namespace lemon {

 /*!

 \page graph-io-page Graph Input-Output

 The standard graph IO enables one to store graphs and additional maps

 (i.e. functions on the nodes or edges) in a flexible and efficient way. 

 Before you read this page you should be familiar with LEMON 

 \ref graphs "graphs" and \ref maps-page "maps".

 \section format The general file format

 The file contains sections in the following order:

 \li nodeset

 \li edgeset

 \li nodes

 \li edges

 \li attributes

 Some of these sections can be omitted, but you will basicly need the nodeset

 section (unless your graph has no nodes at all) and the edgeset section

 (unless your graph has no edges at all). 

 The nodeset section describes the nodes of your graph: it identifies the nodes

 and gives the maps defined on them, if any. It starts with the

 following line:

 <tt>\@nodeset</tt>

 The next line contains the names of the nodemaps, separated by whitespaces.  Each

 following line describes a node in the graph: it contains the values of the

 maps in the right order. The map named "label" should contain unique values

 because it is regarded as a label map. These labels need not be numbers but they

 must identify the nodes uniquely for later reference. For example:

 \code

 @nodeset

 label  x-coord  y-coord  color

 3   1.0      4.0      blue

 5   2.3      5.7      red

 12  7.8      2.3      green

 \endcode

 The edgeset section is very similar to the nodeset section, it has

 the same coloumn oriented structure. It starts with the line 

 <tt>\@edgeset</tt>

 The next line contains the whitespace separated list of names of the edge

 maps.  Each of the next lines describes one edge. The first two elements in

 the line are the labels of the source and target (or tail and head) nodes of the

 edge as they occur in the label node map of the nodeset section. You can also

 have an optional label map on the edges for later reference (which has to be

 unique in this case).

 \code

 @edgeset

              label      weight   note

 3   5        a          4.3      a-edge

 5   12       c          2.6      c-edge

 3   12       g          3.4      g-edge

 \endcode

 The \e nodes section contains <em>labeled (distinguished) nodes</em> 

 (i.e. nodes having a special

 label on them). The section starts with

 <tt> \@nodes </tt>

 Each of the next lines contains a label for a node in the graph 

 and then the label as described in the \e nodeset section.

 \code

 @nodes 

 source 3

 target 12

 \endcode

 The last section describes the <em>labeled (distinguished) edges</em>

 (i.e. edges having a special label on them). It starts with \c \@edges

 and then each line contains the name of the edge and the label.

 \code

 @edges 

 observed c

 \endcode

 The file may contain empty lines and comment lines. The comment lines

 start with an \c # character.

 The attributes section can handle some information about the graph. It

 contains key-value pairs in each line (a key and the mapped value to key). The

 key should be a string without whitespaces, the value can be of various types.

 \code

 @attributes

 title "Four colored planar graph"

 author "Balazs DEZSO"

 copyright "Lemon Library"

 version 12

 \endcode

 Finally, the file should be closed with \c \@end line.

 \section use Using graph input-output

 The graph input and output is based on <em> reading and writing

 commands</em>. The user gives reading and writing commands to the reader or

 writer class, then he calls the \c run() method that executes all the given

 commands.

 \subsection write Writing a graph

 The \ref lemon::GraphWriter "GraphWriter" template class

 provides the graph output. To write a graph

 you should first give writing commands to the writer. You can declare

 writing command as \c NodeMap or \c EdgeMap writing and labeled Node and

 Edge writing.

 \code

 GraphWriter<ListGraph> writer(std::cout, graph);

 \endcode

 The \ref lemon::GraphWriter::writeNodeMap() "writeNodeMap()"

 function declares a \c NodeMap writing command in the

 \ref lemon::GraphWriter "GraphWriter".

 You should give a name to the map and the map

 object as parameters. The NodeMap writing command with name "label" should write a 

 unique map because it will be regarded as a label map.

 \see IdMap, DescriptorMap  

 \code

 IdMap<ListGraph, Node> nodeLabelMap;

 writer.writeNodeMap("label", nodeLabelMap);

 writer.writeNodeMap("x-coord", xCoordMap);

 writer.writeNodeMap("y-coord", yCoordMap);

 writer.writeNodeMap("color", colorMap);

 \endcode

 With the \ref lemon::GraphWriter::writeEdgeMap() "writeEdgeMap()"

 member function you can give an edge map

 writing command similar to the NodeMaps.

 \see IdMap, DescriptorMap  

 \code

 DescriptorMap<ListGraph, Edge, ListGraph::EdgeMap<int> > edgeDescMap(graph);

 writer.writeEdgeMap("descriptor", edgeDescMap);

 writer.writeEdgeMap("weight", weightMap);

 writer.writeEdgeMap("note", noteMap);

 \endcode

 With \ref lemon::GraphWriter::writeNode() "writeNode()"

 and \ref lemon::GraphWriter::writeEdge() "writeEdge()"

 functions you can designate Nodes and

 Edges in the graph. For example, you can write out the source and target node

 of a maximum flow instance.

