// 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/>.

#pragma once

#include <iterator>
#include <vector>

class GraphBase
{
public:
    size_t getNVertices() const;
    size_t getNEdges() const;
    bool areAdjacent(const int vertex1, const int vertex2) const;

    struct Incidence
    {
    public:
        Incidence(const int vertex, const int edge)
            : vertex{ vertex }, edge{ edge }
        {
        }
        int vertex;
        int edge;
    };

    struct NeighborIterator
    {
    public:
        // Iterator traits.
        using difference_type = std::ptrdiff_t;
        using value_type = Incidence;
        using pointer = const value_type*;
        using reference = const value_type&;
        using iterator_category = std::forward_iterator_tag;

        NeighborIterator(const GraphBase& graph, const int vertex)
            : graph_{ graph }, incidence_{ -1 }, value_{ -1, -1 }
        {
            if (vertex >= 0 && vertex < graph_.firstVertexIncidences.size())
            {
                setIncidence(graph_.firstVertexIncidences[vertex]);
            }
        };
        NeighborIterator& operator++()
        {
            if (incidence_ >= 0)
            {
                setIncidence(graph_.nextIncidences[incidence_]);
            }
            return *this;
        }
        NeighborIterator operator++(int)
        {
            NeighborIterator it = *this;
            ++(*this);
            return it;
        }
        bool operator==(NeighborIterator other) const
        {
            return incidence_ == other.incidence_;
        }
        bool operator!=(NeighborIterator other) const
        {
            return !(*this == other);
        }
        const reference operator*() const
        {
            return value_;
        }
        const pointer operator->() const
        {
            return &value_;
        }

    private:
        const GraphBase& graph_;
        int incidence_;
        value_type value_;
        void setIncidence(const int incidence)
        {
            incidence_ = incidence;
            if (incidence_ >= 0)
            {
                value_ = { graph_.incidenceVertices[incidence_], incidence_ >> 1 };
            }
        }
    };

    NeighborIterator begin(const int vertex) const;
    NeighborIterator end() const;

protected:
    std::vector<int> firstVertexIncidences{};
    std::vector<int> nextIncidences{};
    std::vector<int> incidenceVertices{};
    int addVertexInternal();
    void addEdgeInternal(const int vertex1, const int vertex2);
};