advent-of-code/2024/15/main-2.cpp

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

set<pair<int, int>> boxes;
set<pair<int, int>> walls;

inline void incpair(pair<int, int> &a, pair<int, int> b) {
	a.first += b.first;
	a.second += b.second;
}

inline pair<int, int> addpair(pair<int, int> a, pair<int, int> b) {
	incpair(a, b);
	return a;
}

bool rec(pair<int, int> pos, pair<int, int> dir, set<pair<int, int>> &acc) {
	if (acc.contains(pos)) {
		return true;
	}

	if (walls.contains(pos)) {
		return false;
	}

	if (!boxes.contains(pos)) {
		incpair(pos, {0, -1});
		if (!boxes.contains(pos)) {
			return true;
		}
	}
	acc.insert(pos);
	incpair(pos, dir);

	bool other;
	switch (dir.second) {
	case 0:
		other = rec(addpair(pos, {0, 1}), dir, acc);
		break;
	case 1:
		pos.second += 1;
	default:
		other = true;
	}

	return other && rec(pos, dir, acc);
}

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

	vector<string> grid;
	string line;
	pair<int, int> robot;
	double x = 0;
	while (getline(cin, line) && !line.empty()) {
		double y = 0;
		for (char c : line) {
			switch (c) {
			case '#':
				walls.insert({x, y});
				walls.insert({x, y + 1});
				break;
			case 'O':
				boxes.insert({x, y});
				break;
			case '@':
				robot = {x, y};
				break;
			}
			y += 2;
		}
		x++;
	}

	while (getline(cin, line) && !line.empty()) {
		for (char c : line) {
			pair<int, int> dir;
			switch (c) {
			case '<':
				dir = {0, -1};
				break;
			case '>':
				dir = {0, 1};
				break;
			case '^':
				dir = {-1, 0};
				break;
			case 'v':
				dir = {1, 0};
				break;
			}

			set<pair<int, int>> r;
			auto first = addpair(robot, dir);
			if (!rec(first, dir, r)) {
				continue;
			}

			robot = first;
			for (auto box : r) {
				boxes.erase(box);
			}
			for (auto box : r) {
				boxes.insert(addpair(box, dir));
			}
		}
	}

	int sum = 0;
	for (auto box : boxes) {
		sum += 100 * box.first + box.second;
	}

	cout << sum << '\n';
}