Q_iterator.hpp

/*********************************************************************
 *
 * Linear Arrangement Library - A library that implements a collection
 * algorithms for linear arrangments of graphs.
 *
 * Copyright (C) 2019 - 2023
 *
 * This file is part of Linear Arrangement Library. The full code is available
 * at:
 *      https://github.com/LAL-project/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 <limits>
#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), m_n(m_G.get_num_nodes()) {
        reset();
    }
 
    ~Q_iterator() = default;
 
    /* GETTERS */
 
    bool end() const noexcept { return m_reached_end; }
 
    const edge_pair& get_edge_pair() const noexcept { return m_cur_pair; }
 
    edge_pair_t get_edge_pair_t() const noexcept { return m_cur_pair; }
 
    edge_pair yield_edge_pair() noexcept {
        const auto e = get_edge_pair();
        next();
        return e;
    }
 
    /* MODIFIERS */
 
    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.first.first, m_cur_pair.first.second,
                   m_cur_pair.second.first, m_cur_pair.second.second));
#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;
    }
 
    void reset() noexcept {
        __reset();
        next();
    }
 
private:
    typedef std::pair<node,std::size_t> E_pointer;
 
private:
 
    const GRAPH& m_G;
    const uint64_t m_n;
 
    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:
    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.first = m_cur1.second = 0;
        m_cur2.first = 1;
        m_cur2.second = std::numeric_limits<std::size_t>::max();
 
        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_pointer(
                   new_cur1.first, new_cur1.second,
                   new_cur2.first, new_cur2.second));
#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;
        }
    }
 
    edge_pair make_current_pair() const noexcept {
        node s,t,u,v;
        s = m_cur1.first;
        u = m_cur2.first;
        if constexpr (is_directed) {
            t = m_G.get_out_neighbours(s)[m_cur1.second];
            v = m_G.get_out_neighbours(u)[m_cur2.second];
        }
        else {
            t = m_G.get_neighbours(s)[m_cur1.second];
            v = m_G.get_neighbours(u)[m_cur2.second];
        }
        return {{s,t}, {u,v}};
    }
 
    bool share_nodes_pointer(
        const node s, const std::size_t pt,
        const node u, const std::size_t pv
    )
    const noexcept
    {
        node t, v;
        if constexpr (is_directed) {
            t = m_G.get_out_neighbours(s)[pt];
            v = m_G.get_out_neighbours(u)[pv];
        }
        else {
            t = m_G.get_neighbours(s)[pt];
            v = m_G.get_neighbours(u)[pv];
        }
        return share_nodes(s,t, u,v);
    }
 
    bool share_nodes(const node s, const node t, const node u, const node v)
    const noexcept
    { return s == u or s == v or t == u or t == v; }
 
    template <bool isdir = is_directed, std::enable_if_t<isdir, bool> = true>
    std::tuple<bool, E_pointer, E_pointer>
    find_next_pair(node s, std::size_t pt, node u, std::size_t pv)
    noexcept
    {
        // base case 1: consumed all pairs
        if (s == m_n) {
            return {false, {s,pt}, {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 == m_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_pointer(s,pt, u,pv)) {
            return find_next_pair(s, pt, u, pv + 1);
        }
 
        return {true, {s,pt}, {u,pv}};
    }
 
    template <bool isdir = is_directed, std::enable_if_t<not isdir, bool> = true>
    std::tuple<bool, E_pointer, E_pointer>
    find_next_pair(node s, std::size_t pt, node u, std::size_t pv)
    noexcept
    {
        // base case 1: consumed all pairs
        if (s == m_n) {
            return {false, {s,pt}, {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 == m_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);
        }
 
        const auto& Ns = m_G.get_neighbours(s);
        if (s > Ns[pt]) {
            return find_next_pair(s, pt+1, u, 0);
        }
 
        const auto& Nu = m_G.get_neighbours(u);
        if (u > Nu[pv] or share_nodes_pointer(s,pt, u,pv)) {
            return find_next_pair(s, pt, u, pv + 1);
        }
 
        return {true, {s,pt}, {u,pv}};
    }
 
};
 
} // -- namespace iterators
} // -- namespace lal