diff --git a/2022/09.cpp b/2022/09.cpp
new file mode 100644
index 0000000..7b84acf
--- /dev/null
+++ b/2022/09.cpp
@@ -0,0 +1,96 @@
+#include <algorithm>
+#include <cstdint>
+#include <functional>
+#include <iostream>
+#include <set>
+#include <sstream>
+#include <tuple>
+
+#include <doctest.h>
+
+#include <aocpp/Coord.hpp>
+#include <aocpp/Startup.hpp>
+
+using namespace aocpp;
+
+namespace {
+
+using Dir = Coord(*)(Coord);
+
+auto ToDir(char c) -> Dir {
+  switch (c) {
+    case 'L': return Left;
+    case 'R': return Right;
+    case 'D': return Down;
+    case 'U': return Up;
+    default: throw std::runtime_error{"bad input"};
+  }
+}
+
+/// @brief Return the sign of the number
+/// @param x number
+/// @return -1, 0, or 1 matching the sign of the input
+auto Sign(std::int64_t x) -> std::int64_t
+{
+  return (x>0) - (x<0);
+}
+
+auto Solve(std::istream & in) -> std::pair<std::int64_t, std::int64_t>
+{
+  std::size_t constexpr knotn {10};
+  Coord knots[knotn] {};
+  std::set<Coord> p1{knots[1]}, p2{knots[9]};
+  char d;
+  long n;
+
+  while (in >> d >> n) {
+    auto const dir = ToDir(d);
+    while (n--) {
+      knots[0] = dir(knots[0]);
+      
+      std::size_t k;
+      for (k = 1; k < knotn; ++k) {
+        auto const delta {knots[k-1] - knots[k]};
+        if (NormInf(delta) <= 1) {
+          break;
+        }
+        knots[k] += {Sign(delta.x), Sign(delta.y)};
+      }
+
+      if (k > 1) {
+        p1.insert(knots[1]);
+        if (k > 9) {
+          p2.insert(knots[9]);
+        }
+      }
+    }
+  }
+
+  return {p1.size(), p2.size()};
+}
+
+} // namespace
+
+TEST_SUITE("2022-09 examples") {
+  TEST_CASE("example") {
+    std::istringstream in {
+      "R 5\n"
+      "U 8\n"
+      "L 8\n"
+      "D 3\n"
+      "R 17\n"
+      "D 10\n"
+      "L 25\n"
+      "U 20\n"};
+    auto [p1,p2] = Solve(in);
+    CHECK(p1 == 88);
+    CHECK(p2 == 36);
+  }
+}
+
+auto main(int argc, char** argv) -> int
+{
+  auto [p1,p2] = Solve(*aocpp::Startup(argc, argv));
+  std::cout << "Part 1: " << p1 << std::endl;
+  std::cout << "Part 2: " << p2 << std::endl;
+}
diff --git a/2022/CMakeLists.txt b/2022/CMakeLists.txt
index fa84705..0a3ddfb 100644
--- a/2022/CMakeLists.txt
+++ b/2022/CMakeLists.txt
@@ -10,6 +10,9 @@ target_link_libraries(2022_04 aocpp)
 add_executable(2022_08 08.cpp)
 target_link_libraries(2022_08 aocpp)
 
+add_executable(2022_09 09.cpp)
+target_link_libraries(2022_09 aocpp)
+
 add_executable(2022_12 12.cpp)
 target_link_libraries(2022_12 aocpp)