From 17728edb4429d6724004c40ad3d59195e649fb6f Mon Sep 17 00:00:00 2001
From: Eric Mertens <emertens@gmail.com>
Date: Thu, 10 Nov 2022 21:01:24 -0800
Subject: [PATCH] 20

---
 2019/20.cpp         | 175 ++++++++++++++++++++++++++++++++++++++++++++
 2019/CMakeLists.txt |   3 +
 2 files changed, 178 insertions(+)
 create mode 100644 2019/20.cpp

diff --git a/2019/20.cpp b/2019/20.cpp
new file mode 100644
index 0000000..95525cc
--- /dev/null
+++ b/2019/20.cpp
@@ -0,0 +1,175 @@
+#include <algorithm>
+#include <cstdint>
+#include <iostream>
+#include <iomanip>
+#include <fstream>
+#include <iterator>
+#include <stdexcept>
+#include <vector>
+#include <set>
+#include <string>
+#include <tuple>
+#include <bitset>
+#include <deque>
+#include <cctype>
+#include <queue>
+#include <map>
+
+#include <aocpp/Startup.hpp>
+#include <aocpp/Coord.hpp>
+
+using namespace aocpp;
+
+namespace {
+
+Coord(* const directions[4])(Coord) = {Up, Down, Left, Right};
+
+using Name = std::string;
+using Portals = std::map<std::string, Coord>;
+using Distances = std::map<Name,std::vector<std::pair<Name, std::int64_t>>>;
+
+struct Map {
+  std::vector<std::string> rows;
+
+  auto operator[](Coord c)       -> char& { return rows[c.y][c.x]; }
+  auto operator[](Coord c) const -> char  { return rows[c.y][c.x]; }
+
+  static auto Parse(std::istream & in) -> Map {
+    Map result;
+    std::string line;
+    while (std::getline(in, line)) {
+      result.rows.emplace_back(std::move(line));
+    }
+    return {result};
+  }
+};
+
+auto FindPortals(Map const& map) -> Portals {
+  Portals portals;
+
+  std::int64_t w = map.rows[0].size();
+  std::int64_t h = map.rows.size();
+
+  for (std::int64_t x = 1; x < w-1; x++) {
+    for (std::int64_t y = 1; y < h-1; y++) {
+      Coord const c = {x,y};
+      auto const v = map[c];
+      if (std::isupper(v)) {
+        char polarity = x==1 || x==w-2 || y==1 || y==h-2
+                      ? '-' : '+';
+        if (map[Up(c)] == '.') {
+          portals[{polarity, v, map[Down(c)]}] = Up(c);
+        } else if (map[Down(c)] == '.') {
+          portals[{polarity, map[Up(c)], v}] = Down(c);
+        } else if (map[Left(c)] == '.') {
+          portals[{polarity, v, map[Right(c)]}] = Left(c);
+        } else if (map[Right(c)] == '.') {
+          portals[{polarity, map[Left(c)], v}] = Right(c);
+        }
+      }
+    }
+  }
+  return portals;
+}
+
+auto FindDistancesFrom(
+  Map const& map,
+  std::map<Coord, Name> const& names,
+  Distances & distances,
+  std::string const start_name,
+  Coord const start
+) {
+  std::vector<std::pair<Name, std::int64_t>> result;
+
+  std::deque<std::pair<std::int64_t, Coord>> todo {{0, start}};
+
+  std::set<Coord> seen;
+
+  for (; !todo.empty(); todo.pop_front()) {
+    auto const [steps, here] = todo.front();
+
+    // Don't visit the same coordinate twice
+    if (!seen.insert(here).second) continue;
+
+    auto const c = map[here];
+    if (c != '.') continue; // avoid walls
+
+    // success, we've found a key, record the path
+    if (auto it = names.find(here); it != names.end() && it->second != start_name) {
+        result.emplace_back(it->second, steps);
+        continue; // don't walk beyond the portal
+    }
+
+    // Visit all neighbors
+    for (auto const fn : directions) {
+      todo.emplace_back(steps+1, fn(here));
+    }
+  }
+
+  return result;
+}
+
+auto OtherName(Name name) {
+  name[0] = name[0] == '+' ? '-' : '+';
+  return name;
+}
+
+auto FindDistances(Map const& map, Portals const& portals) {
+  Distances distances;
+
+  std::map<Coord, Name> names;
+  for (auto [k,v] : portals) {
+    names[v] = k;
+  }
+
+  for (auto const [start_name, start_coord] : portals) {
+    distances[start_name] = FindDistancesFrom(map, names, distances, start_name, start_coord);
+  }
+
+  return distances;
+}
+
+auto SolveMaze(Distances const& distances, bool const recursive) -> std::int64_t
+{
+  // Track current positions and current set of keys in easy to compare form
+  using Visited = std::pair<int, Name>;
+  std::set<Visited> seen;
+
+  // Priority queue returning lowest path cost states first.
+  using PqElt = std::tuple<std::int64_t, int, Name>;
+  using PqCmp = decltype([](PqElt const& x, PqElt const& y) {
+                return std::get<0>(x) > std::get<0>(y); });
+  std::priority_queue<PqElt, std::vector<PqElt>, PqCmp> todo;
+
+  todo.emplace(0, 0, "-AA");
+
+  while(!todo.empty()) {
+    auto [steps, depth, name] = todo.top();
+    todo.pop();
+    if (name == "-ZZ") { return steps; }
+    if (seen.emplace(depth, name).second) {
+      if (auto const it = distances.find(name); it != distances.end()) {
+        for (auto const [next, cost] : it->second) {
+          if (next == "-ZZ") {
+            if (depth == 0) todo.emplace(steps + cost, depth, "-ZZ");
+          } else {
+            auto const depth_ = depth + (recursive ? (next[0]=='+' ? 1 : -1) : 0);
+            if (depth_ >= 0) todo.emplace(steps + cost + 1, depth_, OtherName(next));
+          }
+        }
+      }
+    }
+  }
+  throw std::runtime_error{"no solution"};
+}
+
+} // namespace
+
+auto main(int argc, char** argv) -> int {
+  auto map = Map::Parse(Startup(argc, argv));
+  auto portals = FindPortals(map);
+  auto distances = FindDistances(map, portals);
+
+  std::cout << "Part 1: " << SolveMaze(distances, false) << std::endl;
+  std::cout << "Part 2: " << SolveMaze(distances, true) << std::endl;
+}
diff --git a/2019/CMakeLists.txt b/2019/CMakeLists.txt
index 504fee5..173a95a 100644
--- a/2019/CMakeLists.txt
+++ b/2019/CMakeLists.txt
@@ -43,5 +43,8 @@ target_link_libraries(15 aocpp intcode)
 add_executable(18 18.cpp)
 target_link_libraries(18 aocpp)
 
+add_executable(20 20.cpp)
+target_link_libraries(20 aocpp)
+
 add_executable(23 23.cpp)
 target_link_libraries(23 aocpp intcode)