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Directed Graph Library

by GRASS Development Team


The Directed Graph Library or DGLib (Micarelli 2002) provides functionality for vector network analysis. This library released under GPL is hosted by the GRASS project (in the CVS server). As stand-alone library it may also be used by other software project.

A graph is a system of logical connections between a collection of objects called vertices. Graphs are usually represented by a picture, so that each vertex is shown as a point, with the connections shown as line segments. These vertices are also commonly referred to as nodes, edges referred to as arcs. A directed graph (digraph) consists of a finite set of vertices, and a finite set of edges, where an edge is an ordered pair of vertices. A directed graph has the property that edges have a direction, this is the reason for defining an edge as an ordered pair of vertices often referred to as the head and the tail of the edge.

The original design idea behind DGLib was to support middle sized graphs in RAM with a near-static structure that doesn't need to be dynamically modified by the user program; ability to read graphs from input streams and process them with no needle to rebuild internal trees. A representation has been defined, where graph data is stored in 32bit word arrays and each element pointer is converted to a relative offset. This representation is serializable from/to input/output streams and allows fast load-and-use processing. Graphs need to be de-serialized in order to be edited. In further refactorings the library has evolved to support dynamic changes and state-independent algorithm (algorithms can be run on both serializable or editable graphs).

DGLib defines a serializable graph as being in FLAT state and a editable graph as being in TREE state. The implementation makes intensive use of libavl ( AVL data structures to support TREE state.

So far DGLib defines three different graph versions, version 1 supports directed graph with a weak concept of the edge, it can support many applications where one doesn't need to know about the input edges of a node (in-degree) and where there is no requirement to directly retrieve edges by their identifier but only by head/tail combinations. Version 2 adds in-degree support and a true edge addressing, yet supporting directed graph. Version 3 uses the same internal representation of version 2 but activates code branches to support undirected graphs.

The DGLib user can control a number of static features and can attach a arbitrary amount of data to each node (node-attributes) and each edge (edge-attributes). Attributes are not considered to be part of the graph structure and can be edited also when the graph is in FLAT state.

Graph traversal in neither recursive nor hook (callback) based, but built on the use of traversers for nodes and edges. By default, traversal is ordered by node and edge identifiers but can optionally be ordered by other means. For example, it is often useful to visit edges on a weight order} basis (greedy algorithm), this is possible via prioritizers that are activated by setting specific graph options.

Both preemptive (blocking) and non-preemptive (non-blocking/multiplexed) I/O is supported, although GRASS does not actually use graph storage it may be easily required by any other library user. Thread safety is so far ensured by a data separation design that keeps all application context states into stack containers, whose life cycle is controlled by the user program. Each graph is a separate container and two or more graphs never conflict. In addition algorithms (ie. shortest path) can safely share the same graph, while concurrent editing on the same graph is unsafe.

As DGLib is under development, only a bunch of polynomial time algorithms have been implemented, and the basic structure is being stressed to be a mature core to possibly time wasting computations. Current algorithms are: shortest path, depth spanning, and minimum spanning. Spanning algorithms silently behave as arborescenses when applied to directed graphs. A clip callback function, optionally supplied by the user, comes called by the library while traversing the graph in order to alter default algorithm behavior (i.e. user can control access to specific graph segments while computing shortest path).

The Directed Graph Library library provides functionality to assign costs to lines and/or nodes. That means that costs can be accumulated while traveling along polylines. The user can assign individual costs to all lines and/or nodes of a vector map and later calculate shortest path connections based on the accumulated costs. Applications are transport analysis, connectivity and more.

Text based on:

R. Blazek, M. Neteler, and R. Micarelli. The new GRASS 5.1 vector architecture. In Open source GIS - GRASS users conference 2002, Trento, Italy, 11-13 September 2002. University of Trento, Italy, 2002.

Code Walk Through explanation

Directed graph library is responsible for network analysis operations done on vector maps. For this purpose directed graph library converts the vector attibutes into form of a graph. This graph structure and related structures are defined in dglib/graph.h This header file contains defination of all structers required for storing components of graph, transversing, accessing and modifying it.

Map Structure Map_info contains a instance of dglGraph_s as one of its elements named 'graph'. This element is populated call to api Vect_net_build_graph(). This API takes several arguments as defined in lib/vector/Vlib/net.c (see GRASS 6 Vector Architecture).

Following structure declares dglGraph_s, main datastructure for storing graph.

typedef struct _dglGraph
        int                             iErrno; // used to report what kind of error while operating on it lately.

