Add 2021 solutions

src/2021/Day13.cpp

@@ -0,0 +1,100 @@
+#include "common/aoc.h"
+
+namespace y2021::day13
+{
+
+  void Fold(std::set<Index2D>& indices, bool x, int fold)
+  {
+    std::set<Index2D> newIndices;
+    for (auto& index : indices)
+    {
+      // y axis
+      if (x)
+      {
+        Index2D newIndex = index;
+        if (newIndex.x > fold)
+          newIndex.x = fold - (newIndex.x - 1) % fold - 1;
+        newIndices.emplace(newIndex);
+      }
+      else
+      {
+        Index2D newIndex = index;
+        if (newIndex.y > fold)
+          newIndex.y = fold - (newIndex.y - 1) % fold - 1;
+        newIndices.emplace(newIndex);
+      }
+    }
+    indices = newIndices;
+  }
+
+  struct Paper
+  {
+    std::set<Index2D> indices;
+    std::vector<std::pair<bool, int>> folds;
+  };
+
+  REGISTER_DAY(2021, Day13, Paper, int);
+
+  REGISTER_TEST_EXAMPLE(2021, Day13, ExampleInput, 1, 17);
+  REGISTER_TEST(2021, Day13, Input, 1, 716);
+  REGISTER_TEST_EXAMPLE(2021, Day13, ExampleInput, 2, 16);
+  REGISTER_TEST(2021, Day13, Input, 2, 97);
+
+  READ_INPUT(input)
+  {
+    Paper paper;
+    std::string str;
+    while (getline(input, str))
+    {
+      if (str.empty())
+        break;
+      Index2D index;
+      std::stringstream ss{str};
+      ss >> index.x >> "," >> index.y;
+      paper.indices.emplace(index);
+    }
+
+    while (getline(input, str))
+    {
+      char c, c2;
+      std::string s;
+      int i;
+      std::stringstream ss{str};
+      ss >> s >> s >> c >> c2 >> i;
+      paper.folds.emplace_back(c == 'x', i);
+    }
+    return paper;
+  }
+
+  OUTPUT1(input)
+  {
+    std::set<Index2D> indices = input.indices;
+    Fold(indices, input.folds.front().first, input.folds.front().second);
+    return indices.size();
+  }
+
+  OUTPUT2(input)
+  {
+    std::set<Index2D> indices = input.indices;
+    for (auto& fold : input.folds)
+    {
+      Fold(indices, fold.first, fold.second);
+    }
+
+    Index2D max = *indices.begin();
+    for (auto index : indices)
+    {
+      max.x = std::max(index.x, max.x);
+      max.y = std::max(index.y, max.y);
+    }
+    Array2D<char> board{max.x + 1, max.y + 1};
+    board.Fill(' ');
+    for (auto& index : indices)
+    {
+      board[index] = '#';
+    }
+    std::cout << board;
+    // Not the "correct" output to part 2, but it verifies something at least
+    return indices.size();
+  }
+}