# The &rocs; Handbook

Tomaz Canabrava tomaz.canabrava@gmail.com
Andreas Cord-Landwehr cordlandwehr@kde.org
2021-10-23
KDE Gear 21.08
&FDLNotice;

&rocs; is a graph theory tool.

KDE kdeedu mathematics math graphs node edge Rocs

## Introduction

This chapter provides an overview of the core features and the typical workflows. The most important parts are and , which together should allow every new user to directly start using &rocs;.

### Goals, Target Audience, and Workflows

&rocs; is a Graph Theory Tool for everybody interested in designing and studying graph algorithms. In particular, those are

- lecturers, who want to demonstrate algorithms to their students,

- students and researchers, who want to see how their algorithm perform, and

- everybody who is interested in data structures and algorithms.

For all them, &rocs; provides an easy to use graphical editor for creating graphs, a powerful scripting engine to execute algorithms, and several helper tools for simulations, experiments, and graph exports. The typical way of using &rocs; is to create a graph, either by hand (&ie;, dragging nodes and edges to the whiteboard), or by using one of the graph generators. Graph algorithms then can be implemented and executed on the created graph and all changes, which the algorithm performs, are visible immediately in the graph editor.

A Screenshot of &rocs;.
A Screenshot of &rocs;.

### &rocs; in a Nutshell

Every &rocs; session is a project: when opening &rocs; an empty project is created, when loading some project it becomes the current project. Hereby, a project itself consists of graph documents, scripts/algorithms, and a journal.

#### Graph Documents

A graph document represents the content of a whiteboard in the graph editor. It contains information about the user defined node and edge types, their properties, and about the already created nodes and edges. This is, &rocs; understands the set of all nodes and edges of a graph document to form a (not necessarily connected) graph. Everything belonging to a graph document is accessible by the script engine via the global object ``Document``.

#### Edge Types

In some scenarios, graphs consist of different types of edges (⪚, an undirected graph plus the tree edges computed by a breadh-first-search algorithm) that shall be handled and displayed differently. For this, besides a default edge type, you can define arbitrary other edge types. Each edge type has its individual visual representation, dynamic properties, and can be set to be either undirected or directed. The scripting interface provides convenience methods to specifically access only edges of specific types.

#### Node Types

Analog to edge types, you can define different types of nodes of a graph (⪚, to give some nodes special roles). Each node type has its own visual representation and dynamic properties.

#### Properties

Every (node or edge) element can have properties. Those properties must be setup at the corresponding node or edge type. Properties are identified and accessed by their names and can contain any value. To create new or change existing properties, use the Element Types sidebar and use the Properties button to open the property dialog.

You can also use the scripting engine to access registered properties and change their values. In the following example we assume that the property weight is registered for the default edge type.

    var nodes = Document.nodes()
    for (var i = 0; i < nodes.length; ++i){
        nodes[i].weight = i;
    }
    for (var i = 0; i < nodes.length; ++i){
        Console.log("weight of node " + i + ": " + nodes[i].weight);
    }

### Tutorial

In this section we want to create an example project to explore some of the most important functions of &rocs;. The goal is to create a graph and a script that illustrates a simple 2-approximate algorithm for the minimum vertex cover problem. The minimum vertex cover problem is the problem to find a subset of graph nodes C of minimal size such that each graph edge is connected to at least one node in C. This problem is known to be NP-hard and we want to illustrate how to find an approximation of factor 2 by computing a matching in the given graph.

Our goal is to visualize the relationship of the matching and the minimum vertex cover. For this, we want to specify two edge types, one to display matching edges and one type to display ordinary edges, as well as two node types that we use to distinguish nodes contained in C and those not contained in C.

#### Generating the Graph

For creating the graph, we use a default graph generator provided by &rocs;. This can be found in the main menu at Graph Document Tools Generate Graph. There, we select a Random Graph with 30 nodes, 90 edges, and with seed 1 (the seed is the starting seed for the random graph generator; using the same seed multiple times results in same and reproducible graphs).

#### Creating the Element Types

We use the Element Types and create a second node type as well as a second edge type. For both new types we open the properties dialog by using the respective Properties buttons and set the IDs to `2`. Furthermore, we change the colors of elements of these two new types (to distinguish them from the default types). Finally, we set all edge types to be bidirectional, and the IDs of the default types to `1`.

