AdventOfCode2024/src/ReindeerMaze.cpp

// Solutions to the Advent Of Code 2024.
// Copyright (C) 2025  Stefan Müller
//
// This program is free software: you can redistribute it and/or modify it under
// the terms of the GNU General Public License as published by the Free Software
// Foundation, either version 3 of the License, or (at your option) any later
// version.
//
// This program is distributed in the hope that it will be useful, but WITHOUT
// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
// FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License along with
// this program.  If not, see <http://www.gnu.org/licenses/>.

#include <aoc/ReindeerMaze.hpp>

ReindeerMaze::ReindeerMaze(const int inputFileNameSuffix)
    : LinesSolver{ inputFileNameSuffix }
{
}

const std::string ReindeerMaze::getPuzzleName() const
{
    return "Reindeer Maze";
}

const int ReindeerMaze::getPuzzleDay() const
{
    return 16;
}

void ReindeerMaze::finish()
{
    WeightedEdgeGraph graph{};
    auto entry = graph.addVertex();
    auto exit = graph.addVertex();
    buildPathSegmentGraph(graph, entry, exit);

    part1 = graph.dijkstra(entry, exit) / 2;
}

constexpr char ReindeerMaze::getStartChar()
{
    return 'S';
}

constexpr char ReindeerMaze::getEndChar()
{
    return 'E';
}

constexpr char ReindeerMaze::getWallChar()
{
    return '#';
}

constexpr int ReindeerMaze::getTurnCost()
{
    return 1000;
}

// Constructs the graph of path segment incidences, starting with a graph that already contains the entry and the exit
// vertices.
void ReindeerMaze::buildPathSegmentGraph(WeightedEdgeGraph& graph, const int entry, const int exit)
{
    // Uses list for work items to prevent invalidation of iterators on add.
    std::list<ReindeerMazeCrossing> crossings{};
    initializeWorkList(crossings, entry);

    while (!crossings.empty())
    {
        auto startCrossing{ --crossings.end() };
        Point2 startPosition = startCrossing->getPosition();
        auto incidence = std::find_if(startCrossing->incidences.begin(), startCrossing->incidences.end(),
            [](auto& x) { return x.getPathVertex() == -1; });
        Point2 direction{ incidence->getDirection() };
        Point2 backwards{ -direction };
        Point2 position{ startPosition + direction };

        int pathCost{ 1 };
        bool isSkipped{ false };

        int nNext{ 0 };
        while (getCharAt(position) != getEndChar() &&
               std::find_if(crossings.begin(), crossings.end(),
                   [position](auto& x) { return x.getPosition() == position; }) == crossings.end())
        {
            nNext = 0;
            Point2 nextPosition, nextDirection;
            for (const auto& checkDirection : Point2::cardinalDirections)
            {
                if (checkDirection != backwards)
                {
                    Point2 checkPosition{ checkDirection + position };
                    if (getCharAt(checkPosition) != getWallChar())
                    {
                        nNext++;
                        if (nNext == 1)
                        {
                            // Found a possible next step.
                            nextPosition = checkPosition;
                            nextDirection = checkDirection;
                        }
                        else if (nNext == 2)
                        {
                            // Found a second possible step, i.e. a new crossing. Will stop processing this path
                            // segment.
                            crossings.emplace_back(position);
                            crossings.back().incidences.emplace_back(backwards);
                            crossings.back().incidences.emplace_back(nextDirection);
                            crossings.back().incidences.emplace_back(checkDirection);
                        }
                        else
                        {
                            // Found another possible step. Adds the incidence to the new crossing.
                            crossings.back().incidences.emplace_back(checkDirection);
                        }
                    }
                }
            }

            if (nNext == 1)
            {
                position = nextPosition;
                pathCost++;
                if (direction != nextDirection)
                {
                    pathCost += getTurnCost();
                    direction = nextDirection;
                    backwards = -direction;
                }

