Add 2021 day 13 puzzles

2021/puzzle-13-01.cc

@@ -0,0 +1,70 @@
+//
+// Created by Matthew Gretton-Dann on 13/12/2021.
+//
+
+#include <iostream>
+#include <regex>
+#include <set>
+#include <string>
+
+using Coord = std::pair<long, long>;
+using LightSet = std::set<Coord>;
+
+auto fold(LightSet const& lights, Coord const& fold_coord) -> LightSet
+{
+  LightSet result;
+  Coord shift{0, 0};
+  for (auto const& c : lights) {
+    Coord folded{c.first > fold_coord.first ? (2 * fold_coord.first - c.first) : c.first,
+                 c.second > fold_coord.second ? (2 * fold_coord.second - c.second) : c.second};
+    shift = {std::min(shift.first, folded.first), std::min(shift.second, folded.second)};
+    result.insert(folded);
+  }
+
+  if (shift == Coord{0, 0}) {
+    return result;
+  }
+
+  LightSet result2;
+  for (auto const& c : result) {
+    result2.emplace(c.first - shift.first, c.second - shift.second);
+  }
+  return result2;
+}
+
+auto main() -> int
+{
+  static std::regex fold_x{"fold along x=(\\d+)"};
+  static std::regex fold_y{"fold along y=(\\d+)"};
+
+  std::string line;
+  LightSet lights;
+  /* Read coords.  */
+  while (std::getline(std::cin, line) && !line.empty()) {
+    std::size_t end{0};
+    auto x{std::stol(line, &end)};
+    auto y{std::stol(line.substr(end + 1))};
+    lights.emplace(x, y);
+  }
+
+  /* Now do folds. */
+  int count{0};
+  std::cout << "Initial lights: " << lights.size() << '\n';
+  while (std::getline(std::cin, line)) {
+    std::smatch m;
+    Coord fold_coord{std::numeric_limits<long>::max(), std::numeric_limits<long>::max()};
+    if (std::regex_search(line, m, fold_x)) {
+      fold_coord.first = std::stol(m.str(1));
+    }
+    else if (std::regex_search(line, m, fold_y)) {
+      fold_coord.second = std::stol(m.str(1));
+    }
+    else {
+      std::cerr << "Cannot interpret: " << line << "\n";
+      return 1;
+    }
+    ++count;
+    lights = fold(lights, fold_coord);
+    std::cout << "After " << count << " folds: " << lights.size() << '\n';
+  }
+}