boost/graph/push_relabel_max_flow.hpp
//======================================================================= // Copyright 2000 University of Notre Dame. // Authors: Jeremy G. Siek, Andrew Lumsdaine, Lie-Quan Lee // // Distributed under the Boost Software License, Version 1.0. (See // accompanying file LICENSE_1_0.txt or copy at // http://www.boost.org/LICENSE_1_0.txt) //======================================================================= #ifndef BOOST_PUSH_RELABEL_MAX_FLOW_HPP #define BOOST_PUSH_RELABEL_MAX_FLOW_HPP #include <boost/config.hpp> #include <boost/assert.hpp> #include <vector> #include <list> #include <iosfwd> #include <algorithm> // for std::min and std::max #include <boost/pending/queue.hpp> #include <boost/limits.hpp> #include <boost/graph/graph_concepts.hpp> #include <boost/graph/named_function_params.hpp> namespace boost { namespace detail { // This implementation is based on Goldberg's // "On Implementing Push-Relabel Method for the Maximum Flow Problem" // by B.V. Cherkassky and A.V. Goldberg, IPCO '95, pp. 157--171 // and on the h_prf.c and hi_pr.c code written by the above authors. // This implements the highest-label version of the push-relabel method // with the global relabeling and gap relabeling heuristics. // The terms "rank", "distance", "height" are synonyms in // Goldberg's implementation, paper and in the CLR. A "layer" is a // group of vertices with the same distance. The vertices in each // layer are categorized as active or inactive. An active vertex // has positive excess flow and its distance is less than n (it is // not blocked). template < class Vertex > struct preflow_layer { std::list< Vertex > active_vertices; std::list< Vertex > inactive_vertices; }; template < class Graph, class EdgeCapacityMap, // integer value type class ResidualCapacityEdgeMap, class ReverseEdgeMap, class VertexIndexMap, // vertex_descriptor -> integer class FlowValue > class push_relabel { public: typedef graph_traits< Graph > Traits; typedef typename Traits::vertex_descriptor vertex_descriptor; typedef typename Traits::edge_descriptor edge_descriptor; typedef typename Traits::vertex_iterator vertex_iterator; typedef typename Traits::out_edge_iterator out_edge_iterator; typedef typename Traits::vertices_size_type vertices_size_type; typedef typename Traits::edges_size_type edges_size_type; typedef preflow_layer< vertex_descriptor > Layer; typedef std::vector< Layer > LayerArray; typedef typename LayerArray::iterator layer_iterator; typedef typename LayerArray::size_type distance_size_type; typedef color_traits< default_color_type > ColorTraits; //======================================================================= // Some helper predicates inline bool is_admissible(vertex_descriptor u, vertex_descriptor v) { return get(distance, u) == get(distance, v) + 1; } inline bool is_residual_edge(edge_descriptor a) { return 0 < get(residual_capacity, a); } inline bool is_saturated(edge_descriptor a) { return get(residual_capacity, a) == 0; } //======================================================================= // Layer List Management Functions typedef typename std::list< vertex_descriptor >::iterator list_iterator; void add_to_active_list(vertex_descriptor u, Layer& layer) { BOOST_USING_STD_MIN(); BOOST_USING_STD_MAX(); layer.active_vertices.push_front(u); max_active = max BOOST_PREVENT_MACRO_SUBSTITUTION( get(distance, u), max_active); min_active = min BOOST_PREVENT_MACRO_SUBSTITUTION( get(distance, u), min_active); layer_list_ptr[u] = layer.active_vertices.begin(); } void remove_from_active_list(vertex_descriptor u) { layers[get(distance, u)].active_vertices.erase(layer_list_ptr[u]); } void add_to_inactive_list(vertex_descriptor