From 37aabd15f1d944d51c9ddcec5b0dddeea687eeae Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Hugo=20M=C3=A5rdbrink?= <hugo@mardbrink.se>
Date: Mon, 16 Dec 2024 17:40:50 +0100
Subject: [PATCH] Day 16

---
 d16/main.cxx | 221 +++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 215 insertions(+), 6 deletions(-)

diff --git a/d16/main.cxx b/d16/main.cxx
index eee8976..1204697 100644
--- a/d16/main.cxx
+++ b/d16/main.cxx
@@ -1,15 +1,224 @@
+#include <utility>
+#include <vector>
+#include <string>
+#include <sstream>
+#include <chrono>
 #include <iostream>
-#include <fstream>
+#include <queue>
+#include <tuple>
+#include <unordered_set>
 
 using std::cout, std::endl;
 
-int main() {
-    std::ifstream file{"../d16/input.txt"};
-    std::string line;
+constexpr const char content[] = {
+    #embed "input.txt"
+};
 
-    while (std::getline(file, line)) {
-        
+enum class Direction { 
+    N,
+    S,
+    E,
+    W
+};
+
+struct Position {
+    size_t x;
+    size_t y;
+    
+    bool operator==(const Position& other) const {
+        return x == other.x && y == other.y;
+    }
+};
+
+namespace std {
+    template <>
+    struct hash<Position> {
+        std::size_t operator()(const Position& p) const {
+            return std::hash<size_t>{}(p.x) ^ std::hash<size_t>{}(p.y);
+        }
+    };
+}
+
+struct Node {
+    int cost;
+    Position pos;
+    int dir;
+    bool operator>(const Node& other) const {
+        return cost > other.cost;
+    }
+};
+
+std::optional<int> find_shortest_path(Position start, Position end, std::vector<std::string>& grid) {
+int rows = grid.size(), cols = grid[0].size();
+    std::vector<std::vector<std::vector<size_t>>> dist(
+        rows, 
+        std::vector<std::vector<size_t>>(
+            cols, 
+            std::vector<size_t>(4, std::numeric_limits<size_t>::max())
+        )
+    );
+    std::array<std::pair<size_t, size_t>, 4> dirs = {std::pair<size_t, size_t>{0,1}, {1,0}, {0,-1}, {-1,0}};
+    
+    std::priority_queue<Node, std::vector<Node>, std::greater<Node>> pq;
+    
+    for (int d = 0; d < 4; ++d) {
+        dist[start.y][start.x][d] = 0;
+        pq.push({0, {start.y, start.x}, d});
     }
     
+    while (!pq.empty()) {
+        auto [cost, pos, curr_dir] = pq.top();
+        auto [y, x] = pos;
+        pq.pop();
+        
+        if (y == end.y && x == end.x) return cost;
+        
+        if (cost > dist[y][x][curr_dir]) continue;
+        
+        for (int d = 0; d < 4; ++d) {
+            auto ny = y + dirs[d].first;
+            auto nx = x + dirs[d].second;
+            
+            if (grid[ny][nx] == '#') 
+                continue;
+            
+            auto new_cost = cost + 1;
+            if (curr_dir != d) 
+                new_cost += 1000;
+            
+            if (new_cost < dist[ny][nx][d]) {
+                dist[ny][nx][d] = new_cost;
+                pq.push({new_cost, {ny, nx}, d});
+            }
+        }
+    }
+
+    return std::nullopt; 
+}
+
+int part_1() {
+    std::istringstream input_stream(content);
+    std::string line;
+
+    Position start;
+    Position end;
+    std::vector<std::string> grid = {};
+
+    size_t line_num = 0;
+    while(std::getline(input_stream, line)) {
+        if(line.contains("S")) start = {line.find("S"), line_num};
+        if(line.contains("E")) end = {line.find("E"), line_num};
+        
+        grid.push_back(line);
+        line_num++;
+    }
+
+    auto result = find_shortest_path(start, end, grid);
+    if(result.has_value()) {
+        return result.value();
+    } else {
+        return -1;
+    }
+}
+
+struct SittingNode {
+    int cost;
+    std::unordered_set<Position> path = {};
+    Position pos;
+    int dir;
+
+    bool operator>(const SittingNode& other) const {
+        return cost > other.cost;
+    }
+};
+
+std::optional<int> find_best_sitting_place(Position start, Position end, std::vector<std::string>& grid) {
+    std::unordered_set<Position> all_short_paths = {};
+    int rows = grid.size(), cols = grid[0].size();
+    std::vector<std::vector<std::vector<size_t>>> dist(
+        rows, 
+        std::vector<std::vector<size_t>>(
+            cols, 
+            std::vector<size_t>(4, std::numeric_limits<size_t>::max())
+        )
+    );
+    std::array<std::pair<size_t, size_t>, 4> dirs = {std::pair<size_t, size_t>{0,1}, {1,0}, {0,-1}, {-1,0}};
+    
+    std::priority_queue<SittingNode, std::vector<SittingNode>, std::greater<SittingNode>> pq;
+    
+    for (int d = 0; d < 4; ++d) {
+        dist[start.y][start.x][d] = 0;
+        std::unordered_set<Position> path = {start};
+        pq.push({0, path, {start.y, start.x }, d});
+    }
+    
+    while (!pq.empty()) {
+        auto [cost, path, pos, curr_dir] = pq.top();
+        auto [y, x] = pos;
+        pq.pop();
+        
+        if (y == end.y && x == end.x) {
+            path.insert({y, x});
+            all_short_paths.merge(path);
+        }
+
+        if (cost > dist[y][x][curr_dir]) continue;
+        
+        for (int d = 0; d < 4; ++d) {
+            auto ny = y + dirs[d].first;
+            auto nx = x + dirs[d].second;
+            
+            if (grid[ny][nx] == '#') 
+                continue;
+            
+            auto new_cost = cost + 1;
+            auto new_path = path;
+            new_path.insert({ny, nx});
+            if (curr_dir != d) 
+                new_cost += 1000;
+            
+            if (new_cost <= dist[ny][nx][d]) {
+                dist[ny][nx][d] = new_cost;
+                pq.push({new_cost, new_path, {ny, nx}, d});
+            }
+        }
+    }
+    return all_short_paths.size();
+}
+
+int part_2() {
+    std::istringstream input_stream(content);
+    std::string line;
+
+    Position start;
+    Position end;
+    std::vector<std::string> grid = {};
+
+    size_t line_num = 0;
+    while(std::getline(input_stream, line)) {
+        if(line.contains("S")) start = {line.find("S"), line_num};
+        if(line.contains("E")) end = {line.find("E"), line_num};
+        
+        grid.push_back(line);
+        line_num++;
+    }
+
+    auto result = find_best_sitting_place(start, end, grid);
+    if(result.has_value()) {
+        return result.value();
+    } else {
+        return -1;
+    }
+}
+
+int main() {
+    std::chrono::steady_clock::time_point begin = std::chrono::steady_clock::now();
+    auto p1_val = part_1();
+    std::chrono::steady_clock::time_point middle = std::chrono::steady_clock::now();
+    auto p2_val = part_2();
+    std::chrono::steady_clock::time_point end = std::chrono::steady_clock::now();
+    cout << "Part 1: " << p1_val << " (" << std::chrono::duration_cast<std::chrono::microseconds>(middle - begin).count() << "µs)" << endl;  
+    cout << "Part 2: " << p2_val << " (" << std::chrono::duration_cast<std::chrono::microseconds>(end - middle).count() << "µs)" << endl;
     return 0;
 }
+