#### The Algorithm

At last we have to implement the approximation algorithm. For this we use the following implementation:

    for (var i=0; i < Document.nodes.length; i++) {
        Document.nodes[i].type = 1;
    }
    for (var i=0; i < Document.edges.length; i++) {
        Document.edges[i].type = 1;
    }

    var E = Document.edges(); // set of unprocessed edges
    var C = new Array();      // matching edges
    while (E.length > 0) {
        var e = E[0];         // we take first edge e={u,v}
        var u = e.from();
        var v = e.to();
        e.type = 2;           // set edge to be a matching edge
        E.shift();            // remove e (i.e., E[0]) from edge list
        C.push(u);            // add u to C
        C.push(v);            // add v to C

        // mark u,v as nodes in C
        u.type = 2;
        v.type = 2;

        // remove from E all edges incident to u or v
        var adjacent = u.edges();
        for (var i=0; i < adjacent.length; i++) {
            var index = E.indexOf(adjacent[i]); // find the index
            if (index != -1) {
                E.splice(index, 1); // remove it if really found
            }
        }
        var adjacent = v.edges();
        for (var i=0; i < adjacent.length; i++) {
            var index = E.indexOf(adjacent[i]); // find the index
            if (index != -1) {
                E.splice(index, 1); // remove it if really found
            }
        }
    }
    Console.log("Vertex Cover contains " + C.length + " nodes.");

#### Execute the Algorithm

The algorithm can be executed by the Run button at the script control panel.

## The &rocs; User Interface

### Main Elements of the User Interface

The user interface is divided into several logical parts as presented at the screenshot below.

&GUI; elements of the &rocs; interface.
&GUI; elements of the &rocs; interface.

Graph Editor

  The editor provides a whiteboard at that nodes and edges can be placed. Double-clicking at any of its elements opens a corresponding property menu. You can use the tools from the Side Bar Tabs to create and modify graphs.

  Available tools:

  - Top left in the box, your found the following action icons. Clicking at an action means that your mouse pointer applies this action at the graph editor whiteboard:

    - Select and Move: To select elements, either click at unused space at the whiteboard, keep the mouse pressed and draw a rectangle that contains some data elements and/or edges to select these elements or otherwise directly click at an unselected element to select this element. If you click at a selected element or a set of selected elements, respectively, by keeping the mouse pressed and moving around you can move these elements. Moving selected elements is also possible with the arrow keys.

    - Add a Node: Click at an arbitrary position at the graph editor whiteboard to create a new data element that belongs to the currently selected data structure. By keeping the mouse pointer pressed at the button, a menu shows up at which the data type of the new created data elements can be selected (only if different data types exist).

    - Create an Edge: Click at one data element, keep the mouse pressed and draw a line to another data element to which the edge shall point. This action is only successful if the current graph allows to add this edge (⪚, in an undirected graph you are not allowed to add multiple edges between two data elements). By keeping the mouse pointer pressed at the button, a menu shows up at which the edge type of the new created edges can be selected (only if different edge types exist).

    - Delete element: Click at an element to delete it. If you delete a node, all adjacent edges are also deleted.

Side Bar

  At the right, you can find the side bar that provides several tools for your workflow:

  - Element Types: This widget gives you direct access to the available edge and node types.

  - Journal: Each project has its own journal that can be used to, ⪚ note tasks, results, or observations.

  - Scripting API: To get direct access to the script documentation, you can open this widget.

Script Editor

  In this text editor you can write algorithms as explained in detail in . You can work on several script documents simultaneously by using several tabs.

Script output:

  This text area either shows debug information or the script output of your algorithm, depending on the toggled setting at the top of this widget. If the script throws an error, automatically the debug output is presented.

Controls

  Here you can find the controls for executing scripts. You can execute the script that is currently open at the script editor by pressing the Run button. While the script is executed, it is possible to stop execution by pressing the Stop button.

## Scripting

### Executing Algorithms in &rocs;

&rocs; internally uses the QtScript &javascript; engine. This means, all algorithms that you implement must use &javascript;. In the following, we explain how to access and change elements of a graph document from the scripting engine. It is important to note that changes done by the scripting engine are directly reflected at the properties at the graph editor elements.

#### Control Script Execution

There are different execution modes for your algorithms:

- Run: Execute the script until it finishes.

- Stop: Stop script execution (only available while a script is executed).

#### Script Output

During the execution of an algorithm, debug and program output is displayed in the Debug & Script Output. If the scripting engine detects a syntax error in your script, the error is also displayed as debug message. Note that all program messages are also displayed at the debug output (displayed as bold text).