u, Layer& layer) { layer.inactive_vertices.push_front(u); layer_list_ptr[u] = layer.inactive_vertices.begin(); } void remove_from_inactive_list(vertex_descriptor u) { layers[get(distance, u)].inactive_vertices.erase(layer_list_ptr[u]); } //======================================================================= // initialization push_relabel(Graph& g_, EdgeCapacityMap cap, ResidualCapacityEdgeMap res, ReverseEdgeMap rev, vertex_descriptor src_, vertex_descriptor sink_, VertexIndexMap idx) : g(g_) , n(num_vertices(g_)) , capacity(cap) , src(src_) , sink(sink_) , index(idx) , excess_flow_data(num_vertices(g_)) , excess_flow(excess_flow_data.begin(), idx) , current_data(num_vertices(g_), out_edges(*vertices(g_).first, g_)) , current(current_data.begin(), idx) , distance_data(num_vertices(g_)) , distance(distance_data.begin(), idx) , color_data(num_vertices(g_)) , color(color_data.begin(), idx) , reverse_edge(rev) , residual_capacity(res) , layers(num_vertices(g_)) , layer_list_ptr_data( num_vertices(g_), layers.front().inactive_vertices.end()) , layer_list_ptr(layer_list_ptr_data.begin(), idx) , push_count(0) , update_count(0) , relabel_count(0) , gap_count(0) , gap_node_count(0) , work_since_last_update(0) { vertex_iterator u_iter, u_end; // Don't count the reverse edges edges_size_type m = num_edges(g) / 2; nm = alpha() * n + m; // Initialize flow to zero which means initializing // the residual capacity to equal the capacity. out_edge_iterator ei, e_end; for (boost::tie(u_iter, u_end) = vertices(g); u_iter != u_end; ++u_iter) for (boost::tie(ei, e_end) = out_edges(*u_iter, g); ei != e_end; ++ei) { put(residual_capacity, *ei, get(capacity, *ei)); } for (boost::tie(u_iter, u_end) = vertices(g); u_iter != u_end; ++u_iter) { vertex_descriptor u = *u_iter; put(excess_flow, u, 0); current[u] = out_edges(u, g); } bool overflow_detected = false; FlowValue test_excess = 0; out_edge_iterator a_iter, a_end; for (boost::tie(a_iter, a_end) = out_edges(src, g); a_iter != a_end; ++a_iter) if (target(*a_iter, g) != src) test_excess += get(residual_capacity, *a_iter); if (test_excess > (std::numeric_limits< FlowValue >::max)()) overflow_detected = true; if (overflow_detected) put(excess_flow, src, (std::numeric_limits< FlowValue >::max)()); else { put(excess_flow, src, 0); for (boost::tie(a_iter, a_end) = out_edges(src, g); a_iter != a_end; ++a_iter) { edge_descriptor a = *a_iter; vertex_descriptor tgt = target(a, g); if (tgt != src) { ++push_count; FlowValue delta = get(residual_capacity, a); put(residual_capacity, a, get(residual_capacity, a) - delta); edge_descriptor rev = get(reverse_edge, a); put(residual_capacity, rev, get(residual_capacity, rev) + delta); put(excess_flow, tgt, get(excess_flow, tgt) + delta); } } } max_distance = num_vertices(g) - 1; max_active = 0; min_active = n; for (boost::tie(u_iter, u_end) = vertices(g); u_iter != u_end; ++u_iter) { vertex_descriptor u = *u_iter; if (u == sink) { put(distance, u, 0); continue; } else if (u == src && !overflow_detected) put(distance, u, n); else put(distance, u, 1); if (get(excess_flow, u) > 0) add_to_active_list(u, layers[1]); else if (get(distance, u) < n) add_to_inactive_list(u, layers[1]); } } // push_relabel constructor //======================================================================= // This is a breadth-first search over the residual graph // (well, actually the reverse of the residual graph). // Would be cool to have a graph view adaptor for hiding certain // edges, like the saturated (non-residual) edges in this case. // Goldberg's implementation abused "distance" for the coloring. void global_distance_update() { BOOST_USING_STD_MAX(); ++update_count; vertex_iterator u_iter, u_end; for (boost::tie(u_iter, u_end) = vertices(g); u_iter != u_end; ++u_iter) { put(color, *u_iter, ColorTraits::white()); put(distance, *u_iter, n); } put(color, sink, ColorTraits::gray()); put(distance, sink, 0); for (distance_size_type l = 0; l <= max_distance; ++l) { layers[l].active_vertices.clear(); layers[l].inactive_vertices.clear(); } max_distance = max_active = 0; min_active = n; Q.push(sink); while (!Q.empty()) { vertex_descriptor u = Q.top(); Q.pop(); distance_size_type d_v = get(distance, u) + 1; out_edge_iterator ai, a_end; for (boost::tie(ai, a_end) = out_edges(u, g); ai != a_end; ++ai) { edge_descriptor a = *ai; vertex_descriptor v = target(a, g); if (get(color, v) == ColorTraits::white() && is_residual_edge(get(reverse_edge, a))) { put(distance, v, d_v); put(color, v, ColorTraits::gray()); current[v] = out_edges(v, g); max_distance = max BOOST_PREVENT_MACRO_SUBSTITUTION( d_v, max_distance); if (get(excess_flow, v) > 0) add_to_active_list(v, layers[d_v]); else add_to_inactive_list(v, layers[d_v]); Q.push(v); } } } } // global_distance_update() //======================================================================= // This function is called "push" in Goldberg's h_prf implementation, // but it is called "discharge" in the paper and in hi_pr.c. void discharge(vertex_descriptor u) { BOOST_ASSERT(get(excess_flow, u) > 0); while (1) { out_edge_iterator ai, ai_end; for (boost::tie(ai, ai_end) = current[u]; ai != ai_end; ++ai) { edge_descriptor a = *ai; if (is_residual_edge(a)) { vertex_descriptor v = target(a, g); if (is_admissible(u, v)) { ++push_count; if (v != sink && get(excess_flow, v) == 0) { remove_from_inactive_list(v); add_to_active_list(v, layers[get(distance, v)]); } push_flow(a); if (get(excess_flow, u) == 0) break; } } } // for out_edges of i starting from current Layer& layer = layers[get(distance, u)]; distance_size_type du = get(distance, u); if (ai == ai_end) { // i must be relabeled relabel_distance(u); if (layer.active_vertices.empty() && layer.inactive_vertices.empty()) gap(du); if (get(distance, u) == n) break; } else { // i is no longer active current[u].first = ai; add_to_inactive_list(u, layer); break; } } // while (1) } // discharge() //======================================================================= // This corresponds to the "push" update operation of the paper, // not the "push" function in Goldberg's h_prf.c implementation. // The idea is to push the excess flow from from vertex u to v. void push_flow(edge_descriptor u_v) { vertex_descriptor u = source(u_v, g), v = target(u_v, g); BOOST_USING_STD_MIN(); FlowValue flow_delta = min BOOST_PREVENT_MACRO_SUBSTITUTION( get(excess_flow, u), get(residual_capacity, u_v)); put(residual_capacity, u_v, get(residual_capacity, u_v) - flow_delta); edge_descriptor rev = get(reverse_edge, u_v); put(residual_capacity, rev, get(residual_capacity, rev) + flow_delta); put(excess_flow, u, get(excess_flow, u) - flow_delta); put(excess_flow, v, get(excess_flow, v) + flow_delta); } // push_flow() //======================================================================= // The main purpose of this routine is to set distance[v] // to the smallest value allowed by the valid labeling constraints, // which are: // distance[t] = 0 // distance[u] <= distance[v] + 1 for every residual edge (u,v) // distance_size_type relabel_distance(vertex_descriptor u) { BOOST_USING_STD_MAX(); ++relabel_count; work_since_last_update += beta(); distance_size_type min_distance = num_vertices(g); put(distance, u, min_distance); // Examine the residual out-edges of vertex i, choosing the // edge whose target vertex has the minimal distance. out_edge_iterator ai, a_end, min_edge_iter; for (boost::tie(ai, a_end) = out_edges(u, g); ai != a_end; ++ai) { ++work_since_last_update; edge_descriptor a = *ai; vertex_descriptor v = target(a, g); if (is_residual_edge(a) && get(distance, v) < min_distance) { min_distance = get(distance, v); min_edge_iter = ai; } } ++min_distance; if (min_distance < n) { put(distance, u, min_distance); // this is the main action current[u].first = min_edge_iter; max_distance = max BOOST_PREVENT_MACRO_SUBSTITUTION( min_distance, max_distance); } return min_distance; } // relabel_distance() //======================================================================= // cleanup beyond the gap void gap(distance_size_type empty_distance) { ++gap_count; distance_size_type r; // distance of layer before the current layer r = empty_distance - 1; // Set the distance for the vertices beyond the gap to "infinity". for (layer_iterator l = layers.begin() + empty_distance + 1; l < layers.begin() + max_distance; ++l) { list_iterator i; for (i = l->inactive_vertices.begin(); i != l->inactive_vertices.end(); ++i) { put(distance, *i, n); ++gap_node_count; } l->inactive_vertices.clear(); } max_distance = r; max_active = r; } //======================================================================= // This is the core part of the algorithm, "phase one". FlowValue maximum_preflow() { work_since_last_update = 0; while (max_active >= min_active) { // "main" loop Layer& layer = layers[max_active]; list_iterator u_iter = layer.active_vertices.begin(); if (u_iter == layer.active_vertices.end()) --max_active; else { vertex_descriptor u = *u_iter; remove_from_active_list(u); discharge(u); if (work_since_last_update * global_update_frequency() > nm) { global_distance_update(); work_since_last_update = 0; } } } // while (max_active >= min_active) return get(excess_flow, sink); } // maximum_preflow() //======================================================================= // remove excess flow, the "second phase" // This does a DFS on the reverse flow graph of nodes with excess flow. // If a cycle is found, cancel it. // Return the nodes with excess flow in topological order. // // Unlike the prefl_to_flow() implementation, we use // "color" instead of "distance" for the DFS labels // "parent" instead of nl_prev for the DFS tree // "topo_next" instead of nl_next for the topological ordering void convert_preflow_to_flow() { vertex_iterator u_iter, u_end; out_edge_iterator ai, a_end; vertex_descriptor r, restart, u; std::vector< vertex_descriptor > parent(n); std::vector< vertex_descriptor > topo_next(n); vertex_descriptor tos(parent[0]), bos(parent[0]); // bogus initialization, just to avoid warning bool bos_null = true; // handle self-loops for (boost::tie(u_iter, u_end) = vertices(g); u_iter != u_end; ++u_iter) for (boost::tie(ai, a_end) = out_edges(*u_iter, g); ai != a_end; ++ai) if (target(*ai, g) == *u_iter) put(residual_capacity, *ai, get(capacity, *ai)); // initialize for (boost::tie(u_iter, u_end) = vertices(g); u_iter != u_end; ++u_iter) { u = *u_iter; put(color, u, ColorTraits::white()); parent[get(index, u)] = u; current[u] = out_edges(u, g); } // eliminate flow cycles and topologically order the vertices for (boost::tie(u_iter, u_end) = vertices(g); u_iter != u_end; ++u_iter) { u = *u_iter; if (get(color, u) == ColorTraits::white() && get(excess_flow, u) > 0 && u != src && u != sink) { r = u; put(color, r, ColorTraits::gray()); while (1) { for (; current[u].first != current[u].second; ++current[u].first) { edge_descriptor a = *current[u].first; if (get(capacity, a) == 0 && is_residual_edge(a)) { vertex_descriptor v = target(a, g); if (get(color, v) == ColorTraits::white()) { put(color, v, ColorTraits::gray()); parent[get(index, v)] = u; u = v; break; } else if (get(color, v) == ColorTraits::gray()) { // find minimum flow on the cycle FlowValue delta = get(residual_capacity, a); while (1) { BOOST_USING_STD_MIN(); delta = min