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7<title>Parallel BGL Connected Components</title>
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11<div class="document" id="logo-connected-components">
12<h1 class="title"><a class="reference external" href="http://www.osl.iu.edu/research/pbgl"><img align="middle" alt="Parallel BGL" class="align-middle" src="pbgl-logo.png" /></a> Connected Components</h1>
13
14<!-- Copyright (C) 2004-2008 The Trustees of Indiana University.
15Use, modification and distribution is subject to the Boost Software
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18<pre class="literal-block">
19template&lt;typename Graph, typename ComponentMap&gt;
20inline typename property_traits&lt;ComponentMap&gt;::value_type
21strong_components( const Graph&amp; g, ComponentMap c);
22
23namespace graph {
24 template&lt;typename Graph, typename VertexComponentMap&gt;
25 void
26 fleischer_hendrickson_pinar_strong_components(const Graph&amp; g, VertexComponentMap r);
27
28 template&lt;typename Graph, typename ReverseGraph,
29 typename ComponentMap, typename IsoMapFR, typename IsoMapRF&gt;
30 inline typename property_traits&lt;ComponentMap&gt;::value_type
31 fleischer_hendrickson_pinar_strong_components(const Graph&amp; g,
32 ComponentMap c,
33 const ReverseGraph&amp; gr,
34 IsoMapFR fr, IsoMapRF rf);
35}
36</pre>
37<p>The <tt class="docutils literal"><span class="pre">strong_components()</span></tt> function computes the strongly connected
38components of a directed graph. The distributed strong components
39algorithm uses the <a class="reference external" href="http://www.boost.org/libs/graph/doc/strong_components.html">sequential strong components</a> algorithm to
40identify components local to a processor. The distributed portion of
41the algorithm is built on the <a class="reference external" href="breadth_first_search.html">distributed breadth first search</a>
42algorithm and is based on the work of Fleischer, Hendrickson, and
43Pinar <a class="citation-reference" href="#fhp00" id="id1">[FHP00]</a>. The interface is a superset of the interface to the
44BGL <a class="reference external" href="http://www.boost.org/libs/graph/doc/strong_components.html">sequential strong components</a> algorithm. The number of
45strongly-connected components in the graph is returned to all
46processes.</p>
47<p>The distributed strong components algorithm works on both <tt class="docutils literal"><span class="pre">directed</span></tt>
48and <tt class="docutils literal"><span class="pre">bidirectional</span></tt> graphs. In the bidirectional case, a reverse
49graph adapter is used to produce the required reverse graph. In
50the directed case, a separate graph is constructed in which all the
51edges are reversed.</p>
52<div class="contents topic" id="contents">
53<p class="topic-title first">Contents</p>
54<ul class="simple">
55<li><a class="reference internal" href="#where-defined" id="id2">Where Defined</a></li>
56<li><a class="reference internal" href="#parameters" id="id3">Parameters</a></li>
57<li><a class="reference internal" href="#complexity" id="id4">Complexity</a></li>
58<li><a class="reference internal" href="#algorithm-description" id="id5">Algorithm Description</a></li>
59<li><a class="reference internal" href="#bibliography" id="id6">Bibliography</a></li>
60</ul>
61</div>
62<div class="section" id="where-defined">
63<h1><a class="toc-backref" href="#id2">Where Defined</a></h1>
64<p>&lt;<tt class="docutils literal"><span class="pre">boost/graph/distributed/strong_components.hpp</span></tt>&gt;</p>
65<p>also accessible from</p>
66<p>&lt;<tt class="docutils literal"><span class="pre">boost/graph/strong_components.hpp</span></tt>&gt;</p>
67</div>
68<div class="section" id="parameters">
69<h1><a class="toc-backref" href="#id3">Parameters</a></h1>
70<dl class="docutils">
71<dt>IN: <tt class="docutils literal"><span class="pre">const</span> <span class="pre">Graph&amp;</span> <span class="pre">g</span></tt></dt>
72<dd>The graph type must be a model of <a class="reference external" href="DistributedGraph.html">Distributed Graph</a>. The graph
73type must also model the <a class="reference external" href="http://www.boost.org/libs/graph/doc/IncidenceGraph.html">Incidence Graph</a> and be directed.</dd>
74<dt>OUT: <tt class="docutils literal"><span class="pre">ComponentMap</span> <span class="pre">c</span></tt></dt>
75<dd>The algorithm computes how many strongly connected components are in the
76graph, and assigns each component an integer label. The algorithm
77then records to which component each vertex in the graph belongs by
78recording the component number in the component property map. The
79<tt class="docutils literal"><span class="pre">ComponentMap</span></tt> type must be a <a class="reference external" href="distributed_property_map.html">Distributed Property Map</a>. The
80value type must be the <tt class="docutils literal"><span class="pre">vertices_size_type</span></tt> of the graph. The key
81type must be the graph's vertex descriptor type.</dd>
82<dt>UTIL: <tt class="docutils literal"><span class="pre">VertexComponentMap</span> <span class="pre">r</span></tt></dt>
83<dd>The algorithm computes a mapping from each vertex to the
84representative of the strong component, stored in this property map.
