Recently, I've seen a lot of blogs talking about the issues of cheaters. So, I make up a problem about using automatic system to catch them.

Currently, the most well-known automatic system for assisting of detecting plagiarism is MOSS(from Stanford). At first, I asked myself, why didn't Codeforces use them? However, I look at the number of participants of each contests; it turns out that the count is approximately under 30000. So, we have to compare $$$4.5*10^8$$$ pairs of source code!

Assuming that the system can check $$$10^4$$$ pairs per second, we will need $$$45000$$$ seconds, which is just more than half a day, the same length as hacking procedure of Educational Rounds. But I believe that limit is much lower (I haven't used it).

Is there any assistance like that could run that fast, if not MOSS? Is there any solutions that can drop the complexity of $$$O(n^2)$$$?