# HG changeset patch
# User Peter Kovacs <kpeter@inf.elte.hu>
# Date 1266193098 -3600
# Node ID bc3ef6652f1bffa5e9d3661ad9a92c43b0cb1a45
# Parent  42b0128ae0a781ec9f206167f70538c16face6c0
Add a preliminary page of graph adaptors

diff -r 42b0128ae0a7 -r bc3ef6652f1b adaptors.dox
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/adaptors.dox	Mon Feb 15 01:18:18 2010 +0100
@@ -0,0 +1,199 @@
+/* -*- mode: C++; indent-tabs-mode: nil; -*-
+ *
+ * This file is a part of LEMON, a generic C++ optimization library.
+ *
+ * Copyright (C) 2003-2009
+ * Egervary Jeno Kombinatorikus Optimalizalasi Kutatocsoport
+ * (Egervary Research Group on Combinatorial Optimization, EGRES).
+ *
+ * Permission to use, modify and distribute this software is granted
+ * provided that this copyright notice appears in all copies. For
+ * precise terms see the accompanying LICENSE file.
+ *
+ * This software is provided "AS IS" with no warranty of any kind,
+ * express or implied, and with no claim as to its suitability for any
+ * purpose.
+ *
+ */
+
+namespace lemon {
+/**
+[PAGE]sec_graph_adaptors[PAGE] Graph Adaptors
+
+\todo Clarify this section.
+
+Alteration of standard containers need a very limited number of
+operations, these together satisfy the everyday requirements.
+In the case of graph structures, different operations are needed which do
+not alter the physical graph, but gives another view. If some nodes or
+arcs have to be hidden or the reverse oriented graph have to be used, then
+this is the case. It also may happen that in a flow implementation
+the residual graph can be accessed by another algorithm, or a node-set
+is to be shrunk for another algorithm.
+LEMON also provides a variety of graphs for these requirements called
+\ref graph_adaptors "graph adaptors". Adaptors cannot be used alone but only
+in conjunction with other graph representations.
+
+The main parts of LEMON are the different graph structures, generic
+graph algorithms, graph concepts, which couple them, and graph
+adaptors. While the previous notions are more or less clear, the
+latter one needs further explanation. Graph adaptors are graph classes
+which serve for considering graph structures in different ways.
+
+A short example makes this much clearer.  Suppose that we have an
+instance \c g of a directed graph type, say ListDigraph and an algorithm
+\code
+template <typename Digraph>
+int algorithm(const Digraph&);
+\endcode
+is needed to run on the reverse oriented graph.  It may be expensive
+(in time or in memory usage) to copy \c g with the reversed
+arcs.  In this case, an adaptor class is used, which (according
+to LEMON \ref concepts::Digraph "digraph concepts") works as a digraph.
+The adaptor uses the original digraph structure and digraph operations when
+methods of the reversed oriented graph are called.  This means that the adaptor
+have minor memory usage, and do not perform sophisticated algorithmic
+actions.  The purpose of it is to give a tool for the cases when a
+graph have to be used in a specific alteration.  If this alteration is
+obtained by a usual construction like filtering the node or the arc set or
+considering a new orientation, then an adaptor is worthwhile to use.
+To come back to the reverse oriented graph, in this situation
+\code
+template<typename Digraph> class ReverseDigraph;
+\endcode
+template class can be used. The code looks as follows
+\code
+ListDigraph g;
+ReverseDigraph<ListDigraph> rg(g);
+int result = algorithm(rg);
+\endcode
+During running the algorithm, the original digraph \c g is untouched.
+This techniques give rise to an elegant code, and based on stable
+graph adaptors, complex algorithms can be implemented easily.
+
+In flow, circulation and matching problems, the residual
+graph is of particular importance. Combining an adaptor implementing
+this with shortest path algorithms or minimum mean cycle algorithms,
+a range of weighted and cardinality optimization algorithms can be
+obtained. For other examples, the interested user is referred to the
+detailed documentation of particular adaptors.
+
+The behavior of graph adaptors can be very different. Some of them keep
+capabilities of the original graph while in other cases this would be
+meaningless. This means that the concepts that they meet depend
+on the graph adaptor, and the wrapped graph.
+For example, if an arc of a reversed digraph is deleted, this is carried
+out by deleting the corresponding arc of the original digraph, thus the
+adaptor modifies the original digraph.
+However in case of a residual digraph, this operation has no sense.
+
+Let us stand one more example here to simplify your work.
+ReverseDigraph has constructor
+\code
+ReverseDigraph(Digraph& digraph);
+\endcode
+This means that in a situation, when a <tt>const %ListDigraph&</tt>
+reference to a graph is given, then it have to be instantiated with
+<tt>Digraph=const %ListDigraph</tt>.
