/* -*- mode: C++; indent-tabs-mode: nil; -*-

 *

 * This file is a part of LEMON, a generic C++ optimization library.

 *

 * Copyright (C) 2003-2010

 * 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_lgf[PAGE] LEMON Graph Format

LEMON provides a versatile file format for storing graphs and related

node and arc maps. Such a format should be really flexible, it should be

able to store arbitrary number of maps of arbitrary value types.

On the other hand, the file size and the ease of processing are also critical

to support huge graphs, which is a major goal of LEMON.

These requirements forbid using complicated and deeply structured formats

like XML. That is why a compact text file format is designed for LEMON

instead of using hierarchical formats (such as GraphML, GXL or GML).

The LEMON Graph Format (LGF) consists of several sections, for example a

digraph is stored in a <tt>\@nodes</tt> and an <tt>\@arcs</tt> section.

These parts use column oriented formats, each column belongs to a map in

the graph. The first line of the section associates names to these maps,

which can be used to refer them. Note that this simple idea makes it

possible to extend the files with new maps (columns) at any position

without having to modify the codes using these files.

Other data can also be added to an LGF file as individual properties

in an <tt>\@attributes</tt> section. This part can be used to specify

certain nodes or arcs, or store meta data for the file, such as copyright

notice.

For example, a shortest path problem, which is represented as a directed

graph with some node and arc maps, can be stored in LGF format as follows.

\code

  @nodes

  label coordinate

  0     (20,100)

  1     (40,120)

  ...

  41    (600,100)

  @arcs

            label length

  0    1    0     16

  0    2    1     12

  2    12   2     20

  ...

  36   41   123   21

  @attributes

  source 0

  caption "A shortest path problem"

\endcode

In the <tt>\@nodes</tt> section, the \c label map has special role,

it must store unique values, which in turn can be used to refer to the nodes

in the file.

The first two columns of the <tt>\@arcs</tt> section are anonymous, they

store the source and target nodes, respectively.

The \ref DigraphReader and \ref DigraphWriter classes can used

to read and write data in LGF format.

For example, the above file can be read as follows.

\code

  ListDigraph g;

  ListDigraph::NodeMap<dim2::Point<int> > coord(g);

  ListDigraph::ArcMap<int> length(g);

  ListDigraph::Node src;

  std::string title;

  digraphReader(g, "digraph.lgf")

    .nodeMap("coordinate", coord)

    .arcMap("length", length)

    .node("source", src)

    .attribute("caption", title)

    .run();

\endcode

Note that the \ref DigraphReader class is used through the \ref digraphReader()

function with several named parameters.

\note By default, a map can be used with these classes if its value type

has standard I/O operators (\c operator<<(ostream&, T) and \c

operator>>(istream&, T&)). Otherwise, a function object can be specified

which converts the value type to \c std::string.

The following code demonstrates how the above digraph with the maps and

attributes can be written into the standard output in LGF Format.

\code

  digraphWriter(g, std::cout)

    .nodeMap("coordinate", coord)

    .arcMap("length", length)

    .node("source", src)

    .attribute("caption", title)

    .run();

\endcode

Undirected graphs can be stored in LGF format in almost the same way.

The <tt>\@arcs</tt> section can also be called <tt>\@edges</tt>, they are

identical. The only speciality is that arc maps can be distinguished from

edge maps using a \c + or \c - prefix before the name of the map.

For example,

\code

  @edges

            label  +length  -length  

  0    1    0      10       20

  ...

\endcode

In conjunction with undirected graphs, the classes \ref GraphReader and

\ref GraphWriter can be used.

For more information, see the \ref lgf-format "description of the LGF format"

and the \ref io_group module in the reference manual.

For a working example, see \ref lgf_demo.cc in the demo directory

of the LEMON source.

\note Apart from LGF, the library can also handle other graph

formats, such as the well-known DIMACS format.

[TRAILER]

*/

}