Q_iterator.hpp

/*********************************************************************
 *
 *  Linear Arrangement Library - A library that implements a collection
 *  algorithms for linear arrangments of graphs.
 *
 *  Copyright (C) 2019 - 2021
 *
 *  This file is part of Linear Arrangement Library. To see the full code
 *  visit the webpage:
 *      https://github.com/lluisalemanypuig/linear-arrangement-library.git
 *
 *  Linear Arrangement Library is free software: you can redistribute it
 *  and/or modify it under the terms of the GNU Affero General Public License
 *  as published by the Free Software Foundation, either version 3 of the
 *  License, or (at your option) any later version.
 *
 *  Linear Arrangement Library 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 Affero General Public License for more details.
 *
 *  You should have received a copy of the GNU Affero General Public License
 *  along with Linear Arrangement Library.  If not, see <http://www.gnu.org/licenses/>.
 *
 *  Contact:
 *
 *      Lluís Alemany Puig (lalemany@cs.upc.edu)
 *          LARCA (Laboratory for Relational Algorithmics, Complexity and Learning)
 *          CQL (Complexity and Quantitative Linguistics Lab)
 *          Jordi Girona St 1-3, Campus Nord UPC, 08034 Barcelona.   CATALONIA, SPAIN
 *          Webpage: https://cqllab.upc.edu/people/lalemany/
 *
 *      Ramon Ferrer i Cancho (rferrericancho@cs.upc.edu)
 *          LARCA (Laboratory for Relational Algorithmics, Complexity and Learning)
 *          CQL (Complexity and Quantitative Linguistics Lab)
 *          Office S124, Omega building
 *          Jordi Girona St 1-3, Campus Nord UPC, 08034 Barcelona.   CATALONIA, SPAIN
 *          Webpage: https://cqllab.upc.edu/people/rferrericancho/
 *
 ********************************************************************/
 
#pragma once
 
// C++ includes
#if defined DEBUG
#include <cassert>
#endif
#include <type_traits>
#include <tuple>
 
// lal includes
#include <lal/graphs/directed_graph.hpp>
#include <lal/graphs/undirected_graph.hpp>
 
namespace lal {
namespace iterators {
 
template<
    typename GRAPH,
    bool is_directed = std::is_base_of_v<graphs::directed_graph, GRAPH>,
    std::enable_if_t<
        std::is_base_of_v<graphs::directed_graph, GRAPH> ||
        std::is_base_of_v<graphs::undirected_graph, GRAPH>
        , bool
    > = true
>
class Q_iterator {
public:
    /* CONSTRUCTORS */
 
    Q_iterator(const GRAPH& g) noexcept : m_G(g) {
        reset();
    }
 
    ~Q_iterator() = default;
 
    /* GETTERS */
 
    inline bool end() const noexcept { return m_reached_end; }
 
    inline edge_pair get_edge_pair() const noexcept { return m_cur_pair; }
 
    /* MODIFIERS */
 
    inline void next() noexcept {
        if (not m_exists_next) {
            m_reached_end = true;
            return;
        }
 
        m_cur_pair = make_current_pair();
#if defined DEBUG
        assert(not share_nodes(m_cur_pair));
#endif
 
        // find the next edge
        auto [found, new_cur1, new_cur2] =
            find_next_pair(
                m_cur1.first, m_cur1.second,
                m_cur2.first, m_cur2.second + 1
            );
        m_exists_next = found;
        m_cur1 = new_cur1;
        m_cur2 = new_cur2;
    }
 
    inline void reset() noexcept {
        __reset();
        next();
    }
 
private:
    typedef std::pair<node,std::size_t> E_pointer;
 
private:
 
    const GRAPH& m_G;
 
    E_pointer m_cur1 = E_pointer(0,0);
    E_pointer m_cur2 = E_pointer(0,0);
 
    bool m_exists_next = true;
    bool m_reached_end = false;
    edge_pair m_cur_pair = { {0,0}, {0,0} };
 
private:
    inline void __reset() noexcept {
        // there are not enough edges to have |Q| > 0
        if (m_G.get_num_edges() <= 1) {
            m_exists_next = false;
            m_reached_end = true;
            return;
        }
 
        m_cur1 = E_pointer(0, 0);
        m_cur2 = E_pointer(1, static_cast<size_t>(-1));
 
        const auto [found, new_cur1, new_cur2] =
            find_next_pair(
                m_cur1.first, m_cur1.second,
                m_cur2.first, m_cur2.second + 1
            );
        if (not found) {
            // if we can't find the next pair, then there is no next...
            m_exists_next = false;
            m_reached_end = true;
            return;
        }
 
#if defined DEBUG
        assert(not share_nodes(new_cur1, new_cur2));
#endif
 
        // since a pair was found, store it in current
        m_cur1 = new_cur1;
        m_cur2 = new_cur2;
        m_cur_pair = make_current_pair();
 
