diff --git a/2023/puzzle-10-01.cc b/2023/puzzle-10-01.cc
new file mode 100644
index 0000000..e9cc5cf
--- /dev/null
+++ b/2023/puzzle-10-01.cc
@@ -0,0 +1,135 @@
+#include <cassert>
+#include <cstdint>
+#include <cstdlib>
+#include <iostream>
+#include <string>
+#include <utility>
+#include <vector>
+
+using Int = std::int64_t;
+using NumVec = std::vector<Int>;
+using Pos = std::pair<std::size_t, std::size_t>; // (x, y) pair
+
+[[nodiscard]] constexpr auto operator==(Pos const& lhs, Pos const& rhs) -> bool
+{
+  return lhs.first == rhs.first && lhs.second == rhs.second;
+}
+
+struct Grid
+{
+  void push_line(std::string const& line)
+  {
+    if (auto x{line.find('S')}; x != std::string::npos) {
+      assert(animal_.first == std::string::npos);
+      animal_.first = x;
+      animal_.second = grid_.size();
+    }
+    grid_.push_back(line);
+  }
+
+  [[nodiscard]] auto animal_pos() const -> Pos { return animal_; }
+
+  [[nodiscard]] auto width() const -> std::size_t { return grid_.front().size(); }
+  [[nodiscard]] auto height() const -> std::size_t { return grid_.size(); }
+
+  [[nodiscard]] auto walk_pipes(Pos const& start, Pos pos) const -> std::pair<Pos, std::size_t>
+  {
+    auto last{start};
+    std::size_t steps{0};
+    while (true) {
+      auto const [x, y] = pos;
+      auto const c{grid_[y][x]};
+      if (c == '.' || c == 'S') { break; }
+      if (start == pos) { break; }
+
+      if (c == '|' || c == 'L' || c == 'J') {
+        if (Pos const next_pos{x, y - 1}; y > 0 && next_pos != last) {
+          last = pos;
+          pos = next_pos;
+          ++steps;
+          continue;
+        }
+      }
+
+      if (c == '|' || c == 'F' || c == '7') {
+        if (Pos const next_pos{x, y + 1}; y < height() - 1 && next_pos != last) {
+          last = pos;
+          pos = next_pos;
+          ++steps;
+          continue;
+        }
+      }
+
+      if (c == '-' || c == '7' || c == 'J') {
+        if (Pos const next_pos{x - 1, y}; x > 0 && next_pos != last) {
+          last = pos;
+          pos = next_pos;
+          ++steps;
+          continue;
+        }
+      }
+
+      if (c == '-' || c == 'L' || c == 'F') {
+        if (Pos const next_pos{x + 1, y}; x < width() - 1 && next_pos != last) {
+          last = pos;
+          pos = next_pos;
+          ++steps;
+          continue;
+        }
+      }
+
+      std::abort();
+    }
+
+    return std::make_pair(pos, steps);
+  }
+
+private:
+  std::vector<std::string> grid_;
+  Pos animal_{std::string::npos, std::string::npos};
+};
+
+auto main() -> int try {
+  std::string line;
+  Grid grid;
+
+  while (std::getline(std::cin, line)) {
+    grid.push_line(line);
+  }
+
+  auto animal = grid.animal_pos();
+
+  if (animal.second > 0) {
+    auto [end_pos, length] = grid.walk_pipes(animal, {animal.first, animal.second - 1});
+    if (end_pos == animal) {
+      std::cout << "Started going north ended back at animal: " << length / 2 + 1 << '\n';
+    }
+  }
+
+  if (animal.first > 0) {
+    auto [end_pos, length] = grid.walk_pipes(animal, {animal.first - 1, animal.second});
+    if (end_pos == animal) {
+      std::cout << "Started going west ended back at animal: " << length / 2 + 1 << '\n';
+    }
+  }
+
+  if (animal.second < grid.height() - 1) {
+    auto [end_pos, length] = grid.walk_pipes(animal, {animal.first, animal.second + 1});
+    if (end_pos == animal) {
+      std::cout << "Started going south ended back at animal: " << length / 2 + 1 << '\n';
+    }
+  }
+
+  if (animal.first < grid.width() - 1) {
+    auto [end_pos, length] = grid.walk_pipes(animal, {animal.first + 1, animal.second});
+    if (end_pos == animal) {
+      std::cout << "Started going east ended back at animal: " << length / 2 + 1 << '\n';
+    }
+  }
+
+  return EXIT_SUCCESS;
+}
+catch (...) {
+  std::cerr << "Uncaught exception.\n";
+  return EXIT_FAILURE;
+}
diff --git a/2023/puzzle-10-02.cc b/2023/puzzle-10-02.cc
new file mode 100644
index 0000000..7794b5f
--- /dev/null
+++ b/2023/puzzle-10-02.cc
@@ -0,0 +1,374 @@
+#include <algorithm>
+#include <cassert>
+#include <cstdint>
