Sunday, July 23, 2023

An EGOI week

EGOI 2023 in Lund, Sweden was the main event of the week (problems, results, top 5 on the left, discussions). Congratulations to all participants and especially to the medalists!

This was the third overall and the second onsite edition of the European Girls' Olympiad in Informatics, and I'm very happy to support this great event. I was onsite as a problem author, but my problem did not end up being used for the competition, therefore my contribution this time was not very significant. Still, being there and feeling the awesome onsite programming contest atmosphere again finally gave me enough motivation to restart this blog :) I was also very impressed with Lund, even though we mostly explored its playgrounds.

Of the problems that did make it to the problemset, I'd like to highlight problem C from the first day: you are given a HxW grid (H,W <= 50), where some unknown cell of the grid contains an obstacle: a box that cannot be moved. You can send a robot onto the grid in order to determine the location of the box. In one visit, you give the robot a program consisting of characters <, >, ^ and v. The robot starts the visit at the cell (0,0), and then executes your program (> corresponds to moving one cell right, and so on). If a certain move is not possible, either because it would cause the robot to move outside the grid, or because the robot would end up in the cell with the obstacle, the corresponding command is ignored and the execution of the rest of the program continues. You do not observe the robot's movements; instead, you only receive the final coordinates of the robot after executing all commands from the program given for the visit. In the next visit, the robot will start from the cell (0,0) again. To get the full score in this problem, you have to determine the location of the box in just 2 visits. Can you see how to do it? Bonus points (not really) if the total number of commands used throughout all visits is H*W+O(H+W).

Codeforces Round 887 wrapped up the week (problems, results, top 5 on the left, analysis). Even rainboy was not able to solve the last problem, but the remaining five provided enough of a challenge even for the top participants. Rebelz was a bit faster but made one wrong submission on E, which proved decisive as it dropped them two points behind jiangly. Congratulations to both on the great performance!

As this blog has been dormant for quite some time, it is a good time to ask: what do you think about the format? Do you have suggestions on what I should remove, add, or do more often? Of course, I don't promise to follow the suggestions, but I will consider :)

Thanks for reading, and check back next week!