Add 2021 day 14 puzzles

2021/puzzle-14-01.cc

@@ -0,0 +1,71 @@
+#include <iostream>
+#include <map>
+#include <string>
+
+using PolymerMap = std::map<std::pair<char, char>, char>;
+
+template<typename Fn>
+void expand1(char c1, char c2, PolymerMap const& polymers, unsigned current_depth,
+             unsigned max_depth, Fn action)
+{
+  if (current_depth == max_depth) {
+    action(c1);
+    return;
+  }
+
+  auto it{polymers.find({c1, c2})};
+  if (it == polymers.end()) {
+    action(c1);
+    return;
+  }
+  expand1(c1, it->second, polymers, current_depth + 1, max_depth, action);
+  expand1(it->second, c2, polymers, current_depth + 1, max_depth, action);
+}
+
+template<typename Fn>
+void expand(std::string const& str, unsigned max_depth, PolymerMap const& polymers, Fn action)
+{
+  for (std::size_t idx{0}; idx < str.size() - 1; ++idx) {
+    expand1(str[idx], str[idx + 1], polymers, 0, max_depth, action);
+  }
+  action(str.back());
+}
+
+auto main() -> int
+{
+  std::string initial;
+  std::string line;
+  PolymerMap polymers;
+
+  if (!std::getline(std::cin, initial)) {
+    std::cerr << "Unable to read initial lines.\n";
+    return 1;
+  }
+
+  if (!std::getline(std::cin, line) && !line.empty()) {
+    std::cerr << "Missing empty line.\n";
+    return 1;
+  }
+
+  while (std::getline(std::cin, line) && !line.empty()) {
+    polymers.insert({{line[0], line[1]}, line.back()});
+  }
+
+  std::map<char, unsigned> counts;
+  auto action = [&counts](char c) {
+    auto [it, success] = counts.insert({c, 0});
+    (it->second)++;
+  };
+
+  expand(initial, 10, polymers, action);
+
+  for (auto const& c : counts) {
+    std::cout << c.first << " -> " << c.second << '\n';
+  }
+  auto [min, max] = std::minmax_element(
+    counts.begin(), counts.end(), [](auto const& l, auto const& r) { return l.second < r.second; });
+  std::cout << "Min: " << min->first << " -> " << min->second << '\n';
+  std::cout << "Max: " << max->first << " -> " << max->second << '\n';
+  std::cout << "Difference: " << max->second - min->second << '\n';
+  return 0;
+}