This is a story about my recent contest participation.

Let's call the contest X. X was an international contest with online qualification rounds and the onsite Finals, with trip and hotel expenses covered by the organizer. In other words, X was a very important contest for me.

Hoping to go to the Finals, I participated in the qualification round. However, when I opened the last problem, I immediately remembered that the exact same problem had previously been submitted to AtCoder. I also knew that the submitter of that problem was among the organizers of X.

Let's call the author of that problem Y. I immediately messaged Y about this issue. At first, I thought that Y had simply forgotten that he had submitted the problem to me, but the situation was a bit more complicated.

Y did remember he had submitted the problem to me but still used it in another contest. Why? Because I had given him permission!

To make things clear, let me show you the relevant chat log between me and Y.

I said X sounds OK. so it's totally my fault, case closed...? Let me defend myself.

When this conversation happened, X was just an online contest and therefore it wasn't an important one for me. Actually, an online version of X happened about a month after this chat. (To be more precise, there was an onsite contest for X but it's for locals and there were no invitations to foreign participants.)

Then, a few months later, it was announced that the next X would invite international contestants. At this point, I completely forgot that I had said X sounds OK.. Instead, I only remembered I'd appreciate it if you could avoid using the remaining problems in important contests., and I thought it was obvious that X was important to me.

As you know, Y didn't understand what I was thinking, and that's how the incident happened.

I should have said "don't use the problem for international onsite contests or its qualifications" in the first place. When chatting, I thought only UCUP could cause a "collision" so I only mentioned UCUP. This is my fault.

Y should also have told me that he's going to use the problem when the contest was announced (or the problem was accepted). He was mentioning X in the chat but the problem was used in the next-next X, which was confusing to me.

I do feel frustrated about this incident, but I also think it's partly due to my fault. That's why I accepted the request from the author not to include the contest name or the author's name. I know you can infer what exactly X and Y are, but please don't comment on them.

After all, what's this blog for? It's a rant to appease myself. It's an excuse for not advancing to the Finals. And last but not least, I want to raise awareness about what to be careful about when submitting your problems to other contests. Specifically, if your problem is used in a contest, everyone who knows the problem should be informed in advance when and where it will appear, and you should let them know as early as possible.

Or more generally, it’s better to over-communicate than to under-communicate!