From 495222c63c18edde04240edeeedb4962fc3332b7 Mon Sep 17 00:00:00 2001
From: Matthew Gretton-Dann <matt@gretton-dann.org.uk>
Date: Mon, 20 Dec 2021 07:46:43 +0000
Subject: [PATCH] Add 2021 day 20 puzzles

---
 2021/puzzle-20-01.cc | 104 ++++++++++++++++++++++++++++++++++++++++++
 2021/puzzle-20-02.cc | 106 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 210 insertions(+)
 create mode 100644 2021/puzzle-20-01.cc
 create mode 100644 2021/puzzle-20-02.cc

diff --git a/2021/puzzle-20-01.cc b/2021/puzzle-20-01.cc
new file mode 100644
index 0000000..7bdcfdf
--- /dev/null
+++ b/2021/puzzle-20-01.cc
@@ -0,0 +1,104 @@
+#include <iostream>
+#include <string>
+#include <vector>
+
+struct State
+{
+  explicit State(std::string const& rules) : rules_(rules)
+  {
+    assert(rules.size() == 512);
+    assert(rules[0] == '.' || rules[511] == '.');
+  }
+
+  void add_row(std::string const& row)
+  {
+    using namespace std::string_literals;
+    lights_.push_back(row);
+  }
+
+  auto size() const noexcept -> std::size_t
+  {
+    std::size_t result{0};
+    for (auto const& row : lights_) {
+      result += std::count(row.begin(), row.end(), '#');
+    }
+    return result;
+  }
+
+  auto next() const -> State
+  {
+    std::ptrdiff_t longest_line(
+      std::max_element(lights_.begin(), lights_.end(), [](auto const& l, auto const& r) {
+        return l.size() < r.size();
+      })->size());
+    State result{rules_};
+    if (rules_[0] == '#') {
+      result.outside_set_ = !outside_set_;
+    }
+    std::ptrdiff_t row_count(lights_.size());
+    for (std::ptrdiff_t y{-1}; y <= row_count; ++y) {
+      std::string next_row;
+      next_row.reserve(longest_line);
+      for (std::ptrdiff_t x{-1}; x <= longest_line; ++x) {
+        next_row.push_back(lit(x, y) ? '#' : '.');
+      }
+      result.add_row(next_row);
+    }
+    return result;
+  }
+
+  [[nodiscard]] auto lit(std::ptrdiff_t x0, std::ptrdiff_t y0) const noexcept -> bool
+  {
+    auto num{0};
+    for (auto y{y0 - 1}; y < y0 + 2; ++y) {
+      for (auto x{x0 - 1}; x < x0 + 2; ++x) {
+        num <<= 1;
+        num |= bit_set(x, y);
+      }
+    }
+    return rules_[num] == '#';
+  }
+
+  void print(std::ostream& os) const
+  {
+    for (auto const& row : lights_) {
+      os << row << '\n';
+    }
+  }
+
+private:
+  auto bit_set(std::ptrdiff_t x, std::ptrdiff_t y) const noexcept -> bool
+  {
+    if (y < 0 || y >= lights_.size()) {
+      return outside_set_;
+    }
+    if (x < 0 || x >= lights_[y].size()) {
+      return outside_set_;
+    }
+    return lights_[y][x] == '#';
+  }
+
+  std::string rules_;
+  std::vector<std::string> lights_;
+  bool outside_set_{false};
+};
+
+auto main() -> int
+{
+  std::string line;
+  if (!std::getline(std::cin, line)) {
+    std::cerr << "Unable to read rules.\n";
+    return 1;
+  }
+  State state{line};
+  std::getline(std::cin, line);
+  assert(line.empty());
+  while (std::getline(std::cin, line) && !line.empty()) {
+    state.add_row(line);
+  }
+
+  state = state.next();
+  state = state.next();
+
+  std::cout << "Number lit: " << state.size() << '\n';
+}
diff --git a/2021/puzzle-20-02.cc b/2021/puzzle-20-02.cc
new file mode 100644
index 0000000..fe3456b
--- /dev/null
+++ b/2021/puzzle-20-02.cc
@@ -0,0 +1,106 @@
+#include <iostream>
+#include <string>
+#include <vector>
+
+struct State
+{
+  explicit State(std::string const& rules) : rules_(rules)
+  {
+    assert(rules.size() == 512);
+    assert(rules[0] == '.' || rules[511] == '.');
+  }
+
+  void add_row(std::string const& row)
+  {
+    using namespace std::string_literals;
+    lights_.push_back(row);
+  }
+
+  auto size() const noexcept -> std::size_t
+  {
+    std::size_t result{0};
+    for (auto const& row : lights_) {
+      result += std::count(row.begin(), row.end(), '#');
+    }
+    return result;
+  }
+
+  auto next() const -> State
+  {
+    std::ptrdiff_t longest_line(
+      std::max_element(lights_.begin(), lights_.end(), [](auto const& l, auto const& r) {
+        return l.size() < r.size();
+      })->size());
+    State result{rules_};
+    if (rules_[0] == '#') {
+      result.outside_set_ = !outside_set_;
+    }
+    std::ptrdiff_t row_count(lights_.size());
+    for (std::ptrdiff_t y{-1}; y <= row_count; ++y) {
+      std::string next_row;
+      next_row.reserve(longest_line);
+      for (std::ptrdiff_t x{-1}; x <= longest_line; ++x) {
+        next_row.push_back(lit(x, y) ? '#' : '.');
+      }
+      result.add_row(next_row);
+    }
+    return result;
+  }
+
+  [[nodiscard]] auto lit(std::ptrdiff_t x0, std::ptrdiff_t y0) const noexcept -> bool
+  {
+    auto num{0};
+    for (auto y{y0 - 1}; y < y0 + 2; ++y) {
+      for (auto x{x0 - 1}; x < x0 + 2; ++x) {
+        num <<= 1;
+        num |= bit_set(x, y);
+      }
+    }
+    return rules_[num] == '#';
+  }
+
+  void print(std::ostream& os) const
+  {
+    for (auto const& row : lights_) {
+      os << row << '\n';
+    }
+  }
+
+private:
+  auto bit_set(std::ptrdiff_t x, std::ptrdiff_t y) const noexcept -> bool
+  {
+    if (y < 0 || y >= lights_.size()) {
+      return outside_set_;
+    }
+    if (x < 0 || x >= lights_[y].size()) {
+      return outside_set_;
+    }
+    return lights_[y][x] == '#';
+  }
+
+  std::string rules_;
+  std::vector<std::string> lights_;
+  bool outside_set_{false};
+};
+
+auto main() -> int
+{
+  std::string line;
+  if (!std::getline(std::cin, line)) {
+    std::cerr << "Unable to read rules.\n";
+    return 1;
+  }
+  State state{line};
+  std::getline(std::cin, line);
+  assert(line.empty());
+  while (std::getline(std::cin, line) && !line.empty()) {
+    state.add_row(line);
+  }
+
+  constexpr unsigned count{50};
+  for (unsigned i{0}; i < count; ++i) {
+    state = state.next();
+  }
+
+  std::cout << "Number lit: " << state.size() << '\n';
+}