From c7881c47b9b4c15571cdd208227bdb76128bbbc4 Mon Sep 17 00:00:00 2001
From: Bert Peters <bert@bertptrs.nl>
Date: Wed, 25 Dec 2019 16:56:51 +0100
Subject: [PATCH] Implement day 20 part 2.

---
 2019/src/day20.cpp            | 57 ++++++++++++++++++++++++++++++++++-
 2019/tests/samples/20-2-2.in  | 37 +++++++++++++++++++++++
 2019/tests/samples/20-2-2.out |  1 +
 3 files changed, 94 insertions(+), 1 deletion(-)
 create mode 100644 2019/tests/samples/20-2-2.in
 create mode 100644 2019/tests/samples/20-2-2.out

diff --git a/2019/src/day20.cpp b/2019/src/day20.cpp
index d5d71b1..d8087f6 100644
--- a/2019/src/day20.cpp
+++ b/2019/src/day20.cpp
@@ -3,6 +3,7 @@
 #include <map>
 #include <unordered_set>
 #include <queue>
+#include <set>
 #include "days.hpp"
 #include "point.hpp"
 
@@ -156,5 +157,59 @@ void aoc2019::day20_part1(std::istream &input, std::ostream &output) {
 }
 
 void aoc2019::day20_part2(std::istream &input, std::ostream &output) {
-    output << "Not implemented\n";
+    const auto map = read_map(input);
+    const auto portals = get_portals(map);
+
+    using state_t = std::pair<int, point_t>;
+
+    const auto starting_point = std::find_if(portals.begin(), portals.end(), [](auto &x) {
+        return x.second == "AA";
+    })->first;
+
+    auto graph = get_implicit_graph(map, portals);
+
+    std::set<state_t> visited;
+    std::priority_queue<std::tuple<int, int, point_t>, std::vector<std::tuple<int, int, point_t>>, std::greater<>> todo;
+    todo.emplace(0, 0, starting_point);
+
+    const int outer_x_min = 2;
+    const int outer_x_max = map[0].size() - 3;
+    const int outer_y_min = 2;
+    const int outer_y_max = map.size() - 3;
+
+    while (!todo.empty()) {
+        const auto[dist, level, pos] = todo.top();
+        todo.pop();
+
+        if (visited.count({level, pos})) {
+            continue;
+        }
+
+        visited.emplace(level, pos);
+
+        if (level == 0 && portals.at(pos) == "ZZ") {
+            output << dist << std::endl;
+            return;
+        }
+
+        for (auto &edge : graph[pos]) {
+            int mod = 0;
+            if (edge.first == 1) {
+                // Taking a portal, determine which level we're going to
+                if (pos[0] == outer_x_max || pos[0] == outer_x_min || pos[1] == outer_y_max || pos[1] == outer_y_min) {
+                    mod = -1;
+                } else {
+                    mod = 1;
+                }
+            }
+
+            if (level + mod < 0 || visited.count({level + mod, edge.second})) {
+                continue;
+            }
+
+            todo.emplace(dist + edge.first, level + mod, edge.second);
+        }
+    }
+
+    throw std::domain_error("No valid route.");
 }
diff --git a/2019/tests/samples/20-2-2.in b/2019/tests/samples/20-2-2.in
new file mode 100644
index 0000000..d051133
--- /dev/null
+++ b/2019/tests/samples/20-2-2.in
@@ -0,0 +1,37 @@
+             Z L X W       C                 
+             Z P Q B       K                 
+  ###########.#.#.#.#######.###############  
+  #...#.......#.#.......#.#.......#.#.#...#  
+  ###.#.#.#.#.#.#.#.###.#.#.#######.#.#.###  
+  #.#...#.#.#...#.#.#...#...#...#.#.......#  
+  #.###.#######.###.###.#.###.###.#.#######  
+  #...#.......#.#...#...#.............#...#  
+  #.#########.#######.#.#######.#######.###  
+  #...#.#    F       R I       Z    #.#.#.#  
+  #.###.#    D       E C       H    #.#.#.#  
+  #.#...#                           #...#.#  
+  #.###.#                           #.###.#  
+  #.#....OA                       WB..#.#..ZH
+  #.###.#                           #.#.#.#  
+CJ......#                           #.....#  
+  #######                           #######  
+  #.#....CK                         #......IC
+  #.###.#                           #.###.#  
+  #.....#                           #...#.#  
+  ###.###                           #.#.#.#  
+XF....#.#                         RF..#.#.#  
+  #####.#                           #######  
+  #......CJ                       NM..#...#  
+  ###.#.#                           #.###.#  
+RE....#.#                           #......RF
+  ###.###        X   X       L      #.#.#.#  
+  #.....#        F   Q       P      #.#.#.#  
+  ###.###########.###.#######.#########.###  
+  #.....#...#.....#.......#...#.....#.#...#  
+  #####.#.###.#######.#######.###.###.#.#.#  
+  #.......#.......#.#.#.#.#...#...#...#.#.#  
+  #####.###.#####.#.#.#.#.###.###.#.###.###  
+  #.......#.....#.#...#...............#...#  
+  #############.#.#.###.###################  
+               A O F   N                     
+               A A D   M                     
diff --git a/2019/tests/samples/20-2-2.out b/2019/tests/samples/20-2-2.out
new file mode 100644
index 0000000..4391a33
--- /dev/null
+++ b/2019/tests/samples/20-2-2.out
@@ -0,0 +1 @@
+396