                if (position == startPosition)
                {
                    // Stops processing this path segment because it is a loop, and remove both incidences from the
                    // start crossing. Both must exist here.
                    startCrossing->incidences.erase(incidence);
                    auto endIncidence = std::find_if(startCrossing->incidences.begin(), startCrossing->incidences.end(),
                        [backwards](auto& x) { return x.getDirection() == backwards; });
                    startCrossing->incidences.erase(endIncidence);
                    isSkipped = true;
                    break;
                }
            }
            else if (nNext == 0)
            {
                // Stops processing this path segment because it is a dead-end, and remove it from the start crossing.
                startCrossing->incidences.erase(incidence);
                isSkipped = true;
                break;
            }
            else
            {
                // Stops processing this path segment because a new crossing has been found. This check avoids the
                // find_if() call in the loop condition.
                break;
            }
        }

        if (!isSkipped)
        {
            // Adds the new maze path segment as a vertex.
            auto pathVertex = graph.addVertex();

            // Updates start crossing.
            incidence->setPathVertex(pathVertex);
            incidence->setPathCost(pathCost);

            // Determines the end crossing of the new path segment and its incidence.
            std::list<ReindeerMazeCrossing>::iterator endCrossing;
            std::vector<ReindeerMazePathIncidence>::iterator endIncidence;
            endCrossing = nNext == 1 ? endCrossing = std::find_if(crossings.begin(), crossings.end(),
                                           [position](auto& x) { return x.getPosition() == position; })
                                     : --crossings.end();
            if (endCrossing != crossings.end())
            {
                // This incidence must exist, no need to check the incidence iterator.
                endIncidence = std::find_if(endCrossing->incidences.begin(), endCrossing->incidences.end(),
                    [backwards](auto& x) { return x.getDirection() == backwards; });
                endIncidence->setPathVertex(pathVertex);
                endIncidence->setPathCost(pathCost);
            }

            // Connects the new path segment to all adjacent path segments.
            AddPathSegmentEdges(graph, *incidence, startCrossing->incidences);
            if (endCrossing != crossings.end())
            {
                AddPathSegmentEdges(graph, *endIncidence, endCrossing->incidences);
            }
            else
            {
                graph.addEdge(pathVertex, exit, pathCost);
            }

            // Checks if end crossing is finished.
            // checkFinishedCrossing(crossings, endCrossing);
            if (endCrossing != crossings.end() && endCrossing->isFinished())
            {
                crossings.erase(endCrossing);
            }
        }

        // Checks if start crossing is finished.
        if (startCrossing->isFinished())
        {
            crossings.erase(startCrossing);
        }
    }
}

// Initializes the work list of crossing incidences.
void ReindeerMaze::initializeWorkList(std::list<ReindeerMazeCrossing>& crossings, const int entryVertex)
{
    Point2 start{ findStart() };
    crossings.emplace_back(start);
    crossings.back().incidences.emplace_back(Point2::left, entryVertex);
    addCheckedIncidence(crossings.back().incidences, start, Point2::right);
    addCheckedIncidence(crossings.back().incidences, start, Point2::up);
}

void ReindeerMaze::addCheckedIncidence(std::vector<ReindeerMazePathIncidence>& incidences, const Point2 start,
    const Point2 direction)
{
    if (getCharAt(start + direction) != getWallChar())
    {
        incidences.emplace_back(direction);
    }
}

Point2 ReindeerMaze::findStart() const
{
    for (int j = 0; j < lines.size(); j++)
    {
        for (int i = 0; i < lines[j].size(); i++)
        {
            if (lines[j][i] == getStartChar())
            {
                return { i, j };
            }
        }
    }
    return { 0, 0 };
}

void ReindeerMaze::AddPathSegmentEdges(WeightedEdgeGraph& graph, const ReindeerMazePathIncidence& pathIncidence,
    const std::vector<ReindeerMazePathIncidence>& otherPathIncidences)
{
    for (auto& otherIncidence : otherPathIncidences)
    {
        if (otherIncidence.getPathVertex() > -1 && otherIncidence.getPathVertex() != pathIncidence.getPathVertex())
        {
            int weight{ pathIncidence.getPathCost() + otherIncidence.getPathCost() };
            // Checks for turn, i.e. perpendicular directions.
            if (otherIncidence.getDirection().x != -pathIncidence.getDirection().x)
            {
                weight += 2 * getTurnCost();
            }
            graph.addEdge(pathIncidence.getPathVertex(), otherIncidence.getPathVertex(), weight);
        }
    }
}