        dglByte_t                       Version;        //graph version 1 2 or 3
        dglByte_t                       Endian;         // big or small 
        dglInt32_t                      NodeAttrSize;    //node attr sze
        dglInt32_t                      EdgeAttrSize; //edge attribute size
        dglInt32_t                      aOpaqueSet[ 16 ];       

        dglInt32_t                      cNode;  // count of nodes
        dglInt32_t                      cHead;  // count of head nodes ( nodes from those edge orignates)
        dglInt32_t                      cTail;  // count of tail nodes (nodes at whom edge terimate )
        dglInt32_t                      cAlone; // component nodes ( which are not connected to anyone )
        dglInt32_t                      cEdge;  //count of edges
        dglInt64_t                      nnCost; //total cost of edges

        dglInt32_t                      Flags;  // flags for graph . This conveys whether graph is of what kind.
        dglInt32_t                      nFamily;
        dglInt32_t                      nOptions;

        void *                          pNodeTree;      // pointer to tree of nodes     
        void *                          pEdgeTree;      //pointer to tree of edges
        dglByte_t *                     pNodeBuffer;    //pointer to node buffer. this points contiguous memory location containing pointers to nodes of  graph.
        dglInt32_t                      iNodeBuffer; //current index of node being accessed. This is used as offset index from pNodeBuffer
        dglByte_t *                     pEdgeBuffer; //pointer to edge buffer. this points to contiguous memory ,containing pointer to edgesets passing through each node. Here as per i remeber means orignating.
        dglInt32_t                      iEdgeBuffer; // offset index counter for edge buffer.

                        //rest i have not studied yet (NKD).
        dglEdgePrioritizer_s edgePrioritizer;
        dglNodePrioritizer_s nodePrioritizer;

 //so far statistics are only computed by dglAddEdge() 
#ifdef DGL_STATS
        clock_t                         clkAddEdge;  // cycles spent during the last addedge execution 
        int                                     cAddEdge;    // # of calls to dglAddEdge() 
        clock_t                         clkNodeTree; // cycles spent in accessing the node binary tree 
        int                                     cNodeTree;   // # of probes in the node tree 

As mentioned earlier, graphs are stored as FLAT form and for all operations they are first converted to TREE. In FLAT form, pnodeBuffer contains all information about nodes. They are stored in ascending order as per nodeid. pnodeBuffer points to a contiguous memory area contains pointer to node type structure. This node type structure is a memory area indexed as

// Node macros - addresses in a flat node ( as defined in dglib/graph_v1.h ,
// similar for version 2)

#define DGL_IN_NODEID_v1                0
#define DGL_IN_STATUS_v1                1
#define DGL_IN_TAIL_OFFSET_v1   2// this gives where edgeset pointer is in pnode 
#define DGL_IN_ATTR_v1                  3
#define DGL_IN_SIZE_v1                  DGL_IN_ATTR_v1

Similarly pEdgeBuffer is a contiguous memory locations contains pointer to edgesets passing through each node. These edgesets can be transversed using dglEdgesetTraverser_s structure defined in dglib/graph.h .

// Edge macros - addresses in a flat edge (as defined in dglib/graph_v1.h and similar for version 2)
#define DGL_IL_HEAD_OFFSET_v1   0
#define DGL_IL_TAIL_OFFSET_v1   1
#define DGL_IL_COST_v1                  2
#define DGL_IL_ID_v1                    3
#define DGL_IL_ATTR_v1                  4
#define DGL_IL_SIZE_v1                  DGL_IL_ATTR_v1

For any operations on graph, these first need to be converted to TREE from XXX. Dglib first convert FLAT to AVL trees using libavl. This operation is called unflattening.

This has been explained earlier that graph may be one of three versions depending on which operations will be supported. APIs for these has been diffrentiated from each other through a suffix _v1 and _v2 . All generic graph operations are defined in dglib/graph.h . These APIs depending on graph version call respective version APIs. For version 1 they resolve into *_v1() ones declared and defined in dglib/graph_v1.h and dglib/graph_v1.c . For version 2, they are is in dglib/graph_v2.h and dglib/graph_v2.c . For version 3, DOCUMENT THIS. Operations on various permitives are defined in template files.

  • Node operations : nodemgmt-template.c ->This contains operations like getting, setting a node, finding in and out edgeset from node.

  • Edge operations: edgemgmt-template.c -> add edge, delete edge, get edge etc.

  • Spanning tree : span-template.c -> Build the depth-first spanning tree, minimum spanning tree using Prims algorithm

  • Shortest path : sp-template.c -> it implement Dijkastra's algo using min heap. Algorithm needs to be studied more.

  • misc operations: misc-template.c -> operations like flattening or unflattening of graph, edgeset transversing, node set transversing etc.

All these files contains defination using/for macros.These macro definations are there to make graph version transparent and provide uniform interface. Macro declaration happens in v1-defs.h for version 1 and v2-defs.h for version 2 graphs. If you are looking for any API, if it is all capital, it is a macro. You need to look for its expansion. This expansion will be true function for that operation. Just a piece of advice , always keep programmers manual with you. Just search in it for what you want otherwise one may spend quite substantial time in finding desired code.

Vlib/level-two.c defines various APIs used for operation done on nodes and edges . For example finding node coordinates, edges coming from head nodes. etc.

dglib/example folder contains some good examples which work only on graph (I mean graph is required to be input). They are easy and almost self explanatory once you know what API are for.


(yet incomplete, all functions need to be doxygenized...)















Roberto Micarelli, Italy
GRASS Development Team
Nitin K Dhiman, AINN group, CAIR (code walkthrough documentation)

See Also

GRASS 6 Vector Architecture GRASS 6 Vector Architecture

Last change:

2008-07-12 07:39:09 -0700 (Sat, 12 Jul 2008)