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

  *

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

  *

  * Copyright (C) 2003-2008

  * 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] Input-Output for Graphs

 \todo Clarify this section.

 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 storing 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 file format is designed for LEMON instead of using

 hierarchical formats, such as GraphML, GXL or GML.

 The LEMON Graph Format (LGF) comprises different sections, for

 example a digraph is stored in a \c @nodes and an \c @arcs

 section. These parts use column oriented formats, each

 column belongs to a map in the graph. The first line of the section associate

 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.

 The \c label map has special role, it must store unique values, which in turn

 can be used to refer

 to the nodes and arcs in the file. Finally, the first two column of the

 \c @arcs section is anonymous, they indicate the source and target nodes,

 respectively.

 \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

   target 41

   caption "A shortest path problem"

 \endcode

 The \ref DigraphReader and \ref DigraphWriter classes are used

 to read and write a digraph and corresponding maps. 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 above LGF file can be scanned 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, std::cin)

     .nodeMap("coord", coord)

     .arcMap("length", length)

     .attribute("caption", title)

     .node("source", src)

     .run();

 \endcode

 Apart from LGF, the library can also interpret other graph

 formats, such as the well-known DIMACS format or the NAUTY graph6

 format.

 <hr>

 The \e LGF is a <em>column oriented</em>

 file format for storing graphs and associated data like

 node and edge maps.

 Each line with \c '#' first non-whitespace

 character is considered as a comment line.

 Otherwise the file consists of sections starting with

 a header line. The header lines starts with an \c '@' character followed by the

 type of section. The standard section types are \c \@nodes, \c

 \@arcs and \c \@edges

 and \@attributes. Each header line may also have an optional

 \e name, which can be use to distinguish the sections of the same

 type.

 The standard sections are column oriented, each line consists of

 <em>token</em>s separated by whitespaces. A token can be \e plain or

 \e quoted. A plain token is just a sequence of non-whitespace characters,

 while a quoted token is a

 character sequence surrounded by double quotes, and it can also

 contain whitespaces and escape sequences.

 The \c \@nodes section describes a set of nodes and associated

 maps. The first is a header line, its columns are the names of the

 maps appearing in the following lines.

 One of the maps must be called \c

 "label", which plays special role in the file.

 The following

 non-empty lines until the next section describes nodes of the

 graph. Each line contains the values of the node maps

 associated to the current node.

 \code

  @nodes

  label  coordinates  size    title

  1      (10,20)      10      "First node"

  2      (80,80)      8       "Second node"

  3      (40,10)      10      "Third node"

 \endcode

 The \c \@arcs section is very similar to the \c \@nodes section,

 it again starts with a header line describing the names of the maps,

 but the \c "label" map is not obligatory here. The following lines

 describe the arcs. The first two tokens of each line are

 the source and the target node of the arc, respectively, then come the map

 values. The source and target tokens must be node labels.

 \code

  @arcs

          capacity

  1   2   16

  1   3   12

  2   3   18

 \endcode

 The \c \@edges is just a synonym of \c \@arcs. The \@arcs section can

 also store the edge set of an undirected graph. In such case there is

 a conventional method for store arc maps in the file, if two columns

 has the same caption with \c '+' and \c '-' prefix, then these columns

 can be regarded as the values of an arc map.

 The \c \@attributes section contains key-value pairs, each line

 consists of two tokens, an attribute name, and then an attribute

 value. The value of the attribute could be also a label value of a

 node or an edge, or even an edge label prefixed with \c '+' or \c '-',

 which regards to the forward or backward directed arc of the

 corresponding edge.

 \code

  @attributes

  source 1

  target 3

  caption "LEMON test digraph"

 \endcode

 The \e LGF can contain extra sections, but there is no restriction on

 the format of such sections.

 */

 }