advent-of-code/2024/16/main-2.cpp

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

unordered_map<icoord, unordered_map<int, pair<int, set<int>>>> m;

inline icoord posdir(int p) {
	switch (p) {
	case 0:
		return {0, 1};
	case 1:
		return {1, 0};
	case 2:
		return {0, -1};
	case 3:
		return {-1, 0};
	default:
		return {};
	}
}

void rec(icoord pos, int dir, int score, int follow) {
	if (!m.contains(pos)) {
		return;
	}

	auto &mp = m[pos];

	if (mp.contains(dir)) {
		auto &[s, f] = mp[dir];
		if (s < score) {
			return;
		}

		if (s > score) {
			f.clear();
		}
		f.insert(follow);
		if (s == score) {
			return;
		}

		s = score;
	} else {
		mp[dir] = {score, {follow}};
	}

	mp[dir].second.insert(follow);

	rec(pos + posdir(dir), dir, score + 1, dir);
	rec(pos, (dir + 1) & 3, score + 1000, follow);
	rec(pos, (dir - 1) & 3, score + 1000, follow);
}

void rec2(unordered_set<icoord> &mm, icoord pos, int dir) {
	mm.insert(pos);
	auto &[s, f] = m[pos][dir];
	for (auto follow : f) {
		if (follow == -1) {
			continue;
		}

		rec2(mm, pos + posdir((follow + 2) & 3), follow);
	}
}

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

	icoord pos, s, e;
	string line;
	while (getline(cin, line) && !line.empty()) {
		for (pos.y = 0; pos.y < line.size(); pos.y++) {
			switch (line[pos.y]) {
			case '#':
				continue;
			case 'S':
				s = pos;
				break;
			case 'E':
				e = pos;
				break;
			}
			m[pos];
		}
		pos.x++;
	}

	rec(s, 0, 0, -1);

	int minscore = -1;
	vector<int> dirs;
	for (auto &[dir, sf] : m[e]) {
		auto &[s, f] = sf;
		if (minscore == -1 || s < minscore) {
			minscore = s;
			dirs.clear();
		} else if (s > minscore) {
			continue;
		}
		dirs.push_back(dir);
	}

	unordered_set<icoord> mm;
	for (auto dir : dirs) {
		rec2(mm, e, dir);
	}

	cout << mm.size() << '\n';
}