diff --git a/2017/puzzle-21-01.cc b/2017/puzzle-21-01.cc
new file mode 100644
index 0000000..a6b706a
--- /dev/null
+++ b/2017/puzzle-21-01.cc
@@ -0,0 +1,142 @@
+#include <cassert>
+#include <iostream>
+#include <map>
+#include <regex>
+#include <string>
+
+[[nodiscard]] auto rotate(std::string const& s) -> std::string
+{
+  if (s.size() == 4) {
+    return std::string{s[2], s[0], s[3], s[1]};
+  }
+  if (s.size() == 9) {
+    return std::string{s[6], s[3], s[0], s[7], s[4], s[1], s[8], s[5], s[2]};
+  }
+  abort();
+}
+
+[[nodiscard]] auto flip(std::string const& s) -> std::string
+{
+  if (s.size() == 4) {
+    return s.substr(2) + s.substr(0, 2);
+  }
+  if (s.size() == 9) {
+    return s.substr(6) + s.substr(3, 3) + s.substr(0, 3);
+  }
+  abort();
+}
+
+struct Patterns
+{
+  void add(std::string const& s1, std::string const& s2, std::string const& target)
+  {
+    auto s{s1 + s2};
+    for (auto j{0}; j < 2; ++j) {
+      for (auto i{0}; i < 4; ++i) {
+        map_.insert({s, target});
+        s = rotate(s);
+      }
+      s = flip(s);
+    }
+  }
+
+  void add(std::string const& s1, std::string const& s2, std::string const& s3,
+           std::string const& target)
+  {
+    auto s{s1 + s2 + s3};
+    for (auto j{0}; j < 2; ++j) {
+      for (auto i{0}; i < 4; ++i) {
+        map_.insert({s, target});
+        s = rotate(s);
+      }
+      s = flip(s);
+    }
+  }
+
+  std::string const& find(std::string const& s)
+  {
+    static std::string empty;
+    auto it{map_.find(s)};
+    if (it == map_.end()) {
+      return empty;
+    }
+    return it->second;
+  }
+
+  std::map<std::string, std::string> map_;
+};
+
+auto main() -> int
+{
+  static std::regex re2{"([.#][.#])/([.#][.#]) => ([.#][.#][.#])/([.#][.#][.#])/([.#][.#][.#])"};
+  static std::regex re3{
+    "([.#][.#][.#])/([.#][.#][.#])/([.#][.#][.#]) => "
+    "([.#][.#][.#][.#])/([.#][.#][.#][.#])/([.#][.#][.#][.#])/([.#][.#][.#][.#])"};
+
+  Patterns patterns;
+  std::string line;
+  while (std::getline(std::cin, line) && !line.empty()) {
+    std::smatch m;
+    if (std::regex_search(line, m, re2)) {
+      patterns.add(m.str(1), m.str(2), m.str(3) + m.str(4) + m.str(5));
+    }
+    else if (std::regex_search(line, m, re3)) {
+      patterns.add(m.str(1), m.str(2), m.str(3), m.str(4) + m.str(5) + m.str(6) + m.str(7));
+    }
+    else {
+      std::cerr << "Unable to interpret:" << line << '\n';
+      return 1;
+    }
+  }
+
+  std::string grid{".#...####"};
+  unsigned width{3};
+
+  for (unsigned i{0}; i < 5; ++i) {
+    assert(grid.size() == width * width);
+    std::string new_grid;
+    if (width % 2 == 0) {
+      for (unsigned y{0}; y < grid.size(); y += width * 2) {
+        std::string l1;
+        std::string l2;
+        std::string l3;
+        for (unsigned x{0}; x < width; x += 2) {
+          auto pattern{grid.substr(y + x, 2) + grid.substr(y + width + x, 2)};
+          auto expand{patterns.find(pattern)};
+          assert(!expand.empty());
+          l1 += expand.substr(0, 3);
+          l2 += expand.substr(3, 3);
+          l3 += expand.substr(6);
+        }
+        new_grid += l1 + l2 + l3;
+      }
+      width = (width / 2) * 3;
+    }
+    else {
+      assert(width % 3 == 0);
+      for (unsigned y{0}; y < grid.size(); y += width * 3) {
+        std::string l1;
+        std::string l2;
+        std::string l3;
+        std::string l4;
+        for (unsigned x{0}; x < width; x += 3) {
+          auto pattern{grid.substr(y + x, 3) + grid.substr(y + width + x, 3) +
+                       grid.substr(y + width * 2 + x, 3)};
+          auto expand{patterns.find(pattern)};
+          assert(!expand.empty());
+          l1 += expand.substr(0, 4);
+          l2 += expand.substr(4, 4);
+          l3 += expand.substr(8, 4);
+          l4 += expand.substr(12);
+        }
+        new_grid += l1 + l2 + l3 + l4;
+      }
+      width = (width / 3) * 4;
+    }
+
+    std::swap(grid, new_grid);
+  }
+
+  std::cout << "Number of lit cells: " << std::count(grid.begin(), grid.end(), '#') << '\n';
+  return 0;
+}
