hv_fair_r_policy.cpp

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 *   Copyright (C) 2004-2015 The PaGMO development team,                     *
 *   Advanced Concepts Team (ACT), European Space Agency (ESA)               *
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#include <algorithm>
#include <boost/numeric/conversion/cast.hpp>
#include <utility>
#include <vector>
#include <algorithm>

#include "../population.h"
#include "base.h"
#include "base_r_policy.h"
#include "hv_fair_r_policy.h"
#include "fair_r_policy.h"

#include "../util/hypervolume.h"


using namespace pagmo::util;

namespace pagmo { namespace migration {


hv_fair_r_policy::hv_fair_r_policy(const double &rate, rate_type type, const double nadir_eps):base_r_policy(rate,type), m_nadir_eps(nadir_eps) {}

base_r_policy_ptr hv_fair_r_policy::clone() const
{
        return base_r_policy_ptr(new hv_fair_r_policy(*this));
}

// Selection implementation.
std::vector<std::pair<population::size_type,std::vector<population::individual_type>::size_type> >
        hv_fair_r_policy::select(const std::vector<population::individual_type> &immigrants, const population &dest) const
{
        // Fall back to fair_r_policy when facing a single-objective problem.
        if (dest.problem().get_f_dimension() == 1) {
                return fair_r_policy(m_rate, m_type).select(immigrants, dest);
        }

        std::vector<population::individual_type> filtered_immigrants;
        filtered_immigrants.reserve(immigrants.size());

        // Keeps information on the original indexing of immigrants after we filter out the duplicates
        std::vector<unsigned int> original_immigrant_indices;
        original_immigrant_indices.reserve(immigrants.size());

        // Remove the duplicates from the set of immigrants
        std::vector<population::individual_type>::iterator im_it = (const_cast<std::vector<population::individual_type> &>(immigrants)).begin();
        unsigned int im_idx = 0;
        for( ; im_it != immigrants.end() ; ++im_it) {
                decision_vector im_x((*im_it).cur_x);

                bool equal = true;
                for ( unsigned int idx = 0 ; idx < dest.size() ; ++idx ) {
                        decision_vector isl_x(dest.get_individual(idx).cur_x);
                        equal = true;
                        for (unsigned int d_idx = 0 ; d_idx < im_x.size() ; ++d_idx) {
                                if (im_x[d_idx] != isl_x[d_idx]) {
                                        equal = false;
                                        break;
                                }
                        }
                        if (equal) {
                                break;
                        }
                }
                if (!equal) {
                        filtered_immigrants.push_back(*im_it);
                        original_immigrant_indices.push_back(im_idx);
                }
                ++im_idx;
        }

        // Computes the number of immigrants to be selected (accounting for the destination pop size)
        const population::size_type rate_limit = std::min<population::size_type>(get_n_individuals(dest), boost::numeric_cast<population::size_type>(filtered_immigrants.size()));

        // Defines the retvalue
        std::vector<std::pair<population::size_type, std::vector<population::individual_type>::size_type> > result;

        // Skip the remaining computation if there's nothing to do
        if (rate_limit == 0) {
                return result;
        }

        // Makes a copy of the destination population
        population pop_copy(dest);

        // Merge the immigrants to the copy of the destination population
        for (population::size_type i  = 0; i < rate_limit; ++i) {
                pop_copy.push_back(filtered_immigrants[i].cur_x);
        }

        // Population fronts stored as indices of individuals.
        std::vector< std::vector<population::size_type> > fronts_i = pop_copy.compute_pareto_fronts();

        // Population fronts stored as fitness vectors of individuals.
        std::vector< std::vector<fitness_vector> > fronts_f (fronts_i.size());

        // Nadir point is established manually later, first point is a first "safe" candidate.
        fitness_vector refpoint(pop_copy.get_individual(0).cur_f);

        // Fill fronts_f with fitness vectors and establish the nadir point
        for (unsigned int f_idx = 0 ; f_idx < fronts_i.size() ; ++f_idx) {
                fronts_f[f_idx].resize(fronts_i[f_idx].size());
                for (unsigned int p_idx = 0 ; p_idx < fronts_i[f_idx].size() ; ++p_idx) {
                        fronts_f[f_idx][p_idx] = fitness_vector(pop_copy.get_individual(fronts_i[f_idx][p_idx]).cur_f);

                        // Update the nadir point manually for efficiency.
                        for (unsigned int d_idx = 0 ; d_idx < fronts_f[f_idx][p_idx].size() ; ++d_idx) {
                                refpoint[d_idx] = std::max(refpoint[d_idx], fronts_f[f_idx][p_idx][d_idx]);
                        }
                }
        }

        // Epsilon is added to nadir point
        for (unsigned int d_idx = 0 ; d_idx < refpoint.size() ; ++d_idx) {
                refpoint[d_idx] += m_nadir_eps;
        }

        // Vector for maintaining the original indices of points for augmented population as 0 and 1
        std::vector<unsigned int> g_orig_indices(pop_copy.size(), 1);

        unsigned int no_discarded_immigrants = 0;

        // Store which front we process (start with the last front) and the number of processed individuals.
        unsigned int front_idx = fronts_i.size(); // front_idx is equal to the size, since it's decremented right in the main loop
        unsigned int processed_individuals = 0;

