Add solution for "Day 8: Resonant Collinearity", part 2

The Point2::operator<() was introduced for set<Point2>, which is not actually required.

src/Point2.cpp

@@ -47,11 +47,6 @@ bool Point2::operator!=(const Point2& rhs) const
     return !(*this == rhs);
 }
 
-bool Point2::operator<(const Point2& rhs) const
-{
-    return x < rhs.x || (x == rhs.x && y < rhs.y);
-}
-
 Point2 Point2::operator+(const Point2& rhs) const
 {
     return Point2(x + rhs.x, y + rhs.y);

src/ResonantCollinearity.cpp

@@ -29,18 +29,21 @@ const std::string ResonantCollinearity::getInputFileName() const
 
 void ResonantCollinearity::finish()
 {
+    Grid<bool> antinodeGrid1{ lines.size(), lines[0].size() };
+    antinodeGrid1.fill(false);
+    Grid<bool> antinodeGrid2{ lines.size(), lines[0].size() };
+    antinodeGrid2.fill(false);
+
     for (int j = 0; j < lines.size(); j++)
     {
         for (int i = 0; i < lines[j].size(); i++)
         {
             if (lines[j][i] != getEmptyChar())
             {
-                addNewAntenna({ i, j });
+                addNewAntenna(antinodeGrid1, antinodeGrid2, { i, j });
             }
         }
     }
-
-    part1 = antinodes_.size();
 }
 
 constexpr char ResonantCollinearity::getEmptyChar()
@@ -48,25 +51,53 @@ constexpr char ResonantCollinearity::getEmptyChar()
     return '.';
 }
 
-void ResonantCollinearity::addNewAntenna(Point2&& newPosition)
+void ResonantCollinearity::addNewAntenna(Grid<bool>& antinodeGrid1, Grid<bool>& antinodeGrid2, Point2&& newPosition)
 {
     const auto [it, success] = antennas_.insert({ getPosition(newPosition), { newPosition } });
     if (!success)
     {
-        for (const auto& oldPosition : it->second)
+        // Adds the new antenna position itself as an antinode for part 2.
+        if (it->second.size() == 1)
         {
-            addNewAntinode(newPosition * 2 - oldPosition);
-            addNewAntinode(oldPosition * 2 - newPosition);
+            addNewAntinode(antinodeGrid2, it->second.front(), part2);
+        }
+        addNewAntinode(antinodeGrid2, newPosition, part2);
+
+        // Adds antinodes for all pairs of the new antenna position and each of the already processed other antenna
+        // positions with the same frequency.
+        for (const auto& other : it->second)
+        {
+            Point2 diff = newPosition - other;
+            findNewAntinodes(antinodeGrid1, antinodeGrid2, newPosition + diff, diff);
+            diff = -diff;
+            findNewAntinodes(antinodeGrid1, antinodeGrid2, other + diff, diff);
         }
 
         it->second.push_back(newPosition);
     }
 }
 
-void ResonantCollinearity::addNewAntinode(Point2&& newPosition)
+void ResonantCollinearity::findNewAntinodes(Grid<bool>& antinodeGrid1, Grid<bool>& antinodeGrid2, Point2&& start,
+    const Point2& increment)
 {
-    if (isInBounds(newPosition))
+    if (isInBounds(start))
     {
-        antinodes_.insert(newPosition);
+        addNewAntinode(antinodeGrid1, start, part1);
+        addNewAntinode(antinodeGrid2, start, part2);
+        start += increment;
+        while (isInBounds(start))
+        {
+            addNewAntinode(antinodeGrid2, start, part2);
+            start += increment;
+        }
+    }
+}
+
+void ResonantCollinearity::addNewAntinode(Grid<bool>& antinodeGrid, Point2& newPosition, long long int& count)
+{
+    if (!antinodeGrid[newPosition.y][newPosition.x])
+    {
+        antinodeGrid[newPosition.y][newPosition.x] = true;
+        count++;
     }
 }