BOOST_PREVENT_MACRO_SUBSTITUTION( delta, get(residual_capacity, *current[v].first)); if (v == u) break; else v = target(*current[v].first, g); } // remove delta flow units v = u; while (1) { a = *current[v].first; put(residual_capacity, a, get(residual_capacity, a) - delta); edge_descriptor rev = get(reverse_edge, a); put(residual_capacity, rev, get(residual_capacity, rev) + delta); v = target(a, g); if (v == u) break; } // back-out of DFS to the first saturated // edge restart = u; for (v = target(*current[u].first, g); v != u; v = target(a, g)) { a = *current[v].first; if (get(color, v) == ColorTraits::white() || is_saturated(a)) { put(color, target(*current[v].first, g), ColorTraits::white()); if (get(color, v) != ColorTraits::white()) restart = v; } } if (restart != u) { u = restart; ++current[u].first; break; } } // else if (color[v] == ColorTraits::gray()) } // if (get(capacity, a) == 0 ... } // for out_edges(u, g) (though "u" changes during // loop) if (current[u].first == current[u].second) { // scan of i is complete put(color, u, ColorTraits::black()); if (u != src) { if (bos_null) { bos = u; bos_null = false; tos = u; } else { topo_next[get(index, u)] = tos; tos = u; } } if (u != r) { u = parent[get(index, u)]; ++current[u].first; } else break; } } // while (1) } // if (color[u] == white && excess_flow[u] > 0 & ...) } // for all vertices in g // return excess flows // note that the sink is not on the stack if (!bos_null) { for (u = tos; u != bos; u = topo_next[get(index, u)]) { boost::tie(ai, a_end) = out_edges(u, g); while (get(excess_flow, u) > 0 && ai != a_end) { if (get(capacity, *ai) == 0 && is_residual_edge(*ai)) push_flow(*ai); ++ai; } } // do the bottom u = bos; boost::tie(ai, a_end) = out_edges(u, g); while (get(excess_flow, u) > 0 && ai != a_end) { if (get(capacity, *ai) == 0 && is_residual_edge(*ai)) push_flow(*ai); ++ai; } } } // convert_preflow_to_flow() //======================================================================= inline bool is_flow() { vertex_iterator u_iter, u_end; out_edge_iterator ai, a_end; // check edge flow values for (boost::tie(u_iter, u_end) = vertices(g); u_iter != u_end; ++u_iter) { for (boost::tie(ai, a_end) = out_edges(*u_iter, g); ai != a_end; ++ai) { edge_descriptor a = *ai; if (get(capacity, a) > 0) if ((get(residual_capacity, a) + get( residual_capacity, get(reverse_edge, a)) != get(capacity, a) + get(capacity, get(reverse_edge, a))) || (get(residual_capacity, a) < 0) || (get(residual_capacity, get(reverse_edge, a)) < 0)) return false; } } // check conservation FlowValue sum; for (boost::tie(u_iter, u_end) = vertices(g); u_iter != u_end; ++u_iter) { vertex_descriptor u = *u_iter; if (u != src && u != sink) { if (get(excess_flow, u) != 0) return false; sum = 0; for (boost::tie(ai, a_end) = out_edges(u, g); ai != a_end; ++ai) if (get(capacity, *ai) > 0) sum -= get(capacity, *ai) - get(residual_capacity, *ai); else sum += get(residual_capacity, *ai); if (get(excess_flow, u) != sum) return false; } } return true; } // is_flow() bool is_optimal() { // check if mincut is saturated... global_distance_update(); return get(distance, src) >= n; } void print_statistics(std::ostream& os) const { os << "pushes: " << push_count << std::endl << "relabels: " << relabel_count << std::endl << "updates: " << update_count << std::endl << "gaps: " << gap_count << std::endl << "gap nodes: " << gap_node_count << std::endl << std::endl; } void print_flow_values(std::ostream& os) const { os << "flow values" << std::endl; vertex_iterator u_iter, u_end; out_edge_iterator ei, e_end; for (boost::tie(u_iter, u_end) = vertices(g); u_iter != u_end; ++u_iter) for (boost::tie(ei, e_end) = out_edges(*u_iter, g); ei != e_end; ++ei) if (get(capacity, *ei) > 0) os << *u_iter << " " << target(*ei, g) << " " << (get(capacity, *ei) - get(residual_capacity, *ei)) << std::endl; os << std::endl; } //======================================================================= Graph& g; vertices_size_type n; vertices_size_type nm; EdgeCapacityMap capacity; vertex_descriptor src; vertex_descriptor sink; VertexIndexMap index; // will need to use random_access_property_map with these std::vector< FlowValue > excess_flow_data; iterator_property_map< typename std::vector< FlowValue >::iterator, VertexIndexMap > excess_flow; std::vector< std::pair< out_edge_iterator, out_edge_iterator > > current_data; iterator_property_map< typename std::vector< std::pair< out_edge_iterator, out_edge_iterator > >::iterator, VertexIndexMap > current; std::vector< distance_size_type > distance_data; iterator_property_map< typename std::vector< distance_size_type >::iterator, VertexIndexMap > distance; std::vector< default_color_type > color_data; iterator_property_map< std::vector< default_color_type >::iterator, VertexIndexMap > color; // Edge Property Maps that must be interior to the graph ReverseEdgeMap reverse_edge; ResidualCapacityEdgeMap residual_capacity; LayerArray layers; std::vector< list_iterator > layer_list_ptr_data; iterator_property_map< typename std::vector< list_iterator >::iterator, VertexIndexMap > layer_list_ptr; distance_size_type max_distance; // maximal distance distance_size_type max_active; // maximal distance with active node distance_size_type min_active; // minimal distance with active node boost::queue< vertex_descriptor > Q; // Statistics counters long push_count; long update_count; long relabel_count; long gap_count; long gap_node_count; inline double global_update_frequency() { return 0.5; } inline vertices_size_type alpha() { return 6; } inline long beta() { return 12; } long work_since_last_update; }; } // namespace detail template < class Graph, class CapacityEdgeMap, class ResidualCapacityEdgeMap, class ReverseEdgeMap, class VertexIndexMap > typename property_traits< CapacityEdgeMap >::value_type push_relabel_max_flow( Graph& g, typename graph_traits< Graph >::vertex_descriptor src, typename graph_traits< Graph >::vertex_descriptor sink, CapacityEdgeMap cap, ResidualCapacityEdgeMap res, ReverseEdgeMap rev, VertexIndexMap index_map) { typedef typename property_traits< CapacityEdgeMap >::value_type FlowValue; detail::push_relabel< Graph, CapacityEdgeMap, ResidualCapacityEdgeMap, ReverseEdgeMap, VertexIndexMap, FlowValue > algo(g, cap, res, rev, src, sink, index_map); FlowValue flow = algo.maximum_preflow(); algo.convert_preflow_to_flow(); BOOST_ASSERT(algo.is_flow()); BOOST_ASSERT(algo.is_optimal()); return flow; } // push_relabel_max_flow() template < class Graph, class P, class T, class R > typename detail::edge_capacity_value< Graph, P, T, R >::type push_relabel_max_flow(Graph& g, typename graph_traits< Graph >::vertex_descriptor src, typename graph_traits< Graph >::vertex_descriptor sink, const bgl_named_params< P, T, R >& params) { return push_relabel_max_flow(g, src, sink, choose_const_pmap(get_param(params, edge_capacity), g, edge_capacity), choose_pmap(get_param(params, edge_residual_capacity), g, edge_residual_capacity), choose_const_pmap(get_param(params, edge_reverse), g, edge_reverse), choose_const_pmap(get_param(params, vertex_index), g, vertex_index)); } template < class Graph > typename property_traits< typename property_map< Graph, edge_capacity_t >::const_type >::value_type push_relabel_max_flow(Graph& g, typename graph_traits< Graph >::vertex_descriptor src, typename graph_traits< Graph >::vertex_descriptor sink) { bgl_named_params< int, buffer_param_t > params(0); // bogus empty param return push_relabel_max_flow(g, src, sink, params); } } // namespace boost #endif // BOOST_PUSH_RELABEL_MAX_FLOW_HPP