source: lemon-0.x/doc/read_write_bg.dox @ 2274:432d0469a87e

namespace lemon {
/*!
\page read_write_bg 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
*/
}