+#include <cstdlib>
+#include <iostream>
+#include <string>
+#include <utility>
+#include <vector>
+
+using Int = std::int64_t;
+using NumVec = std::vector<Int>;
+using Pos = std::pair<std::size_t, std::size_t>; // (x, y) pair
+
+[[nodiscard]] constexpr auto operator==(Pos const& lhs, Pos const& rhs) -> bool
+{
+  return lhs.first == rhs.first && lhs.second == rhs.second;
+}
+
+struct Grid;
+auto operator<<(std::ostream& os, Grid const& grid) -> std::ostream&;
+
+struct Grid
+{
+  void push_line(std::string const& line)
+  {
+    if (auto x{line.find('S')}; x != std::string::npos) {
+      assert(animal_.first == std::string::npos);
+      animal_.first = x;
+      animal_.second = grid_.size();
+    }
+    grid_.push_back(line);
+  }
+
+  [[nodiscard]] auto animal_pos() const -> Pos { return animal_; }
+
+  [[nodiscard]] auto width() const -> std::size_t { return grid_.front().size(); }
+  [[nodiscard]] auto height() const -> std::size_t { return grid_.size(); }
+
+  [[nodiscard]] auto at(Pos const& pos) const -> char { return grid_[pos.second][pos.first]; }
+  void set(Pos const& pos, char c) { grid_[pos.second][pos.first] = c; }
+
+  [[nodiscard]] auto walk_pipes(Pos const& start, Pos pos) const -> std::pair<Pos, std::size_t>
+  {
+    auto last{start};
+    std::size_t steps{0};
+    while (true) {
+      auto const [x, y] = pos;
+      auto const c{grid_[y][x]};
+      if (c == '.' || c == 'S') { break; }
+      if (start == pos) { break; }
+
+      if (c == '|' || c == 'L' || c == 'J') {
+        if (Pos const next_pos{x, y - 1}; y > 0 && next_pos != last) {
+          last = pos;
+          pos = next_pos;
+          ++steps;
+          continue;
+        }
+      }
+
+      if (c == '|' || c == 'F' || c == '7') {
+        if (Pos const next_pos{x, y + 1}; y < height() - 1 && next_pos != last) {
+          last = pos;
+          pos = next_pos;
+          ++steps;
+          continue;
+        }
+      }
+
+      if (c == '-' || c == '7' || c == 'J') {
+        if (Pos const next_pos{x - 1, y}; x > 0 && next_pos != last) {
+          last = pos;
+          pos = next_pos;
+          ++steps;
+          continue;
+        }
+      }
+
+      if (c == '-' || c == 'L' || c == 'F') {
+        if (Pos const next_pos{x + 1, y}; x < width() - 1 && next_pos != last) {
+          last = pos;
+          pos = next_pos;
+          ++steps;
+          continue;
+        }
+      }
+
+      std::abort();
+    }
+
+    return std::make_pair(pos, steps);
+  }
+
+  void hightlight_pipes(Pos const& start)
+  {
+    auto last{start};
+    auto pos{start};
+
+    auto const c{at(start)};
+    if (c == '|' || c == 'L' || c == 'J') { pos.second -= 1; }
+    else if (c == 'F' || c == '7') { pos.second += 1; }
+    else if (c == '-') { pos.first += 1; }
+    else { std::abort(); }
+    set(start, c == '|' ? 'v' : '>');
+
+    while (start != pos) {
+      auto const [x, y] = pos;
+      auto const c{grid_[y][x]};
+      if (c == '.') { std::abort(); }
+      grid_[y][x] = c == '|' ? 'v' : '>';
+
+      if (c == '|' || c == 'L' || c == 'J') {
+        if (Pos const next_pos{x, y - 1}; y > 0 && next_pos != last) {
+          last = pos;
+          pos = next_pos;
+          continue;
+        }
+      }
+
+      if (c == '|' || c == 'F' || c == '7') {
+        if (Pos const next_pos{x, y + 1}; y < height() - 1 && next_pos != last) {
+          last = pos;
+          pos = next_pos;
+          continue;
+        }
+      }
+
+      if (c == '-' || c == '7' || c == 'J') {
+        if (Pos const next_pos{x - 1, y}; x > 0 && next_pos != last) {
+          last = pos;
+          pos = next_pos;
+          continue;
+        }
+      }
+
+      if (c == '-' || c == 'L' || c == 'F') {
+        if (Pos const next_pos{x + 1, y}; x < width() - 1 && next_pos != last) {
+          last = pos;
+          pos = next_pos;
+          continue;
+        }
+      }
+
+      std::abort();
+    }
+  }
+
+  auto flood_fill()
+  {
+    /* Ensure every character is either an edge, or a '.' for outside, maybe inside, and a '*'
+     * for the initial starting locations.
