AdventOfCode2024/src/LanParty.cpp

// Solutions to the Advent Of Code 2024.
// Copyright (C) 2024-2025  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/LanParty.hpp>

#include <algorithm>
#include <numeric>

const std::string LanParty::getPuzzleName() const
{
    return "LAN Party";
}

const int LanParty::getPuzzleDay() const
{
    return 23;
}

void LanParty::processDataLine(const std::string& line)
{
    lan_.addEdge(findOrAddVertex(line.substr(0, 2)), findOrAddVertex(line.substr(3, 2)));
}

void LanParty::finish()
{
    size_t targetCliqueNumber{ 0 };
    auto itTx = labelMap_.lower_bound(getFirstTxComputerName());
    const auto itTxEnd = labelMap_.upper_bound(getLastTxComputerName());
    while (itTx != itTxEnd)
    {
        computeInterconnectedThreeSetCount(itTx->second);
        targetCliqueNumber = std::max(targetCliqueNumber, lan_.getDegree(itTx->second));
        ++itTx;
    }

    // Assumes that the graph is connected, but not complete. Otherwise we would start with targetCliqueNumber + 1.
    // Also assumes that the clique number of the graph is close to its maximum degree, hence starting with a large
    // target number.
    bool isFound{ false };
    while (!isFound && targetCliqueNumber > 2)
    {
        auto itTx = labelMap_.lower_bound(getFirstTxComputerName());
        while (!isFound && itTx != itTxEnd)
        {
            isFound = findCompleteSubgraph(itTx->second, targetCliqueNumber);
            ++itTx;
        }
        --targetCliqueNumber;
    }
}

constexpr std::string LanParty::getFirstTxComputerName()
{
    return "ta";
}

constexpr std::string LanParty::getLastTxComputerName()
{
    return "tz";
}

int LanParty::findOrAddVertex(const std::string& vertexId)
{
    const auto found = labelMap_.find(vertexId);
    if (found != labelMap_.end())
    {
        return found->second;
    }
    else
    {
        int vertex = lan_.addVertex(vertexId);
        labelMap_.insert({ vertexId, vertex });
        return vertex;
    }
}

void LanParty::computeInterconnectedThreeSetCount(const int vertexTx)
{
    auto itFirstNeighbor = lan_.begin(vertexTx);
    while (itFirstNeighbor != lan_.end())
    {
        if (canProcessVertex(itFirstNeighbor->vertex, vertexTx))
        {
            auto itSecondNeighbor = itFirstNeighbor;
            ++itSecondNeighbor;
            while (itSecondNeighbor != lan_.end())
            {
                if (canProcessVertex(itSecondNeighbor->vertex, vertexTx))
                {
                    if (lan_.areAdjacent(itFirstNeighbor->vertex, itSecondNeighbor->vertex))
                    {
                        ++part1;
                    }
                }
                ++itSecondNeighbor;
            }
        }
        ++itFirstNeighbor;
    }
}

bool LanParty::canProcessVertex(const int vertexToCheck, const int vertexTx) const
{
    const std::string& label{ lan_.getVertexData(vertexToCheck) };
    return (label[0] != 't' || label > lan_.getVertexData(vertexTx));
}

bool LanParty::findCompleteSubgraph(const int vertex, const size_t targetSize)
{
    // Validates that the degree of the start vertex is actually high enough for the target clique size.
    if (targetSize > lan_.getDegree(vertex) + 1)
    {
        return false;
    }

    // Neighbors of start vertex.
    std::vector<int> neighbors{};
    neighbors.reserve(lan_.getDegree(vertex));
    auto it = lan_.begin(vertex);
    while (it != lan_.end())
    {
        neighbors.push_back(it->vertex);
        ++it;
    }

    // Combination of neighbors, i.e. 'neighbor[i]' is selected if and only if 'combination[i]' is true.
    std::vector<bool> combination(targetSize - 1, true);
    combination.resize(neighbors.size());

    do
    {
        if (isSubgraphComplete(neighbors, combination))
        {
            part2 = calcPassword(neighbors, combination, vertex);
            return true;
        }
    } while (std::prev_permutation(combination.begin(), combination.end()));

    return false;
}

bool LanParty::isSubgraphComplete(const std::vector<int>& neighbors, const std::vector<bool>& combination) const
{
    for (size_t i = 0; i < combination.size(); ++i)
    {
        if (combination[i])
        {
            for (size_t j = i + 1; j < combination.size(); ++j)
            {
                if (combination[j] && !lan_.areAdjacent(neighbors[i], neighbors[j]))
                {
                    return false;
                }
            }
        }
    }
    return true;
}

std::string LanParty::calcPassword(const std::vector<int>& neighbors, const std::vector<bool>& combination,
    const int vertex) const
{
    std::vector<std::string> completeSubgraph{ lan_.getVertexData(vertex) };

    for (size_t i = 0; i < combination.size(); ++i)
    {
        if (combination[i])
        {
            completeSubgraph.push_back(lan_.getVertexData(neighbors[i]));
        }
    }
    std::sort(completeSubgraph.begin(), completeSubgraph.end());
    return std::accumulate(++completeSubgraph.begin(), completeSubgraph.end(), *completeSubgraph.begin(),
        [](const std::string& acc, const std::string& x) { return acc + "," + x; });
}