// Copyright 2010 The Trustees of Indiana University.
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// Distributed under the Boost Software License, Version 1.0.
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// (See accompanying file LICENSE_1_0.txt or copy at
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// http://www.boost.org/LICENSE_1_0.txt)
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// Authors: Jeremiah Willcock
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// Andrew Lumsdaine
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#ifndef BOOST_GRAPH_LOOP_ERASED_RANDOM_WALK_HPP
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#define BOOST_GRAPH_LOOP_ERASED_RANDOM_WALK_HPP
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#include <boost/graph/graph_traits.hpp>
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#include <boost/graph/properties.hpp>
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#include <boost/graph/random.hpp>
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#include <boost/next_prior.hpp>
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#include <vector>
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#include <boost/assert.hpp>
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namespace boost
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{
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struct BOOST_SYMBOL_VISIBLE loop_erased_random_walk_stuck
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: public std::exception
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{
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virtual ~loop_erased_random_walk_stuck() throw() {}
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inline virtual const char* what() const throw()
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{
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return "Loop-erased random walk found a vertex with no out-edges";
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}
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};
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// Do a loop-erased random walk from vertex s to any vertex colored black (or
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// actually any color other than white or gray) in the color map. The color
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// white is for vertices that are not part of the path, while gray is for
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// those that are on the path (for cycle detection). The vector path is used
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// for temporary storage and as the result of the algorithm; while all
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// elements of the path except the last have their colors set to gray upon
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// return. Vertex s must start off colored white.
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//
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// Useful references:
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// http://everything2.com/title/loop-erased+random+walk
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// Wikipedia page on "Loop-Erased Random Walk"
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template < typename Graph, typename ColorMap, typename NextEdge >
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void loop_erased_random_walk(const Graph& g,
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typename boost::graph_traits< Graph >::vertex_descriptor s,
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NextEdge next_edge, ColorMap color,
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std::vector< typename boost::graph_traits< Graph >::vertex_descriptor >&
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path)
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{
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typedef typename boost::graph_traits< Graph >::vertex_descriptor
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vertex_descriptor;
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typedef
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typename boost::graph_traits< Graph >::edge_descriptor edge_descriptor;
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typedef typename boost::property_traits< ColorMap >::value_type color_t;
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typedef boost::color_traits< color_t > color_gen;
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BOOST_ASSERT(get(color, s) == color_gen::white());
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path.clear();
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path.push_back(s);
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put(color, s, color_gen::gray());
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while (true)
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{
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edge_descriptor e = next_edge(s, g);
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vertex_descriptor t = target(e, g);
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color_t t_color = get(color, t);
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if (t_color == color_gen::white())
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{
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path.push_back(t);
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put(color, t, color_gen::gray());
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s = t;
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}
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else if (t_color == color_gen::gray())
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{
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// Found a loop; delete from path from the first occurrence of t to
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// the end, coloring vertices white.
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typename std::vector< vertex_descriptor >::iterator it
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= std::find(path.begin(), path.end(), t);
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BOOST_ASSERT(it != path.end());
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++it;
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for (typename std::vector< vertex_descriptor >::iterator j = it;
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j != path.end(); ++j)
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{
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put(color, *j, color_gen::white());
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}
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path.erase(it, path.end());
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s = t;
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}
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else
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{
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// Done
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path.push_back(t);
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break;
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}
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}
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}
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template < typename Graph, typename Gen > class unweighted_random_out_edge_gen
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{
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Gen& gen;
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typedef boost::graph_traits< Graph > gt;
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public:
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unweighted_random_out_edge_gen(Gen& gen) : gen(gen) {}
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typename gt::edge_descriptor operator()(
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typename gt::vertex_descriptor src, const Graph& g) const
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{
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if (out_degree(src, g) == 0)
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throw loop_erased_random_walk_stuck();
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return boost::random_out_edge(g, src, gen);
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}
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};
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template < typename Graph, typename WeightMap, typename Gen >
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class weighted_random_out_edge_gen
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{
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WeightMap weight;
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Gen& gen;
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typedef boost::graph_traits< Graph > gt;
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public:
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weighted_random_out_edge_gen(const WeightMap& weight, Gen& gen)
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: weight(weight), gen(gen)
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{
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}
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typename gt::edge_descriptor operator()(
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typename gt::vertex_descriptor src, const Graph& g) const
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{
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if (out_degree(src, g) == 0)
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throw loop_erased_random_walk_stuck();
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return boost::weighted_random_out_edge(g, src, weight, gen);
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}
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};
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}
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#endif // BOOST_GRAPH_LOOP_ERASED_RANDOM_WALK_HPP
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