+\code
+int algorithm1(const ListDigraph& g) {
+  ReverseDigraph<const ListDigraph> rg(g);
+  return algorithm2(rg);
+}
+\endcode
+
+<hr>
+
+The LEMON graph adaptor classes serve for considering graphs in
+different ways. The adaptors can be used exactly the same as "real"
+graphs (i.e., they conform to the graph concepts), thus all generic
+algorithms can be performed on them. However, the adaptor classes use
+the underlying graph structures and operations when their methods are
+called, thus they have only negligible memory usage and do not perform
+sophisticated algorithmic actions. This technique yields convenient and
+elegant tools for the cases when a graph has to be used in a specific
+alteration, but copying it would be too expensive (in time or in memory
+usage) compared to the algorithm that should be executed on it. The
+following example shows how the \ref ReverseDigraph adaptor can be used
+to run Dijksta's algorithm on the reverse oriented graph. Note that the
+maps of the original graph can be used in connection with the adaptor,
+since the node and arc types of the adaptors convert to the original
+item types. 
+
+\code
+dijkstra(reverseDigraph(g), length).distMap(dist).run(s);
+\endcode
+
+Using \ref ReverseDigraph could be as efficient as working with the
+original graph, but not all adaptors can be so fast, of course. For
+example, the subgraph adaptors have to access filter maps for the nodes
+and/or the arcs, thus their iterators are significantly slower than the
+original iterators. LEMON also provides some more complex adaptors, for
+instance, \ref SplitNodes, which can be used for splitting each node in
+a directed graph and \ref ResidualDigraph for modeling the residual
+network for flow and matching problems. 
+
+Therefore, in cases when rather complex algorithms have to be used
+on a subgraph (e.g. when the nodes and arcs have to be traversed several
+times), it could worth copying the altered graph into an efficient structure
+and run the algorithm on it.
+Note that the adaptor classes can also be used for doing this easily,
+without having to copy the graph manually, as shown in the following
+example.
+
+\code
+  ListDigraph g;
+  ListDigraph::NodeMap<bool> filter_map(g);
+  // construct the graph and fill the filter map
+
+  {
+    SmartDigraph temp_graph;
+    ListDigraph::NodeMap<SmartDigraph::Node> node_ref(g);
+    digraphCopy(filterNodes(g, filter_map), temp_graph)
+      .nodeRef(node_ref).run();
+
+    // use temp_graph
+  }
+\endcode
+
+<hr>
+
+Another interesting adaptor in LEMON is \ref SplitNodes.
+It can be used for splitting each node into an in-node and an out-node
+in a directed graph. Formally, the adaptor replaces each node
+u in the graph with two nodes, namely node u<sub>in</sub> and node
+u<sub>out</sub>. Each arc (u,c) in the original graph will correspond to an
+arc (u<sub>out</sub>,v<sub>in</sub>). The adaptor also adds an
+additional bind arc (u<sub>in</sub>,u<sub>out</sub>) for each node u
+of the original digraph.
+
+The aim of this class is to assign costs to the nodes when using
+algorithms which would otherwise consider arc costs only.
+For example, let us suppose that we have a directed graph with costs
+given for both the nodes and the arcs.
+Then Dijkstra's algorithm can be used in connection with \ref SplitNodes
+as follows.
+
+\code
+  typedef SplitNodes<ListDigraph> SplitGraph;
+  SplitGraph sg(g);
+  SplitGraph::CombinedArcMap<NodeCostMap, ArcCostMap>
+    combined_cost(node_cost, arc_cost);
+  SplitGraph::NodeMap<double> dist(sg);
+  dijkstra(sg, combined_cost).distMap(dist).run(sg.outNode(u));
+\endcode
+
+Note that this problem can be solved more efficiently with
+map adaptors.
+
+These techniques help writing compact and elegant code, and makes it possible
+to easily implement complex algorithms based on well tested standard components.
+For instance, in flow and matching problems the residual graph is of
+particular importance.
+Combining \ref ResidualDigraph adaptor with various algorithms, a
+range of weighted and cardinality optimization methods can be obtained
+directly.
+
+[TRAILER]
+*/
+}
\ No newline at end of file
diff -r 42b0128ae0a7 -r bc3ef6652f1b toc.txt
--- a/toc.txt	Mon Feb 15 00:36:27 2010 +0100
+++ b/toc.txt	Mon Feb 15 01:18:18 2010 +0100
@@ -16,7 +16,7 @@
 ** sec_digraph_types
 ** sec_undir_graphs
 ** sec_special_graphs
-*_sec_graph_adaptors
+* sec_graph_adaptors
 *_sec_tools
 **_sec_lgf
 **_sec_time_count