        // how do we know there is a next edge?
        // well, find it!
        const auto [f2, _, __] =
            find_next_pair(
                m_cur1.first, m_cur1.second,
                m_cur2.first, m_cur2.second + 1
            );
 
        m_exists_next = f2;
 
        // this is the case of the graph
        // having only one independent pair of edges
        if (not m_exists_next) {
            m_exists_next = true;
            m_reached_end = false;
        }
    }
 
    inline edge_pair make_current_pair() const noexcept {
        node s,t,u,v;
        if constexpr (is_directed) {
            s = m_cur1.first;
            t = m_G.get_out_neighbours(s)[m_cur1.second];
            u = m_cur2.first;
            v = m_G.get_out_neighbours(u)[m_cur2.second];
        }
        else {
            s = m_cur1.first;
            t = m_G.get_neighbours(s)[m_cur1.second];
            u = m_cur2.first;
            v = m_G.get_neighbours(u)[m_cur2.second];
        }
        return edge_pair(edge(s,t), edge(u,v));
    }
 
    static inline bool share_nodes(const edge_pair& st_uv) noexcept {
        const auto [s,t] = st_uv.first;
        const auto [u,v] = st_uv.second;
        return s == u or s == v or t == u or t == v;
    }
 
    inline
    bool share_nodes(const E_pointer& p1, const E_pointer& p2)
    const noexcept
    {
        node s, t, u, v;
        if constexpr (is_directed) {
            s = p1.first;
            t = m_G.get_out_neighbours(s)[p1.second];
            u = p2.first;
            v = m_G.get_out_neighbours(u)[p2.second];
        }
        else {
            s = p1.first;
            t = m_G.get_neighbours(s)[p1.second];
            u = p2.first;
            v = m_G.get_neighbours(u)[p2.second];
        }
        return share_nodes(edge_pair(edge(s,t),edge(u,v)));
    }
 
    template<bool isdir = is_directed, std::enable_if_t<isdir, bool> = true>
    inline std::tuple<bool, E_pointer, E_pointer>
    find_next_pair(node s, std::size_t pt, node u, std::size_t pv)
    noexcept
    {
        const uint32_t n = m_G.get_num_nodes();
 
        // base case 1: consumed all pairs
        if (s == n) {
            return make_tuple(false, E_pointer(s,pt), E_pointer(u,pv));
        }
        // base case 2: consumed neighbours of 's'
        if (pt >= m_G.get_out_degree(s)) {
            return find_next_pair(s+1, 0, s+2, 0);
        }
        // base case 3: consumed second pointer
        if (u == n) {
            // advance the first pointer
            return find_next_pair(s, pt + 1, s + 1, 0);
        }
        // base case 4: consumed neighbours of 'u'
        if (pv >= m_G.get_out_degree(u)) {
            // advance second pointer
            return find_next_pair(s, pt, u + 1, 0);
        }
 
        if (share_nodes(E_pointer(s,pt), E_pointer(u,pv))) {
            return find_next_pair(s, pt, u, pv + 1);
        }
 
        return make_tuple(true, E_pointer(s,pt), E_pointer(u,pv));
    }
 
    template<bool isdir = is_directed, std::enable_if_t<not isdir, bool> = true>
    inline std::tuple<bool, E_pointer, E_pointer>
    find_next_pair(node s, std::size_t pt, node u, std::size_t pv)
    noexcept
    {
        // FOR GOD'S SAKE! DO NOT USE 'STATIC'!!!
        const uint32_t n = m_G.get_num_nodes();
 
        // base case 1: consumed all pairs
        if (s == n) {
            return std::make_tuple(false, E_pointer(s,pt), E_pointer(u,pv));
        }
        // base case 2: consumed neighbours of 's'
        if (pt >= m_G.get_degree(s)) {
            return find_next_pair(s+1, 0, s+2, 0);
        }
        // base case 3: consumed second pointer
        if (u == n) {
            // advance the first pointer
            return find_next_pair(s, pt + 1, s + 1, 0);
        }
        // base case 4: consumed neighbours of 'u'
        if (pv >= m_G.get_degree(u)) {
            // advance second pointer
            return find_next_pair(s, pt, u + 1, 0);
        }
 
        auto Ns = m_G.get_neighbours(s);
        if (s > Ns[pt]) {
            return find_next_pair(s, pt+1, u, 0);
        }
 
        auto Nu = m_G.get_neighbours(u);
        if (u > Nu[pv] or share_nodes(E_pointer(s,pt), E_pointer(u,pv))) {
            return find_next_pair(s, pt, u, pv + 1);
        }
 
        return make_tuple(true, E_pointer(s,pt), E_pointer(u,pv));
    }
 
};
 
} // -- namespace iterators
} // -- namespace lal