Add solution for "Day 18: RAM Run", part 2

README.md

@@ -132,6 +132,14 @@ For part 1, the solver implements the eight defined instructions and a runtime c
 
 For part 2, the algorithm appends iteratively 3-digit binary numbers in such a way, that the desired output is approached. The required search patterns and mappings were determined manually, and are predefined for the specific given program in code. It is not a generic solution.
 
+### Day 18: RAM Run
+
+:mag_right: Puzzle: <https://adventofcode.com/2024/day/18>, :white_check_mark: Solver: [`RamRun.cpp`](src/RamRun.cpp)
+
+After the solver tracked the first 1024 corrupted memory positions, it constructs a graph of all remaining positions and their adjacencies, and runs Dijkstra's algorithm I already used for [day 16](#day-16-reindeer-maze) to calculate the length of the shortest path to the exit.
+
+Conveniently, Dijkstra's algorithm also finds the shortest path itself, so after each additional falling byte, the algorithm checks whether it hit the current shortest path and if so, tries to find a new one to the exit. The solution for part two is found once this there is no path any more.
+
 ## Thanks
 
 * [Alexander Brouwer](https://github.com/Bromvlieg) for getting the project set up with CMake.

include/aoc/RamRun.hpp

@@ -33,8 +33,11 @@ class RamRun
     int maxBytes_;
     int nBytes_;
     Grid<int> vertexReferences_;
+    WeightedEdgeGraph::PathsResult dijkstraResult_;
     static constexpr char getUnknownVertexReference();
     static constexpr char getNoVertexReference();
-    void buildGraph(WeightedEdgeGraph& graph);
+    bool tryMarkCorrupted(const std::string& line, int& corruptedVertex);
+    WeightedEdgeGraph buildGraph();
     int getVertex(WeightedEdgeGraph& graph, const size_t x, const size_t y);
+    void resetVertexReferences();
 };

include/aoc/common/WeightedEdgeGraph.hpp

@@ -28,7 +28,23 @@ class WeightedEdgeGraph
     void addEdge(const int vertex1, const int vertex2, const int weight);
     int getVertexWeight(const int vertex) const;
     int getEdgeWeight(const int edge) const;
-    std::vector<int> dijkstra(const int source) const;
+
+    struct PathsResult
+    {
+        PathsResult()
+            : distances{}, predecessors{}
+        {
+        }
+        PathsResult(const size_t size, const int initialDistance, const int initialPredecessor)
+            : distances(size, initialDistance), predecessors(size, initialPredecessor)
+        {
+        }
+        std::vector<int> distances;
+        std::vector<int> predecessors;
+    };
+
+    PathsResult dijkstra(const int source) const;
+    static constexpr int getInfiniteDistance();
 
     struct Incidence
     {

src/RamRun.cpp

@@ -35,22 +35,42 @@ const int RamRun::getPuzzleDay() const
 
 void RamRun::processDataLine(const std::string& line)
 {
-    if (maxBytes_ > nBytes_++)
+    if (part2 == "")
     {
-        std::istringstream stream{ line };
-        Point2 position;
-        char c;
-        if (stream >> position.x >> c >> position.y)
+        int corrupted{ -1 };
+        if (!tryMarkCorrupted(line, corrupted))
         {
-            vertexReferences_.cell(position) = getNoVertexReference();
+            return;
         }
 
-        if (maxBytes_ == nBytes_)
+        if (maxBytes_ == ++nBytes_)
         {
-            WeightedEdgeGraph graph{};
-            buildGraph(graph);
-            auto distances = graph.dijkstra(vertexReferences_[0][0]);
-            part1 = distances[vertexReferences_[memorySize_ - 1][memorySize_ - 1]];
+            // Calculates initial distances.
+            auto graph = buildGraph();
+            dijkstraResult_ = graph.dijkstra(vertexReferences_[0][0]);
+            part1 = dijkstraResult_.distances[vertexReferences_[memorySize_ - 1][memorySize_ - 1]];
+        }
+        else if (maxBytes_ < nBytes_)
+        {
+            int pathVertex = vertexReferences_[memorySize_ - 1][memorySize_ - 1];
+            while (pathVertex >= 0 && pathVertex != corrupted)
+            {
+                pathVertex = dijkstraResult_.predecessors[pathVertex];
+            }
+            if (pathVertex == corrupted)
+            {
+                // Calculates new paths and distances after path was interruped by corrupted position, and checks if path
+                // is now blocked.
+                resetVertexReferences();
+                auto graph = buildGraph();
+                dijkstraResult_ = graph.dijkstra(vertexReferences_[0][0]);
+                if (dijkstraResult_.distances[vertexReferences_[memorySize_ - 1][memorySize_ - 1]]
+                    == WeightedEdgeGraph::getInfiniteDistance())
+                {
+                    // Path is blocked.
+                    part2 = line;
+                }
+            }
         }
     }
 }
@@ -69,8 +89,25 @@ constexpr char RamRun::getNoVertexReference()
     return -2;
 }
 