85The <tt class="docutils literal"><span class="pre">VertexComponentMap</span></tt> type must be a <a class="reference external" href="distributed_property_map.html">Distributed Property Map</a>.
86The value and key types must be the vertex descriptor of the graph.</dd>
87<dt>IN: <tt class="docutils literal"><span class="pre">const</span> <span class="pre">ReverseGraph&amp;</span> <span class="pre">gr</span></tt></dt>
88<dd><p class="first">The reverse (or transpose) graph of <tt class="docutils literal"><span class="pre">g</span></tt>, such that for each
89directed edge <em>(u, v)</em> in <tt class="docutils literal"><span class="pre">g</span></tt> there exists a directed edge
90<em>(fr(v), fr(u))</em> in <tt class="docutils literal"><span class="pre">gr</span></tt> and for each edge <em>(v', u')</em> in <em>gr</em>
91there exists an edge <em>(rf(u'), rf(v'))</em> in <tt class="docutils literal"><span class="pre">g</span></tt>. The functions
92<em>fr</em> and <em>rf</em> map from vertices in the graph to the reverse graph
93and vice-verse, and are represented as property map arguments. The
94concept requirements on this graph type are equivalent to those on
95the <tt class="docutils literal"><span class="pre">Graph</span></tt> type, but the types need not be the same.</p>
96<p class="last"><strong>Default</strong>: Either a <tt class="docutils literal"><span class="pre">reverse_graph</span></tt> adaptor over the original
97graph (if the graph type is bidirectional, i.e., models the
98<a class="reference external" href="http://www.boost.org/libs/graph/doc/BidirectionalGraph.html">Bidirectional Graph</a> concept) or a <a class="reference external" href="distributed_adjacency_list.html">distributed adjacency list</a>
99constructed from the input graph.</p>
100</dd>
101<dt>IN: <tt class="docutils literal"><span class="pre">IsoMapFR</span> <span class="pre">fr</span></tt></dt>
102<dd><p class="first">A property map that maps from vertices in the input graph <tt class="docutils literal"><span class="pre">g</span></tt> to
103vertices in the reversed graph <tt class="docutils literal"><span class="pre">gr</span></tt>. The type <tt class="docutils literal"><span class="pre">IsoMapFR</span></tt> must
104model the <a class="reference external" href="http://www.boost.org/libs/property_map/ReadablePropertyMap.html">Readable Property Map</a> concept and have the graph's
105vertex descriptor as its key type and the reverse graph's vertex
106descriptor as its value type.</p>
107<p class="last"><strong>Default</strong>: An identity property map (if the graph type is
108bidirectional) or a distributed <tt class="docutils literal"><span class="pre">iterator_property_map</span></tt> (if the
109graph type is directed).</p>
110</dd>
111<dt>IN: <tt class="docutils literal"><span class="pre">IsoMapRF</span> <span class="pre">rf</span></tt></dt>
112<dd><p class="first">A property map that maps from vertices in the reversed graph <tt class="docutils literal"><span class="pre">gr</span></tt>
113to vertices in the input graph <tt class="docutils literal"><span class="pre">g</span></tt>. The type <tt class="docutils literal"><span class="pre">IsoMapRF</span></tt> must
114model the <a class="reference external" href="http://www.boost.org/libs/property_map/ReadablePropertyMap.html">Readable Property Map</a> concept and have the reverse
115graph's vertex descriptor as its key type and the graph's vertex
116descriptor as its value type.</p>
117<p class="last"><strong>Default</strong>: An identity property map (if the graph type is
118bidirectional) or a distributed <tt class="docutils literal"><span class="pre">iterator_property_map</span></tt> (if the
119graph type is directed).</p>
120</dd>
121</dl>
122</div>
123<div class="section" id="complexity">