diff --git a/2017/puzzle-21-02.cc b/2017/puzzle-21-02.cc
new file mode 100644
index 0000000..87536d8
--- /dev/null
+++ b/2017/puzzle-21-02.cc
@@ -0,0 +1,142 @@
+#include <cassert>
+#include <iostream>
+#include <map>
+#include <regex>
+#include <string>
+
+[[nodiscard]] auto rotate(std::string const& s) -> std::string
+{
+  if (s.size() == 4) {
+    return std::string{s[2], s[0], s[3], s[1]};
+  }
+  if (s.size() == 9) {
+    return std::string{s[6], s[3], s[0], s[7], s[4], s[1], s[8], s[5], s[2]};
+  }
+  abort();
+}
+
+[[nodiscard]] auto flip(std::string const& s) -> std::string
+{
+  if (s.size() == 4) {
+    return s.substr(2) + s.substr(0, 2);
+  }
+  if (s.size() == 9) {
+    return s.substr(6) + s.substr(3, 3) + s.substr(0, 3);
+  }
+  abort();
+}
+
+struct Patterns
+{
+  void add(std::string const& s1, std::string const& s2, std::string const& target)
+  {
+    auto s{s1 + s2};
+    for (auto j{0}; j < 2; ++j) {
+      for (auto i{0}; i < 4; ++i) {
+        map_.insert({s, target});
+        s = rotate(s);
+      }
+      s = flip(s);
+    }
+  }
+
+  void add(std::string const& s1, std::string const& s2, std::string const& s3,
+           std::string const& target)
+  {
+    auto s{s1 + s2 + s3};
+    for (auto j{0}; j < 2; ++j) {
+      for (auto i{0}; i < 4; ++i) {
+        map_.insert({s, target});
+        s = rotate(s);
+      }
+      s = flip(s);
+    }
+  }
+
+  std::string const& find(std::string const& s)
+  {
+    static std::string empty;
+    auto it{map_.find(s)};
+    if (it == map_.end()) {
+      return empty;
+    }
+    return it->second;
+  }
+
+  std::map<std::string, std::string> map_;
+};
+
+auto main() -> int
+{
+  static std::regex re2{"([.#][.#])/([.#][.#]) => ([.#][.#][.#])/([.#][.#][.#])/([.#][.#][.#])"};
+  static std::regex re3{
+    "([.#][.#][.#])/([.#][.#][.#])/([.#][.#][.#]) => "
+    "([.#][.#][.#][.#])/([.#][.#][.#][.#])/([.#][.#][.#][.#])/([.#][.#][.#][.#])"};
+
+  Patterns patterns;
+  std::string line;
+  while (std::getline(std::cin, line) && !line.empty()) {
+    std::smatch m;
+    if (std::regex_search(line, m, re2)) {
+      patterns.add(m.str(1), m.str(2), m.str(3) + m.str(4) + m.str(5));
+    }
+    else if (std::regex_search(line, m, re3)) {
+      patterns.add(m.str(1), m.str(2), m.str(3), m.str(4) + m.str(5) + m.str(6) + m.str(7));
+    }
+    else {
+      std::cerr << "Unable to interpret:" << line << '\n';
+      return 1;
+    }
+  }
+
+  std::string grid{".#...####"};
+  unsigned width{3};
+
+  for (unsigned i{0}; i < 18; ++i) {
+    assert(grid.size() == width * width);
+    std::string new_grid;
+    if (width % 2 == 0) {
+      for (unsigned y{0}; y < grid.size(); y += width * 2) {
+        std::string l1;
+        std::string l2;
+        std::string l3;
+        for (unsigned x{0}; x < width; x += 2) {
+          auto pattern{grid.substr(y + x, 2) + grid.substr(y + width + x, 2)};
+          auto expand{patterns.find(pattern)};
+          assert(!expand.empty());
+          l1 += expand.substr(0, 3);
+          l2 += expand.substr(3, 3);
+          l3 += expand.substr(6);
+        }
+        new_grid += l1 + l2 + l3;
+      }
+      width = (width / 2) * 3;
+    }
+    else {
+      assert(width % 3 == 0);
+      for (unsigned y{0}; y < grid.size(); y += width * 3) {
+        std::string l1;
+        std::string l2;
+        std::string l3;
+        std::string l4;
+        for (unsigned x{0}; x < width; x += 3) {
+          auto pattern{grid.substr(y + x, 3) + grid.substr(y + width + x, 3) +
+                       grid.substr(y + width * 2 + x, 3)};
+          auto expand{patterns.find(pattern)};
+          assert(!expand.empty());
+          l1 += expand.substr(0, 4);
+          l2 += expand.substr(4, 4);
+          l3 += expand.substr(8, 4);
+          l4 += expand.substr(12);
+        }
+        new_grid += l1 + l2 + l3 + l4;
+      }
+      width = (width / 3) * 4;
+    }
+
+    std::swap(grid, new_grid);
+  }
+
+  std::cout << "Number of lit cells: " << std::count(grid.begin(), grid.end(), '#') << '\n';
+  return 0;
+}