-void RamRun::buildGraph(WeightedEdgeGraph& graph)
+// Marks the position extracted from the 'line' as corrupted in the vertex reference grid.
+bool RamRun::tryMarkCorrupted(const std::string& line, int& corruptedVertex)
 {
+    std::istringstream stream{ line };
+    Point2 position;
+    char c;
+    if (stream >> position.x >> c >> position.y)
+    {
+        corruptedVertex = vertexReferences_.cell(position);
+        vertexReferences_.cell(position) = getNoVertexReference();
+        return true;
+    }
+    corruptedVertex = -1;
+    return false;
+}
+
+WeightedEdgeGraph RamRun::buildGraph()
+{
+    WeightedEdgeGraph graph;
     for (size_t j = 0; j < vertexReferences_.getNRows(); j++)
     {
         for (size_t i = 0; i < vertexReferences_.getNColumns(); i++)
@@ -95,6 +132,8 @@ void RamRun::buildGraph(WeightedEdgeGraph& graph)
             }
         }
     }
+
+    return graph;
 }
 
 int RamRun::getVertex(WeightedEdgeGraph& graph, const size_t x, const size_t y)
@@ -105,3 +144,17 @@ int RamRun::getVertex(WeightedEdgeGraph& graph, const size_t x, const size_t y)
     }
     return vertexReferences_[y][x];
 }
+
+void RamRun::resetVertexReferences()
+{
+    for (size_t j = 0; j < vertexReferences_.getNRows(); j++)
+    {
+        for (size_t i = 0; i < vertexReferences_.getNColumns(); i++)
+        {
+            if (vertexReferences_[j][i] != getNoVertexReference())
+            {
+                vertexReferences_[j][i] = getUnknownVertexReference();
+            }
+        }
+    }
+}

src/ReindeerMaze.cpp

@@ -45,9 +45,9 @@ void ReindeerMaze::finish()
     auto exit = graph.addVertex(0);
     buildPathSegmentGraph(graph, vertexAttachedPositions, entry, exit);
 
-    auto shortestDistances = graph.dijkstra(exit);
-    part1 = shortestDistances[entry] / 2;
-    part2 = calcShortestPaths(graph, vertexAttachedPositions, entry, exit, shortestDistances);
+    auto result = graph.dijkstra(exit);
+    part1 = result.distances[entry] / 2;
+    part2 = calcShortestPaths(graph, vertexAttachedPositions, entry, exit, result.distances);
 }
 
 constexpr char ReindeerMaze::getStartChar()

src/common/WeightedEdgeGraph.cpp

@@ -49,13 +49,13 @@ int WeightedEdgeGraph::getEdgeWeight(const int edge) const
     return edgeWeights_[edge];
 }
 
-std::vector<int> WeightedEdgeGraph::dijkstra(const int source) const
+WeightedEdgeGraph::PathsResult WeightedEdgeGraph::dijkstra(const int source) const
 {
-    std::vector<int> distances(firstVertexIncidences_.size(), std::numeric_limits<int>::max());
-    auto compare = [&distances](int left, int right) { return distances[left] > distances[right]; };
+    PathsResult result(firstVertexIncidences_.size(), getInfiniteDistance(), -1);
+    auto compare = [&result](int left, int right) { return result.distances[left] > result.distances[right]; };
     std::priority_queue<int, std::vector<int>, decltype(compare)> queue{ compare };
 
-    distances[source] = 0;
+    result.distances[source] = 0;
     queue.push(source);
 
     while (!queue.empty())
@@ -65,16 +65,22 @@ std::vector<int> WeightedEdgeGraph::dijkstra(const int source) const
 
         for (auto neighbor = begin(v); neighbor != end(); ++neighbor)
         {
-            int newDistance{ distances[v] + edgeWeights_[neighbor->edge] };
-            if (distances[neighbor->vertex] > newDistance)
+            int newDistance{ result.distances[v] + edgeWeights_[neighbor->edge] };
+            if (result.distances[neighbor->vertex] > newDistance)
             {
-                distances[neighbor->vertex] = newDistance;
+                result.distances[neighbor->vertex] = newDistance;
+                result.predecessors[neighbor->vertex] = v;
                 queue.push(neighbor->vertex);
             }
         }
     }
 
-    return distances;
+    return result;
+}
+
+constexpr int WeightedEdgeGraph::getInfiniteDistance()
+{
+    return std::numeric_limits<int>::max();
 }
 
 WeightedEdgeGraph::NeighborIterator WeightedEdgeGraph::begin(const int vertex) const

tests/src/TestCases.cpp

@@ -332,11 +332,11 @@ TEST_CASE("[RamRunTests]")
     TestContext test;
     SECTION("FullData")
     {
-        test.run(std::make_unique<RamRun>(), 248, "", test.getInputPaths());
+        test.run(std::make_unique<RamRun>(), 248, "32,55", test.getInputPaths());
     }
     SECTION("ExampleData")
     {
-        test.run(std::make_unique<RamRun>(7, 12), 22, "", test.getExampleInputPaths());
+        test.run(std::make_unique<RamRun>(7, 12), 22, "6,1", test.getExampleInputPaths());
     }
 }