Add solution for "Day 12: Garden Groups", part 2

src/GardenGroups.cpp

@@ -53,6 +53,7 @@ void GardenGroups::traverseRegion(Grid<bool>& isVisited, std::stack<Point2>& oth
     const char plantType{ getCharAt(start) };
     long long int area{ 0 };
     long long int perimeter{ 0 };
+    long long int nCorners{ 0 };
     std::stack<Point2> regionStack{};
     regionStack.push(start);
 
@@ -65,34 +66,54 @@ void GardenGroups::traverseRegion(Grid<bool>& isVisited, std::stack<Point2>& oth
             isVisited.cell(position) = true;
             area++;
 
+            Point2 previousNeighbor{ position + Point2::cardinalDirections.back() };
+            bool isPreviousNeighborSameRegion{
+                isInBounds(previousNeighbor) && getCharAt(previousNeighbor) == plantType
+            };
             for (const Point2& direction : Point2::cardinalDirections)
             {
-                const Point2 next{ position + direction };
-                if (isInBounds(next))
+                const Point2 neighbor{ position + direction };
+                const bool isNeighborSameRegion{ isInBounds(neighbor) && getCharAt(neighbor) == plantType };
+
+                if (isNeighborSameRegion)
                 {
-                    if (getCharAt(next) == plantType)
+                    if (!isVisited.cell(neighbor))
                     {
-                        if (!isVisited.cell(next))
-                        {
-                            regionStack.push(next);
-                        }
+                        regionStack.push(neighbor);
                     }
-                    else
+
+                    // Increases the corner count if two adjacent neighbors are both inside this region, but the
+                    // diagonal neighbor in between them is outside this region.
+                    if (isPreviousNeighborSameRegion)
                     {
-                        perimeter++;
-                        if (!isVisited.cell(next))
+                        const Point2 diagonalNeighbor{ previousNeighbor + direction };
+                        if (!isInBounds(diagonalNeighbor) || getCharAt(diagonalNeighbor) != plantType)
                         {
-                            otherRegionsStack.push(next);
+                            nCorners++;
                         }
                     }
                 }
                 else
                 {
+                    if (isInBounds(neighbor) && !isVisited.cell(neighbor))
+                    {
+                        otherRegionsStack.push(neighbor);
+                    }
+
                     perimeter++;
+
+                    // Increases the corner count if two adjacent neighbors are both outside this region.
+                    if (!isPreviousNeighborSameRegion)
+                    {
+                        nCorners++;
+                    }
                 }
+                previousNeighbor = neighbor;
+                isPreviousNeighborSameRegion = isNeighborSameRegion;
             }
         }
     }
 
     part1 += area * perimeter;
+    part2 += area * nCorners;
 }