+     */
+    for (auto& row : grid_) {
+      /* We know we're inside if we've passed an odd number of vertical walls and no corners/
+       * horizontal walls.
+       */
+      bool inside = false;
+      bool definite = true;
+      for (auto& c : row) {
+        if (c == 'v') { inside = !inside; }
+        else if (c == '>') { definite = false; }
+        else if (inside && definite) {
+          c = '*';
+        }
+        else { c = '.'; }
+      }
+    }
+
+    /* Very simple algorithm, that just iterates over all squares until we don't change anything */
+    bool again{true};
+    while (again) {
+      again = false;
+      for (std::size_t y{0}; y != height(); ++y) {
+        for (std::size_t x{0}; x != width(); ++x) {
+          if (grid_[y][x] != '.') { continue; }
+          if ((y > 0 && grid_[y - 1][x] == '*') || (x > 0 && grid_[y][x - 1] == '*') || (
+                y < height() - 1 && grid_[y + 1][x] == '*') || (
+                x < width() - 1 && grid_[y][x + 1] == '*')) {
+            grid_[y][x] = '*';
+            again = true;
+          }
+        }
+      }
+    }
+  }
+
+  /* Shrink the grid by a factor of two in all directions.  We take the even places to be the new
+   * entry in the new grid.
+   *
+   * So grid.enlarge().shrink() should produce the same grid as grid.
+   */
+  [[nodiscard]] auto shrink() const -> Grid
+  {
+    Grid new_grid;
+    for (std::size_t y{0}; y < height(); y += 2) {
+      std::string line;
+      for (std::size_t x{0}; x < width(); x += 2) {
+        line += grid_[y][x];
+      }
+      new_grid.push_line(line);
+    }
+    return new_grid;
+  }
+
+  /* Enlarge the grid by a factor of two in each direction.
+   *
+   * The contents of grid_[y][x] are mapped to grid_[y * 2][x * 2] and the appropriate additional
+   * characters are added.
