1
    factors
end

I got the urge to do this after incidentally running across a largish number in the middle of a random sysadmin task, and idly wondering whether or not it was a power of 2. The actual factorization algorithm was fairly easy to come up with and not all that interesting: just test whether every eligible prime number divides the integer evenly and, if so, count how many times it does so. The interesting part is generating the list of prime numbers (which is the job of the "primes_up_to" subroutine).

I vaguely remembered that some ancient Greek guy had come up with a simple method for finding all prime numbers smaller than a given number, and I preferred to look this up rather than reinvent that particular wheel. Some googling revealed that the "sieve of Eratosthenes" was what I had in mind. It goes a little like this: start with the list of all the integers from 2 to whatever; note that the first member of this list is prime and remove it and all its multiples from the list; keep repeating that last step until the list is empty. The code for doing exactly that is actually a bit different from the function mentioned above:

def primes_up_to (n)
    r = (2..n).to_a
    primes = []
    while not r.empty?
        primes.push(r.first)
        r.reject! { |i| i % primes.last == 0 }
    end
    primes
end

The final version differs from this one primarily in the test for terminating the iteration. By tracing out the way that the list changed during each iteration of the sieve, I discovered that this initial implementation did quite a bit of unnecessary filtering. After a certain point relatively early on, there is nothing but prime numbers left in the list, so there's no point in wasting energy trying to weed out their multiples. A serendipitous consultation of my senior-level algorithms textbook unearthed the (unproved) assertion that, "Trial division by all integers up to B is guaranteed to factor completely any number up to B²." After ruminating on why this must be a true theorem [1], it quickly became obvious that a corollary is that an array of the integers from 2 to B² will contain only primes if you eliminate multiples of the primes from 2 to B [2]. This meant that I could terminate the while loop in primes_up_to as soon as the first member in the list is larger than the square root of the largest integer in the list. At that point, all the numbers left in the list can be dumped into the list of primes that's been built up; no further testing needed.

My curiosity led me to another succinct and elegant implementation of Eratosthenes' sieve:

def primes_up_to (n)
    (2..(n**0.5).ceil).inject((2..n).to_a) { |r, i|
        r.select { |j| j == i || j % i != 0 }
    }
end

I'm charmed by the functional style exhibited here. I've yet to see any code that uses Ruby's inject properly that doesn't exude a fanciful cleverness. Unfortunately, measurements clearly showed that the running time for this version is much worse than that of my imperative version. Intractably so, as far as I can tell. It wastes time performing a filtering run for each of the integers up to square root of the maximum integer, regardless of whether they're actually prime.

Perhaps I'll more formally analyze the running time of these two implementations of Eratosthenes' sieve, but that will have to wait for another day.

[1] If B² has a prime factor greater than B (let's call it A), then A can only form B² by multiplying with a number smaller than B (let's call it C), else A*C would be too large. Since C is less than B, it's already been tested as a factor, which simultaneously reveals A as a factor (B²/C = A).

[2] If the list contained a composite number at this point (let's call it X), then that composite number would have only factors larger than B. Since the factors are larger than B, then the result of multiplying the factors (i.e. X) would have to be larger than B². But the list doesn't contain numbers larger than B², by definition, so X can't be in the list.