advent-of-code/2024/12/main-2.cpp

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

vector<vector<tuple<char, bool>>> squares;

bool rec(int x, int y, set<pair<int, int>> &region) {
	auto &[c, g] = squares[x][y];
	if (g) {
		return false;
	}
	g = true;
	region.insert({x, y});
	vector<pair<int, int>> checks = {
		{x - 1, y}, {x, y - 1}, {x + 1, y}, {x, y + 1}};
	for (auto [xx, yy] : checks) {
		if (xx < 0 || yy < 0 || xx >= squares.size() ||
			yy >= squares[0].size()) {
			continue;
		}
		if (get<0>(squares[xx][yy]) == c) {
			rec(xx, yy, region);
		}
	}
	return true;
}

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

	string line;
	while (getline(cin, line) && !line.empty()) {
		vector<tuple<char, bool>> l;
		for (char c : line) {
			l.push_back({c, false});
		}
		squares.push_back(l);
	}

	vector<set<pair<int, int>>> regions;
	for (int x = 0; x < squares.size(); x++) {
		for (int y = 0; y < squares[0].size(); y++) {
			set<pair<int, int>> region;
			if (rec(x, y, region)) {
				regions.push_back(region);
			}
		}
	}

	int total = 0;
	for (auto &region : regions) {
		map<pair<int, int>, bool[4]> sides;
		int area = region.size();
		for (auto [x, y] : region) {
			vector<pair<int, int>> directions = {
				{x - 1, y}, {x + 1, y}, {x, y - 1}, {x, y + 1}};
			for (int i = 0; i < 4; i++) {
				sides[{x, y}][i] = !region.contains(directions[i]);
			}
		}
		int sidec = 0;
		for (auto &[p, b] : sides) {
			auto [x, y] = p;
			for (int i = 0; i < 4; i++) {
				if (!b[i]) {
					continue;
				}
				pair<int, int> other =
					(i < 2) ? make_pair(x, y - 1) : make_pair(x - 1, y);
				if (!sides.contains(other) || !sides[other][i]) {
					sidec++;
				}
			}
		}
		total += area * sidec;
	}
	cout << total << '\n';
}