+   */
+  [[nodiscard]] auto enlarge() const -> Grid
+  {
+    Grid new_grid;
+    for (auto const& row : grid_) {
+      std::string line1;
+      std::string line2;
+      for (auto const c : row) {
+        if (c == '.') {
+          line1 += "..";
+          line2 += "..";
+        }
+        if (c == '-') {
+          line1 += "--";
+          line2 += "..";
+        }
+        if (c == '7') {
+          line1 += "7.";
+          line2 += "|.";
+        }
+        if (c == 'F') {
+          line1 += "F-";
+          line2 += "|.";
+        }
+        if (c == '|') {
+          line1 += "|.";
+          line2 += "|.";
+        }
+        if (c == 'J') {
+          line1 += "J.";
+          line2 += "..";
+        }
+        if (c == 'L') {
+          line1 += "L-";
+          line2 += "..";
+        }
+      }
+      new_grid.push_line(line1);
+      new_grid.push_line(line2);
+    }
+
+    return new_grid;
+  }
+
+  [[nodiscard]] auto count_interior() const -> std::size_t
+  {
+    std::size_t interior{0};
+    for (auto const& row : grid_) {
+      interior += std::ranges::count_if(row, [](char const c) { return c == '*'; });
+    }
+
+    return interior;
+  }
+
+  friend auto operator<<(std::ostream& os, Grid const& grid) -> std::ostream&;
+
+private:
+  std::vector<std::string> grid_;
+  Pos animal_{std::string::npos, std::string::npos};
+};
+
+auto operator<<(std::ostream& os, Grid const& grid) -> std::ostream&
+{
+  for (auto const& row : grid.grid_) {
+    os << row << '\n';
+  }
+  return os;
+}
+
+auto main() -> int try {
+  std::string line;
+  Grid grid;
+
+  while (std::getline(std::cin, line)) {
+    grid.push_line(line);
+  }
+
+  auto animal = grid.animal_pos();
+
+  /* Find the loop in both directions as this tells us what the animal pipe piece should be. */
+  bool east{false};
+  bool south{false};
+  bool west{false};
+  bool north{false};
+  if (animal.second > 0) {
+    Pos const next_pos{animal.first, animal.second - 1};
+    if (auto const c{grid.at(next_pos)}; c == '|' || c == 'F' || c == '7') {
+      auto [end_pos, length] = grid.walk_pipes(animal, next_pos);
+      if (end_pos == animal) {
+        std::cout << "Started going north ended back at animal: " << length / 2 + 1 << '\n';
+        north = true;
+      }
+    }
+  }
+
+  if (animal.first > 0) {
+    Pos const next_pos{animal.first - 1, animal.second};
+    if (auto const c{grid.at(next_pos)}; c == '-' || c == 'F' || c == 'L') {
+      auto [end_pos, length] = grid.walk_pipes(animal, next_pos);
+      if (end_pos == animal) {
+        std::cout << "Started going west ended back at animal: " << length / 2 + 1 << '\n';
+        west = true;
+      }
+    }
+  }
+
+  if (animal.second < grid.height() - 1) {
+    Pos const next_pos{animal.first, animal.second + 1};
+    if (auto const c{grid.at(next_pos)}; c == '|' || c == 'J' || c == 'L') {
+      auto [end_pos, length] = grid.walk_pipes(animal, next_pos);
+      if (end_pos == animal) {
+        std::cout << "Started going south ended back at animal: " << length / 2 + 1 << '\n';
+        south = true;
+      }
+    }
+  }
+
+  if (animal.first < grid.width() - 1) {
+    Pos const next_pos{animal.first + 1, animal.second};
+    if (auto const c{grid.at(next_pos)}; c == '-' || c == 'J' || c == '7') {
+      auto [end_pos, length] = grid.walk_pipes(animal, next_pos);
+      if (end_pos == animal) {
+        std::cout << "Started going east ended back at animal: " << length / 2 + 1 << '\n';
+        east = true;
+      }
+    }
+  }
+
+  /* Replace animal with pipe piece. */
+  if (north && south) { grid.set(animal, '|'); }
+  else if (east && west) { grid.set(animal, '-'); }
+  else if (north && east) { grid.set(animal, 'L'); }
+  else if (north && west) { grid.set(animal, 'J'); }
+  else if (south && east) { grid.set(animal, 'F'); }
+  else if (south && west) { grid.set(animal, '7'); }
+  else {
+    std::abort();
+  }
+
+  /* To ensure that the interior is stongly connected we double the size of the grid in both
+   * directions.  This makes sure that there is always a . between two pipe edges.
+   *
+   * This then means we can do a very simple flood fill to find the interior.
+   *
+   * We then shrink the grid again and to get the interior count we want.
+   */
+  auto big_grid{grid.enlarge()};
+  // Mark the pipes we want to work with using > & v characters
+  big_grid.hightlight_pipes(Pos{animal.first * 2, animal.second * 2});
+  std::cout << "Before flood fill:\n" << big_grid;
+  // Flood fill the interior with * characters
+  big_grid.flood_fill();
+  std::cout << "After flood fill:\n" << big_grid;
+  // Shrink
+  auto const small_grid = big_grid.shrink();
+  // Count the number of * in the interior.
+  auto const interior_count = small_grid.count_interior();
+  std::cout << "Reconstituted grid\n" << small_grid;
+  std::cout << "Interior count: " << interior_count << "\n";
+
+  return EXIT_SUCCESS;;
+}
+catch (...) {
+  std::cerr << "Uncaught exception.\n";
+  return EXIT_FAILURE;
+}