advent-of-code/2024/6/main-2.cpp

#include <bits/stdc++.h>
using namespace std;

int main() {
	ios::sync_with_stdio(0);
	cin.tie(0);

	string line;
	vector<string> lines;
	while (getline(cin, line) && !line.empty()) {
		lines.push_back(line);
	}

	pair<int, int> pos_;
	for (int x = 0; x < lines.size(); x++) {
		for (int y = 0; y < lines[x].size(); y++) {
			if (lines[x][y] == '^') {
				pos_ = {x, y};
				goto posend;
			}
		}
	}
posend:;

	int count = 0;
	for (int x = 0; x < lines.size(); x++) {
		for (int y = 0; y < lines[0].size(); y++) {
			pair<int, int> dir = {-1, 0};
			auto pos = pos_;
			if (lines[x][y] != '.')
				continue;
			lines[x][y] = '#';

			map<pair<int, int>, set<int>> track;
			while (pos.first >= 0 && pos.first < lines.size() &&
				   pos.second >= 0 && pos.second < lines[0].size()) {
				if (lines[pos.first][pos.second] == '#') {
					pos.first -= dir.first;
					pos.second -= dir.second;
					int d = dir.first * 2 + dir.second;
					if (track[pos].contains(d)) {
						count++;
						goto next;
					}
					track[pos].insert(d);
					switch (d) {
					case -2:
						dir = {0, 1};
						break;
					case -1:
						dir = {-1, 0};
						break;
					case 1:
						dir = {1, 0};
						break;
					case 2:
						dir = {0, -1};
						break;
					}
				}
				pos.first += dir.first;
				pos.second += dir.second;
			}
		next:;
			lines[x][y] = '.';
		}
	}

	cout << count << '\n';
}