diff --git a/2021/puzzle-25-01.cc b/2021/puzzle-25-01.cc
new file mode 100644
index 0000000..56d3740
--- /dev/null
+++ b/2021/puzzle-25-01.cc
@@ -0,0 +1,83 @@
+#include <array>
+#include <iostream>
+#include <regex>
+#include <string>
+
+using Int = unsigned long;
+
+struct State
+{
+  void add_line(std::string const& line)
+  {
+    assert(lines_.empty() || lines_[0].size() == line.size());
+    lines_.push_back(line);
+  }
+
+  void print(std::ostream& os) const
+  {
+    for (auto const& line : lines_) {
+      os << line << '\n';
+    }
+  }
+
+  State move_right() const
+  {
+    State result{*this};
+    for (std::size_t row{0}; row < lines_.size(); ++row) {
+      for (std::size_t col{0}; col < lines_[row].size(); ++col) {
+        auto from = lines_[row][col];
+        auto to = lines_[row].at((col + 1) % lines_[row].size());
+        if (from == '>' && to == '.') {
+          result.lines_[row][(col + 1) % lines_[row].size()] = '>';
+          result.lines_[row][col] = '.';
+        }
+      }
+    }
+    return result;
+  }
+
+  State move_down() const
+  {
+    State result{*this};
+    for (std::size_t row{0}; row < lines_.size(); ++row) {
+      auto const& from_row = lines_[row];
+      auto const& to_row = lines_[(row + 1) % lines_.size()];
+      for (std::size_t col{0}; col < from_row.size(); ++col) {
+        if (from_row[col] == 'v' && to_row[col] == '.') {
+          result.lines_[(row + 1) % lines_.size()][col] = 'v';
+          result.lines_[row][col] = '.';
+        }
+      }
+    }
+
+    return result;
+  }
+
+  bool operator==(State const& rhs) const noexcept { return lines_ == rhs.lines_; }
+
+private:
+  std::vector<std::string> lines_;
+};
+
+auto main() -> int
+{
+  std::string line;
+  State state;
+
+  while (std::getline(std::cin, line) && !line.empty()) {
+    state.add_line(line);
+  }
+
+  std::size_t count{0};
+  while (true) {
+    State s1 = state.move_right().move_down();
+    ++count;
+    if (s1 == state) {
+      state.print(std::cout);
+      std::cout << "Done in " << count << " steps\n";
+      return 0;
+    }
+
+    state = s1;
+  }
+}