 \code

 writer.writeNode("source", sourceNode);

 writer.writeNode("target", targetNode);

 writer.writeEdge("observed", edge);

 \endcode

 With \ref lemon::GraphWriter::writeAttribute() "writeAttribute()"

 function you can write an attribute to the file.

 \code

 writer.writeAttribute("author", "Balazs DEZSO");

 writer.writeAttribute("version", 12);

 \endcode

 After you give all write commands you must call the

 \ref lemon::GraphWriter::run() "run()" member

 function, which executes all the writing commands.

 \code

 writer.run();

 \endcode

 \subsection reading Reading a graph

 The file to be read may contain several maps and labeled nodes or edges.

 If you read a graph you need not read all the maps and items just those

 that you need. The interface of the \ref lemon::GraphReader "GraphReader"

 is very similar to

 the \ref lemon::GraphWriter "GraphWriter"

 but the reading method does not depend on the order of the

 given commands.

 The reader object assumes that each not read value does not contain 

 whitespaces, therefore it has some extra possibilities to control how

 it should skip the values when the string representation contains spaces.

 \code

 GraphReader<ListGraph> reader(std::cin, graph);

 \endcode

 The \ref lemon::GraphReader::readNodeMap() "readNodeMap()"

 function reads a map from the \c nodeset section.

 If there is a map that you do not want to read from the file and there are

 whitespaces in the string represenation of the values then you should

 call the \ref lemon::GraphReader::skipNodeMap() "skipNodeMap()"

 template member function with proper parameters.

 \see QuotedStringReader

 \code

 reader.readNodeMap("x-coord", xCoordMap);

 reader.readNodeMap("y-coord", yCoordMap);

 reader.readNodeMap<QuotedStringReader>("label", labelMap);

 reader.skipNodeMap<QuotedStringReader>("description");

 reader.readNodeMap("color", colorMap);

 \endcode

 With the \ref lemon::GraphReader::readEdgeMap() "readEdgeMap()"

 member function you can give an edge map

 reading command similar to the NodeMaps. 

 \code

 reader.readEdgeMap("weight", weightMap);

 reader.readEdgeMap("label", labelMap);

 \endcode

 With \ref lemon::GraphReader::readNode() "readNode()"

 and \ref lemon::GraphReader::readEdge() "readEdge()"

 functions you can read labeled Nodes and

 Edges.

 \code

 reader.readNode("source", sourceNode);

 reader.readNode("target", targetNode);

 reader.readEdge("observed", edge);

 \endcode

 With \ref lemon::GraphReader::readAttribute() "readAttribute()"

 function you can read an attribute from the file.

 \code

 std::string author;

 writer.readAttribute("author", author);

 int version;

 writer.writeAttribute("version", version);

 \endcode

 After you give all read commands you must call the

 \ref lemon::GraphReader::run() "run()" member

 function, which executes all the commands.

 \code

 reader.run();

 \endcode

 \anchor rwbackground

 \section types Background of Reading and Writing

 To read a map (on the nodes or edges)

 the \ref lemon::GraphReader "GraphReader"

 should know how to read a Value from the given map.

 By the default implementation the input operator reads a value from

 the stream and the type of the read value is the value type of the given map.

 When the reader should skip a value in the stream, because you do not

 want to store it in a map, the reader skips a character sequence without 

 whitespaces. 

 If you want to change the functionality of the reader, you can use

 template parameters to specialize it. When you give a reading

 command for a map you can give a Reader type as template parameter.

 With this template parameter you can control how the Reader reads

 a value from the stream.

 The reader has the next structure: 

 \code

 struct TypeReader {

   typedef TypeName Value;

   void read(std::istream& is, Value& value);

 };

 \endcode

 For example, the \c "strings" nodemap contains strings and you do not need

 the value of the string just the length. Then you can implement an own Reader

 struct.

 \code

 struct LengthReader {

   typedef int Value;

   void read(std::istream& is, Value& value) {

     std::string tmp;

     is >> tmp;

     value = tmp.length();

   }

 };

 ...

 reader.readNodeMap<LengthReader>("strings", lengthMap);

 \endcode  

 The global functionality of the reader class can be changed by giving a

 special template parameter to the GraphReader class. By default, the

 template parameter is \c DefaultReaderTraits. A reader traits class 

 should provide a nested template class Reader for each type, and a 

 DefaultReader for skipping a value.

 The specialization of writing is very similar to that of reading.

 \section u Undirected graphs

 In a file describing an undirected graph (ugraph, for short) you find an

 \c uedgeset section instead of the \c edgeset section. The first line of

 the section describes the names of the maps on the undirected egdes and all

 next lines describe one undirected edge with the the incident nodes and the

 values of the map.

 The format handles directed edge maps as a syntactical sugar???, if there

 are two maps with names being the same with a \c '+' and a \c '-' prefix

 then this will be read as a directed map.

