diff --git a/2017/22.cpp b/2017/22.cpp
new file mode 100644
index 0000000..d70bb1b
--- /dev/null
+++ b/2017/22.cpp
@@ -0,0 +1,132 @@
+#include <cstddef>
+#include <cstdint>
+#include <iosfwd>
+#include <stdexcept>
+#include <unordered_map>
+#include <unordered_set>
+
+#include <doctest.h>
+
+#include <aocpp/Grid.hpp>
+#include <aocpp/Startup.hpp>
+#include <aocpp/Coord.hpp>
+
+auto Part1(aocpp::Grid const &input) -> std::int64_t
+{
+  std::unordered_set<aocpp::Coord> infected;
+
+  // Initialize set of infected locations
+  input.each([&infected](auto const pos, auto const cell)
+             {
+    if (cell == '#')
+    {
+      infected.insert(pos);
+    } });
+
+  // Start in the middle
+  aocpp::Coord pos;
+  pos.x = input.Cols() / 2;
+  pos.y = input.Rows() / 2;
+
+  // Start going "up"
+  aocpp::Coord vel{.x = 0, .y = -1};
+
+  // Count the number of iterations that cause an infection
+  std::uint64_t infections = 0;
+
+  for (int i = 0; i < 10'000; ++i)
+  {
+    auto const it = infected.find(pos);
+    if (it == end(infected))
+    {
+      // was clean
+      vel = CCW(vel);       // turn "left"
+      infected.insert(pos); // infect
+      infections++;
+    }
+    else
+    {
+      // was infected
+      vel = CW(vel);      // turn "right"
+      infected.erase(it); // clean
+    }
+    pos += vel; // advance
+  }
+
+  return infections;
+}
+
+auto Part2(aocpp::Grid const &input) -> std::int64_t
+{
+  std::unordered_map<aocpp::Coord, char> cells;
+
+  // Initialize set of infected locations
+  input.each([&cells](auto const pos, auto const cell)
+             {
+    if (cell == '#')
+    {
+      cells.try_emplace(pos, '#');
+    } });
+
+  // Start in the middle
+  aocpp::Coord pos;
+  pos.x = input.Cols() / 2;
+  pos.y = input.Rows() / 2;
+
+  // Start going "up"
+  aocpp::Coord vel{.x = 0, .y = -1};
+
+  // Count the number of iterations that cause an infection
+  std::uint64_t infections = 0;
+
+  for (int i = 0; i < 10'000'000; ++i)
+  {
+    auto const [it, added] = cells.try_emplace(pos, 'W'); // clean becomes weakened
+    if (added)
+    {
+      // already inserted 'W'
+      vel = CCW(vel); // turn "left"
+    }
+    else
+    {
+      switch (it->second)
+      {
+      case 'W':
+        // no turn
+        it->second = '#';
+        infections++;
+        break;
+      case '#':
+        vel = CW(vel); // turn "right"
+        it->second = 'F';
+        break;
+      case 'F':
+        vel = Turn180(vel);
+        cells.erase(it); // clean
+        break;
+      default:
+        throw std::logic_error{"unexpected cell in steps"};
+      }
+    }
+    pos += vel; // advance
+  }
+
+  return infections;
+}
+
+TEST_SUITE("2017-22 examples")
+{
+  TEST_CASE("part 1")
+  {
+  }
+  TEST_CASE("part 2")
+  {
+  }
+}
+
+auto Main(std::istream &in, std::ostream &out) -> void
+{
+  auto const input = aocpp::Grid::Parse(in);
+  out << "Part 1: " << Part1(input) << std::endl;
+  out << "Part 2: " << Part2(input) << std::endl;
+}
diff --git a/2017/CMakeLists.txt b/2017/CMakeLists.txt
index 1bee5ab..83855cc 100644
--- a/2017/CMakeLists.txt
+++ b/2017/CMakeLists.txt
@@ -33,3 +33,6 @@ target_link_libraries(2017_18 aocpp)
 
 add_executable(2017_19 19.cpp)
 target_link_libraries(2017_19 aocpp)
+
+add_executable(2017_22 22.cpp)
+target_link_libraries(2017_22 aocpp)
diff --git a/lib/include/aocpp/Coord.hpp b/lib/include/aocpp/Coord.hpp
index 415668e..5e6fc7a 100644
--- a/lib/include/aocpp/Coord.hpp
+++ b/lib/include/aocpp/Coord.hpp
@@ -80,6 +80,7 @@ auto operator*=(Coord &, std::int64_t) -> Coord &;
 template<>
 struct std::hash<aocpp::Coord>
 {
+  using is_transparent = void;
   auto operator()(aocpp::Coord const& c) const noexcept -> std::size_t {
     std::hash<std::int64_t> h;
     return h(c.x) ^ h(c.y);
diff --git a/lib/include/aocpp/Grid.hpp b/lib/include/aocpp/Grid.hpp
index 5f17b77..8d09f3f 100644
--- a/lib/include/aocpp/Grid.hpp
+++ b/lib/include/aocpp/Grid.hpp
@@ -1,11 +1,12 @@
 #ifndef AOCPP_GRID_HPP_
 #define AOCPP_GRID_HPP_
 
+#include <concepts>
 #include <iostream>
-#include <vector>
-#include <utility>
-#include <type_traits>
 #include <stdexcept>
+#include <type_traits>
+#include <utility>
+#include <vector>
 
 #include <aocpp/Coord.hpp>
 
@@ -62,8 +63,14 @@ struct Grid {
     return result;
   }
 
-  template <typename F>
-  auto each(F f = F{}) const {
+  /**
+   * @brief Apply the callback function across all elements of the grid.
+   * 
+   * @tparam F Type of callback
+   * @param f Callback function
+   */
+  template <std::invocable<Coord, char> F>
+  auto each(F f = F{}) const -> void {
     auto const h = rows.size();
     for (std::size_t y = 0; y < h; y++) {
       auto const& row = rows[y];