124<h1><a class="toc-backref" href="#id4">Complexity</a></h1>
125<p>The local phase of the algorithm is <em>O(V + E)</em>. The parallel phase of
126the algorithm requires at most <em>O(V)</em> supersteps each containing two
127breadth first searches which are <em>O(V + E)</em> each.</p>
128</div>
129<div class="section" id="algorithm-description">
130<h1><a class="toc-backref" href="#id5">Algorithm Description</a></h1>
131<p>Prior to executing the sequential phase of the algorithm, each process
132identifies any completely local strong components which it labels and
133removes from the vertex set considered in the parallel phase of the
134algorithm.</p>
135<p>The parallel phase of the distributed strong components algorithm
136consists of series of supersteps. Each superstep starts with one
137or more vertex sets which are guaranteed to completely contain
138any remaining strong components. A <a class="reference external" href="breadth_first_search.html">distributed breadth first search</a>
139is performed starting from the first vertex in each vertex set.
140All of these breadth first searches are performed in parallel by having
141each processor call <tt class="docutils literal"><span class="pre">breadth_first_search()</span></tt> with a different starting
142vertex, and if necessary inserting additional vertices into the
143<tt class="docutils literal"><span class="pre">distributed</span> <span class="pre">queue</span></tt> used for breadth first search before invoking
144the algorithm. A second <a class="reference external" href="breadth_first_search.html">distributed breadth first search</a> is
145performed on the reverse graph in the same fashion. For each initial
146vertex set, the successor set (the vertices reached in the forward
147breadth first search), and the predecessor set (the vertices reached
148in the backward breadth first search) is computed. The intersection
149of the predecessor and successor sets form a strongly connected
150component which is labeled as such. The remaining vertices in the
151initial vertex set are partitioned into three subsets each guaranteed
152to completely contain any remaining strong components. These three sets
153are the vertices in the predecessor set not contained in the identified
154strongly connected component, the vertices in the successor set not
155in the strongly connected component, and the remaing vertices in the
156initial vertex set not in the predecessor or successor sets. Once
157new vertex sets are identified, the algorithm begins a new superstep.
158The algorithm halts when no vertices remain.</p>
159<p>To boost performance in sparse graphs when identifying small components,
160when less than a given portion of the initial number of vertices
161remain in active vertex sets, a filtered graph adapter is used
162to limit the graph seen by the breadth first search algorithm to the
163active vertices.</p>
164</div>
165<div class="section" id="bibliography">
166<h1><a class="toc-backref" href="#id6">Bibliography</a></h1>
167<table class="docutils citation" frame="void" id="fhp00" rules="none">
168<colgroup><col class="label" /><col /></colgroup>
169<tbody valign="top">
170<tr><td class="label"><a class="fn-backref" href="#id1">[FHP00]</a></td><td>Lisa Fleischer, Bruce Hendrickson, and Ali Pinar. On
171Identifying Strongly Connected Components in Parallel. In Parallel and
172Distributed Processing (IPDPS), volume 1800 of Lecture Notes in
173Computer Science, pages 505--511, 2000. Springer.</td></tr>
174</tbody>
175</table>
176<hr class="docutils" />
177<p>Copyright (C) 2004, 2005 The Trustees of Indiana University.</p>
178<p>Authors: Nick Edmonds, Douglas Gregor, and Andrew Lumsdaine</p>
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