Hello everybody. So I was really bored tonight and looked at so many comments being like "sorry_dreamoon is X" and then people arguing and disagreeing. Since I had nothing better to do, I decided to finally unravel the truth of who he actually is. Here is the full case and how I actually got to him, hope you enjoy! :D

Selection

Let's start by the obvious. He managed to get first place in Div2 and he got second in Div1 (would've been first if the scores were static). So what we know is that he is really good.

Let's look at the past 10 Div1 contests. It turns out that only 8 times it happened that someone with a rating of less than 2500 got in top5. This is 8 times out of 50 people being in top5. But he was second and almost first, so he is better than that.

It turns out actually, that in the past 10 Div1 contests, only 2 times it happened that someone with rating less than 2500 got in top3.

Combining this information with sorry_dreamoon's confidence and great results — we can assume that his rating is 2500+. That leaves us with 76 suspects.

Reducing the suspects

As pointed out a few times, the obvious way is to remove all people that participated in CF rounds #284 and #292, as those are the rounds sorry_dreamoon participated in. Obviously it's hard to perform well on two accounts, so we can be sure those people are not him. Removing those from our suspect list we are left with 36 suspects.

Reducing the suspects even further

As you can notice from the blog post of round #292 dreamoon_love_AA invited sorry_dreamoon to participate 26hrs ago (as of when I was investigating) and the respond (top comment) was almost immediate. This shows that our suspect was browsing codeforces at the time. And since the account was initially made only as a one-time joke, he most likely was browsing from his real account.

So from this, we can remove all our suspects that havent logged in the last 26 hours.

Well, believe it or not, we're down to 18 suspects!

The final conclusion

Now here the things start to get a little bit tricky. But imagine the following scenario:

You create an account with the only purpose to be first in a competition. If you fail, it will be somewhat embarrasing. While you want to keep your identity private, you want to be as good as you can. Would you use your primary programming language?

For me at least, the answer is absolutely yes. If you are used to a language, then your maximum performance is with that language and changing it is incredibly risky.

While not certain as the other two methods of eliminating, this method is still logical — "there is no good investigation without some assumptions".

So, sorry_dreamoon is using Java. Guess how many people from our 18-people-long suspect list use Java? That's right, you got it — 1!

And who is it... inserting drum roll

qwerty787788

Not convinced yet?

You know, when we try to mask our code we would usually change names of variables and functions but the structure and the way your thought goes is hard to change. So, it turns out that sorry_dreamoon has a code including maximum flow — 9251954.

It turns out that qwerty787788 has maximum flow too, in the recent Rockethon — 9762794.

So, looking at the function that actually does the flow you can see the following:

qwerty787788's code :

class Flow {
	int n;
	ArrayList<Edge>[] g;

	Flow(int n) {
		this.n = n;
		g = new ArrayList[n];
		for (int i = 0; i < n; i++) {
			g[i] = new ArrayList<>();
		}
		q = new int[n];
		h = new int[n];
		cur = new int[n];
	}
etc

And this is sorry_dreamoon's code :

static class Flow {
        int n;
        ArrayList<Edge>[] g;

        Flow(int n) {
            this.n = n;
            g = new ArrayList[n];
            for (int i = 0; i < n; i++) {
                g[i] = new ArrayList<>();
            }
            q = new int[n];
            h = new int[n];
            cur = new int[n];
        }
etc

What is the difference? It's literally the word "static".

This is just a small fraction of the flow class. About 90% of it is completely identical.

Conclusion

To be honest I really liked the mysteriousness of sorry_dreamoon but I really had nothing better to do tonight and since I really wanted to live my detective-dream, I hope everybody takes my work with a smile :)

P.S.

And in case dreamoon_love_AA hadn't figured that out by himself yet, I hope it helps him sleep better at night :D

P.S.S.

I hope this doesn't bring any problems to qwerty787788 since he is a very good coder and I think we all can agree that this was a very well carried out joke (though technically my "logic" isn't a solid proof that it's him at all)