Day 14: Restroom Redoubt
Megathread guidelines
- Keep top level comments as only solutions, if you want to say something other than a solution put it in a new post. (replies to comments can be whatever)
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FAQ
- What is this?: Here is a post with a large amount of details: https://programming.dev/post/6637268
- Where do I participate?: https://adventofcode.com/
- Is there a leaderboard for the community?: We have a programming.dev leaderboard with the info on how to join in this post: https://programming.dev/post/6631465
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C#
using System.Text.RegularExpressions;
namespace aoc24;
[ForDay(14)]
public partial class Day14 : Solver
{
[GeneratedRegex(@"^p=(-?\d+),(-?\d+) v=(-?\d+),(-?\d+)$")]
private static partial Regex LineRe();
private List<(int X, int Y, int Vx, int Vy)> robots = [];
private int width = 101, height = 103;
public void Presolve(string input) {
var data = input.Trim();
foreach (var line in data.Split("\n")) {
if (LineRe().Match(line) is not { Success: true } match ) {
throw new InvalidDataException($"parse error: ${line}");
}
robots.Add((
int.Parse(match.Groups[1].Value),
int.Parse(match.Groups[2].Value),
int.Parse(match.Groups[3].Value),
int.Parse(match.Groups[4].Value)
));
}
}
public string SolveFirst() {
Dictionary<(bool, bool), int> quadrants = [];
foreach (var robot in robots) {
int x = (robot.X + 100 * (robot.Vx > 0 ? robot.Vx : robot.Vx + width)) % width;
int y = (robot.Y + 100 * (robot.Vy > 0 ? robot.Vy : robot.Vy + height)) % height;
if (x == width/2 || y == height/2) continue;
var q = (x < width / 2, y < height / 2);
quadrants[q] = quadrants.GetValueOrDefault(q, 0) + 1;
}
return quadrants.Values.Aggregate((a, b) => a * b).ToString();
}
private int CountAdjacentRobots(HashSet<(int, int)> all_robots, (int, int) this_robot) {
var (x, y) = this_robot;
int count = 0;
for (int ax = x - 1; all_robots.Contains((ax, y)); ax--) count++;
for (int ax = x + 1; all_robots.Contains((ax, y)); ax++) count++;
for (int ay = y - 1; all_robots.Contains((x, ay)); ay--) count++;
for (int ay = y + 1; all_robots.Contains((x, ay)); ay++) count++;
return count;
}
public string SolveSecond() {
for (int i = 0; i < int.MaxValue; ++i) {
HashSet<(int, int)> end_positions = [];
foreach (var robot in robots) {
int x = (robot.X + i * (robot.Vx > 0 ? robot.Vx : robot.Vx + width)) % width;
int y = (robot.Y + i * (robot.Vy > 0 ? robot.Vy : robot.Vy + height)) % height;
end_positions.Add((x, y));
}
if (end_positions.Select(r => CountAdjacentRobots(end_positions, r)).Max() > 10) {
return i.ToString();
}
}
throw new ArgumentException();
}
}
