Day 9

d9/main.cxx

@@ -0,0 +1,80 @@
+#include <iostream>
+#include <fstream>
+#include <sstream>
+#include <string>
+#include <vector>
+#include <unordered_set>
+#include <algorithm>
+
+typedef std::pair<int, int> Pos;
+struct PosHash
+{
+  template <class T1, class T2>
+  std::size_t operator()(const std::pair<T1, T2> &pair) const
+  {
+    return std::hash<T1>()(pair.first) ^ std::hash<T2>()(pair.second);
+  }
+};
+
+int signum(int n)
+{
+  return (0 < n) - (n < 0);
+}
+
+int main()
+{
+  std::ifstream file{"../d9/input.txt"};
+  std::string line;
+
+  Pos h{0, 0};
+  std::vector<Pos> knots(9);
+  std::fill(knots.begin(), knots.end(), std::pair<int, int>{0, 0});
+
+  std::unordered_set<Pos, PosHash> visited;
+
+  visited.insert(knots.back());
+  while (std::getline(file, line))
+  {
+    std::stringstream iss(line);
+    char dir;
+    int dis;
+    iss >> dir >> dis;
+
+    for (int i{0}; i < dis; i++)
+    {
+      switch (dir)
+      {
+      case 'U':
+        h.second++;
+        break;
+      case 'D':
+        h.second--;
+        break;
+      case 'L':
+        h.first--;
+        break;
+      case 'R':
+        h.first++;
+        break;
+      }
+
+      Pos *prev = &h;
+      for (int j{0}; j < knots.size(); j++)
+      {
+        Pos deltaPos{prev->first - knots[j].first, prev->second - knots[j].second};
+        if (std::abs(deltaPos.second) == 2 || std::abs(deltaPos.first) == 2)
+        {
+          knots[j].first += signum(deltaPos.first);
+          knots[j].second += signum(deltaPos.second);
+        }
+        prev = &knots[j];
+      }
+
+      visited.insert(knots.back());
+    }
+  }
+
+  std::cout << visited.size() << std::endl;
+
+  return 0;
+}