        // Pairs of (islander index, islander exclusive hypervolume)
        // Second item is updated later
        std::vector<std::pair<unsigned int, double> > discarded_islanders;

        std::vector<std::pair<unsigned int, double> > point_pairs;
        // index of currently processed point in the point_pair vector.
        // Initiated to its size (=0) in order to enforce the initial computation on penultimate front.
        unsigned int current_point = point_pairs.size();

        // Stops when we reduce the augmented population to the size of the original population or when the number of discarded islanders reaches the limit
        while (processed_individuals < filtered_immigrants.size() && discarded_islanders.size() < rate_limit) {

                // if current front was exhausted, load next one
                if (current_point == point_pairs.size()) {
                        --front_idx;

                        // Compute contributions
                        std::vector<double> c;

                        // If there exist a dominated front for front at index front_idx
                        if (front_idx + 1 < fronts_f.size()) {
                                std::vector<fitness_vector> merged_front;
                                // Reserve the memory and copy the fronts
                                merged_front.reserve(fronts_f[front_idx].size() + fronts_f[front_idx + 1].size());

                                copy(fronts_f[front_idx].begin(), fronts_f[front_idx].end(), back_inserter(merged_front));
                                copy(fronts_f[front_idx + 1].begin(), fronts_f[front_idx +1].end(), back_inserter(merged_front));

                                hypervolume hv(merged_front, false);
                                c = hv.contributions(refpoint);
                        } else {
                                hypervolume hv(fronts_f[front_idx], false);
                                c = hv.contributions(refpoint);
                        }

                        // Initiate the pairs and sort by second item (exclusive volume)
                        point_pairs.resize(fronts_f[front_idx].size());
                        for(unsigned int i = 0 ; i < fronts_f[front_idx].size() ; ++i) {
                                point_pairs[i] = std::make_pair(i, c[i]);
                        }
                        current_point = 0;
                        std::sort(point_pairs.begin(), point_pairs.end(), sort_point_pairs_asc);
                }

                unsigned int orig_lc_idx = fronts_i[front_idx][point_pairs[current_point].first];

                if (orig_lc_idx < dest.size()) {
                        discarded_islanders.push_back(std::make_pair(orig_lc_idx, 0.0));
                } else {
                        ++no_discarded_immigrants;
                }

                // Flag given individual as discarded
                g_orig_indices[orig_lc_idx] = 0;

                ++processed_individuals;
                ++current_point;
        }

        // Number of non-discarded immigrants
        unsigned int no_available_immigrants = boost::numeric_cast<unsigned int>(filtered_immigrants.size() - no_discarded_immigrants);

        // Pairs of (immigrant index, immigrant exclusive hypervolume)
        // Second item is updated later
        std::vector<std::pair<unsigned int, double> > available_immigrants;
        available_immigrants.reserve(no_available_immigrants);
        for(unsigned int idx = dest.size() ; idx < pop_copy.size() ; ++idx) {
                // If the immigrant was not discarded add it to the available set
                if ( g_orig_indices[idx] == 1 ) {
                        available_immigrants.push_back(std::make_pair(idx, 0.0));
                }
        }

        // Aggregate all points to establish the hypervolume contribution of available immigrants and discarded islanders
        std::vector<fitness_vector> merged_fronts;
        merged_fronts.reserve(pop_copy.size());

        for(unsigned int idx = 0 ; idx < pop_copy.size() ; ++idx) {
                merged_fronts.push_back(pop_copy.get_individual(idx).cur_f);
        }

        hypervolume hv(merged_fronts, false);
        std::vector<std::pair<unsigned int, double> >::iterator it;

        for(it = available_immigrants.begin() ; it != available_immigrants.end() ; ++it) {
                (*it).second = hv.exclusive((*it).first, refpoint);
        }

        for(it = discarded_islanders.begin() ; it != discarded_islanders.end() ; ++it) {
                (*it).second = hv.exclusive((*it).first, refpoint);
        }

        // Sort islanders and immigrants according to exclusive hypervolume
        sort(available_immigrants.begin(), available_immigrants.end(), hv_fair_r_policy::ind_cmp);
        sort(discarded_islanders.begin(), discarded_islanders.end(), hv_fair_r_policy::ind_cmp);

        // Number of exchanges is the minimum of the number of non discarded immigrants and the number of discarded islanders
        unsigned int no_exchanges = std::min(boost::numeric_cast<unsigned int>(available_immigrants.size()), boost::numeric_cast<unsigned int>(discarded_islanders.size()));

        it = available_immigrants.begin();
        std::vector<std::pair<unsigned int, double> >::reverse_iterator r_it = discarded_islanders.rbegin();

        // Match the best immigrant (forward iterator) with the worst islander (reverse iterator) no_exchanges times.
        for(unsigned int i = 0 ; i < no_exchanges ; ++i) {
                // Break if any islander is better than an immigrant
                if ((*r_it).second > (*it).second) {
                        break;
                }
                // Push the pair (islander_idx, fixed_immigrant_idx) to the results
                result.push_back(std::make_pair((*r_it).first, original_immigrant_indices[(*it).first - dest.size()]));
                ++r_it;
                ++it;
        }

        return result;
}

bool hv_fair_r_policy::sort_point_pairs_asc(const std::pair<unsigned int, double> &a, const std::pair<unsigned int, double> &b)
{
        return a.second < b.second;
}

bool hv_fair_r_policy::ind_cmp(const std::pair<unsigned int, double> &a, const std::pair<unsigned int, double> &b)
{
        return a.second > b.second;
}

}}

BOOST_CLASS_EXPORT_IMPLEMENT(pagmo::migration::hv_fair_r_policy)