tree_diameter.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
 
// lal includes
#include <lal/graphs/tree.hpp>
#include <lal/graphs/rooted_tree.hpp>
#include <lal/detail/graphs/traversal.hpp>
#include <lal/detail/data_array.hpp>
 
namespace lal {
namespace detail {
 
template <
    class tree_t,
    std::enable_if_t< std::is_base_of_v<graphs::tree, tree_t>, bool> = true
>
uint64_t tree_diameter(const tree_t& t, node x) noexcept
{
    {
    const auto ccsize = t.get_num_nodes_component(x);
    if (ccsize == 1) { return 0; }
    if (ccsize == 2) { return 1; }
    if (ccsize == 3) { return 2; }
    }
 
    const uint64_t n = t.get_num_nodes();
 
    BFS<tree_t> bfs(t);
 
    if constexpr (std::is_base_of_v<graphs::rooted_tree, tree_t>) {
    bfs.set_use_rev_edges(true);
    }
    else {
    bfs.set_use_rev_edges(false);
    }
 
    node farthest_from_x;
 
    // calculate the farthest vertex from a starting 'random' vertex
    bfs.set_process_neighbour
    ( [&](const auto&, node, node v, bool) { farthest_from_x = v; } );
    bfs.start_at(x);
 
    // calculate the longest distance from 'farthest_from_0'
    uint64_t diameter = 0;
    data_array<uint64_t> distance(n, 0);
 
    bfs.clear_visited();
    bfs.clear_queue();
 
    bfs.set_process_neighbour(
    [&](const auto&, node u, node v, bool) {
        distance[v] = distance[u] + 1; diameter = std::max(diameter, distance[v]); }
    );
    bfs.start_at(farthest_from_x);
 
    return diameter;
}
 
} // -- namespace detail
} // -- namespace lal