 \code

 @uedgeset

              label      capacity        +flow   -flow

 32   2       1          4.3             2.0     0.0

 21   21      5          2.6             0.0     2.6

 21   12      8          3.4             0.0     0.0

 \endcode

 The \c edges section is changed to \c uedges section. This section

 describes labeled edges and undirected edges. The directed edge label

 should start with a \c '+' or a \c '-' prefix to decide the direction

 of the edge. 

 \code

 @uedges

 uedge 1

 +edge 5

 -back 5

 \endcode

 There are similar classes to the \ref lemon::GraphReader "GraphReader" and

 \ref lemon::GraphWriter "GraphWriter" which

 handle the undirected graphs. These classes are

 the \ref lemon::UGraphReader "UGraphReader"

 and \ref lemon::UGraphWriter "UGraphWriter".

 The \ref lemon::UGraphReader::readUEdgeMap() "readUEdgeMap()"

 function reads an undirected map and the

 \ref lemon::UGraphReader::readUEdge() "readUEdge()"

 reads an undirected edge from the file, 

 \code

 reader.readUEdgeMap("capacity", capacityMap);

 reader.readEdgeMap("flow", flowMap);

 ...

 reader.readUEdge("u_edge", u_edge);

 reader.readEdge("edge", edge);

 \endcode

 \section advanced Advanced features

 The graph reader and writer classes give an easy way to read and write

 graphs. But sometimes we want more advanced features. In this case we can

 use the more general <tt>lemon reader and writer</tt> interface.

 The LEMON file format is a section oriented file format. It contains one or

 more sections, each starting with a line identifying its type 

 (the word starting with the \c \@  character).

 The content of the section this way cannot contain line with \c \@ first

 character. The file may contains comment lines with \c # first character.

 The \ref lemon::LemonReader "LemonReader"

 and \ref lemon::LemonWriter "LemonWriter"

 gives a framework to read and

 write sections. There are various section reader and section writer

 classes which can be attached to a \ref lemon::LemonReader "LemonReader"

 or a \ref lemon::LemonWriter "LemonWriter".

 There are default section readers and writers for reading and writing

 item sets, and labeled items in the graph. These read and write

 the format described above. Other type of data can be handled with own

 section reader and writer classes which are inherited from the

 \c LemonReader::SectionReader or the

 \ref lemon::LemonWriter::SectionWriter "LemonWriter::SectionWriter"

 classes.

 The next example defines a special section reader which reads the

 \c \@description sections into a string:

 \code 

 class DescriptionReader : LemonReader::SectionReader {

 protected:

   virtual bool header(const std::string& line) {

     std::istringstream ls(line);

     std::string head;

     ls >> head;

     return head == "@description";

   }

   virtual void read(std::istream& is) {

     std::string line;

     while (getline(is, line)) {

       desc += line;

     }

   }

 public:

   typedef LemonReader::SectionReader Parent;

   DescriptionReader(LemonReader& reader) : Parent(reader) {}

   const std::string& description() const {

     return description;

   }

 private:

   std::string desc;

 };

 \endcode

 The other advanced stuff of the generalized file format is that 

 multiple edgesets can be stored to the same nodeset. It can be used 

 for example as a network traffic matrix.

 In our example there is a network with symmetric links and there are assymetric

 traffic request on the network. This construction can be stored in an

 undirected graph and in a directed \c ListEdgeSet class. The example

 shows the input with the \ref lemon::LemonReader "LemonReader" class:

 \code

 ListUGraph network;

 ListUGraph::UEdgeMap<double> capacity;

 ListEdgeSet<ListUGraph> traffic(network);

 ListEdgeSet<ListUGraph>::EdgeMap<double> request(network);

 LemonReader reader(std::cin);

 NodeSetReader<ListUGraph> nodesetReader(reader, network);

 UEdgeSetReader<ListUGraph> 

   uEdgesetReader(reader, network, nodesetReader);

 uEdgesetReader.readEdgeMap("capacity", capacity);

 EdgeSetReader<ListEdgeSet<ListUGraph> > 

   edgesetReader(reader, traffic, nodesetReader, "traffic");

 edgesetReader.readEdgeMap("request", request);

 reader.run();

 \endcode

 Because both the \ref lemon::GraphReader "GraphReader"

 and the \ref lemon::UGraphReader "UGraphReader" can be converted

 to \ref lemon::LemonReader "LemonReader"

 and it can resolve the label's of the items, the previous

 result can be achived with the \ref lemon::UGraphReader "UGraphReader"

 class, too.

 \code

 ListUGraph network;

 ListUGraph::UEdgeSet<double> capacity;

 ListEdgeSet<ListUGraph> traffic(network);

 ListEdgeSet<ListUGraph>::EdgeMap<double> request(network);

 UGraphReader<ListUGraph> reader(std::cin, network);

 reader.readEdgeMap("capacity", capacity);

 EdgeSetReader<ListEdgeSet<ListUGraph> > 

   edgesetReader(reader, traffic, reader, "traffic");

 edgesetReader.readEdgeMap("request", request);

 reader.run();

 \endcode

 \author Balazs Dezso

 */

 }