This is a story of 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 was participating in the qualification round. However, when I opened the last problem, I immediately remembered that the exact same problem was submitted to AtCoder before. I also knew that the submitter of that problem was included in the organizers of X.

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

Y did remember he submitted the problem to me and he still used it in another contest. Why? Because I had gaviven 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 contest for me. Actually, an online version of X happened about a month after this chat. (To be more precise, there was a 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 moment, I completely forgot the 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 my thought and 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 for this accident, 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 is X and Y, 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 you should be careful about when submitting your problems to other contests.

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