From f74a1558f1f4234bf120616b22045f2fb6c02484 Mon Sep 17 00:00:00 2001
From: Naomi Amethyst <naomi@amethyst.name>
Date: Sat, 21 Dec 2024 06:22:38 +0000
Subject: [PATCH] +day06 part2

---
 2024/day06/part2.cc | 130 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 130 insertions(+)
 create mode 100644 2024/day06/part2.cc

diff --git a/2024/day06/part2.cc b/2024/day06/part2.cc
new file mode 100644
index 0000000..8dd6fd6
--- /dev/null
+++ b/2024/day06/part2.cc
@@ -0,0 +1,130 @@
+#include <iostream>
+#include <vector>
+#include <string>
+#include <utility>
+#include <set>
+
+class vec {
+public:
+  int x;
+  int y;
+  vec(int x, int y) : x(x), y(y) {}
+  vec operator+(const vec& other) const;
+  int index(int width) const;
+  vec right() const;
+  auto operator<=>(const vec&) const = default;
+};
+
+vec vec::operator+(const vec& other) const {
+  return vec{x + other.x, y + other.y};
+}
+
+int vec::index(int width) const {
+  return y * width + x;
+}
+
+vec vec::right() const {
+  return vec{-y, x};
+}
+
+class grid {
+  int width;
+  int height;
+  std::vector<bool> obstacles;
+  std::vector<bool> visited_pos;
+  std::set<std::pair<vec, vec>> visited;
+public:
+  grid(int width, int height);
+  bool visit(const vec& v, const vec& dir);
+  void obstacle(const vec& v);
+  bool is_obstacle(const vec& v) const;
+  bool is_valid(const vec& v) const;
+  bool is_visited(const vec& v) const;
+};
+
+grid::grid(int width, int height)
+    : width(width), height(height) {
+  obstacles.resize(width * height);
+  visited_pos.resize(width * height);
+}
+
+bool grid::visit(const vec& v, const vec& dir) {
+  visited_pos[v.index(width)] = true;
+  return visited.emplace(v, dir).second;
+}
+
+void grid::obstacle(const vec& v) {
+  obstacles[v.index(width)] = true;
+}
+
+bool grid::is_obstacle(const vec& v) const {
+  return obstacles.at(v.index(width));
+}
+
+bool grid::is_valid(const vec& v) const {
+  return v.x >= 0 && v.x < width
+    && v.y >= 0 && v.y < height;
+}
+
+bool grid::is_visited(const vec& v) const {
+  return visited_pos.at(v.index(width));
+}
+
+bool is_loop(grid& g, vec pos, vec dir) {
+  for (;; pos = pos + dir) {
+    if (!g.is_valid(pos + dir)) {
+      return false;
+    }
+    while (g.is_obstacle(pos + dir)) {
+      dir = dir.right();
+    }
+    if (!g.visit(pos, dir)) {
+      return true;
+    }
+  }
+}
+
+int main() {
+  int width = 0, height = 0, count = 0;
+  std::string line;
+  std::vector<vec> obstacles;
+  std::set<vec> counted;
+  vec pos{0, 0};
+  vec dir{0, -1};
+  for (int line_num = 0; getline(std::cin, line); line_num++) {
+    height++;
+    for (int x = 0; x < line.size(); x++) {
+      if (line[x] == '#') {
+        obstacles.emplace_back(x, line_num);
+      } else if (line[x] == '^') {
+        pos = vec{x, line_num};
+      }
+    }
+  }
+  width = line.size();
+  grid g{width, height};
+  for (const vec& v : obstacles) {
+    g.obstacle(v);
+  }
+  grid tmp{width, height};
+  for (;; pos = pos + dir) {
+    if (!g.is_valid(pos + dir)) {
+      break;
+    }
+    while (g.is_obstacle(pos + dir)) {
+      dir = dir.right();
+    }
+    if (!g.is_visited(pos + dir)) {
+      tmp = g;
+      tmp.obstacle(pos + dir);
+      if (is_loop(tmp, pos, dir)) {
+        if (counted.emplace(pos + dir).second) {
+          count++;
+        }
+      }
+    }
+    g.visit(pos, dir);
+  }
+  std::cout << count << std::endl;
+  return EXIT_SUCCESS;
+}