You can control the text that is displayed at the script output by the following functions:

        Console.log(string message);            // displays the message as script output
        Console.debug(string message);          // displays the message as debug output
        Console.error(string message);          // displays the message as error output

#### Scripting Engine API

The different parts of &rocs; each provide a static element that can be accessed by the scripting engine. These are:

- ``Document`` for the graph document

- ``Console`` for the console log output

For the explicit API use and for a method reference, please see the inline help at the &rocs; side bar.

## Import and Export

### Exchange &rocs; Projects

&rocs; projects can be imported and exported as archived `.tar.gz` files. These archives can be used to exchange projects. Import and Export can be done with the Graph Document Import Graph... and Graph Document Export Graph As... menu items, respectively.

#### Import and Export of Graph Documents

&rocs; currently supports import and export of the following file formats:

- &DOT; files, also known as Graphviz files

- GML files

- Trivial Graph Format files

- Keyhole Markup Language Format

##### Trivial Graph File Format

The Trivial Graph Format (TGF) is a simple text-based file format for describing graphs. A TGF file consists of a list of node definitions, that map the node IDs to labels, followed by a list of the edges. In this format it is only possible to have one label per node and one value per edge. &rocs; interprets imported graphs as undirected graphs. Exported graphs will contain two edges per connection if connections are bidirectional.

###### Format Specification

- The file starts with a list of nodes (one node per line), followed by a line with the only character #, followed by a list of edges (one edge per line).

- A node consists of an integer (identifier), followed by a space, followed by an arbitrary string.

- An edge consists of two integers (identifiers) separated by a space, followed by a space, followed by an arbitrary string. It is assumed that the directed edge points from the first identifier to the second identifier.

###### Example

    1 starting node
    2 transmitter
    3 sink
    #
    1 2 blue
    2 1 red
    2 3 green

##### &DOT; Language / Graphviz Graph File Format

The &DOT; language is a plain text graph description language that allows both,a good human readable representation of graphs as well as an efficient processing by graph layout programs. &DOT; is the default file format for the Graphviz graph visualization suite, but is also widely used by other graph tools. The usual file extensions for &DOT; are `.gv` and `.dot`.

###### Unsupported Features

&rocs; can parse every graph file that contains a graph specified according to the &DOT; language specification

https://graphviz.org/doc/info/lang.html

. The support of language features is complete, despite of the following exceptions:

- subgraph: Due to the lack of a subgraph concept in &rocs;, subgraphs are only imported as a set of date elements and connections. Especially, connections to or from subgraphs are not imported.

- &HTML; and &XML; attributes: Attributes (like labels) that contain &HTML; or &XML; syntax are read unchanged. Especially, not adjustment of fonts and styles are read from those attributes.

###### Example

    digraph myGraph {
        a -> b -> c;
        b -> d;
    }

## Graph Layout

### Laying out graphs automatically in &rocs;

&rocs; can lay out graphs automatically. The &rocs; graph layout tool can be found in the main menu at Graph Document Tools Graph Layout. There are two different layout algorithms that can be applied: Force Based Layout and Radial Tree Layout. To apply one of them, select the corresponding tab of the graph layout tool, choose the desired parameters and execute the algorithm by pressing on the OK button. Details that are specific to each one of the layout algorithms are provided in the next sections.

#### Force Based Layout

The Force Based Layout can be applied to any graph. Intuitively, this algorithm simulates forces acting in each node. There are repelling forces between pairs of nodes and attraction forces between pairs of nodes that are neighbours. The magnitude of these forces can be specified by moving the corresponding sliders in the user interface.

A Screenshot of the Force Based Layout tab of the &rocs; graph layout tool.
A Screenshot of the Force Based Layout tab of the &rocs; graph layout tool.

Another parameter that can be controlled is the Area Factor. This parameter controls how the nodes are spread. Layouts generated with high values of Area Factor have a tendency of having large distances between nodes.

##### Radial Tree Layout

The Radial Tree Layout can only be applied to trees. Any attempt to apply this layout algorithm to other kinds of graph will produce an error message. Parameters for the Radial Tree Layout can be selected using the provided user interface.

A Screenshot of the Radial Tree Layout tab of the &rocs; graph layout tool.
A Screenshot of the Radial Tree Layout tab of the &rocs; graph layout tool.

The tree type parameter selects between a free tree layout and a rooted tree layout. In a free tree layout, nodes are placed freely without any apparent hierarchy between them. In a rooted tree layout, the root node is placed at the top and sub-trees are laid out below it, giving an idea of a hierarchy between nodes.

The center/root parameter defines which node is going be used as root for the rooted tree layout or as center for the free tree layout. The center of a free tree layout is the first node to be placed by the algorithm. All other nodes are placed on circles centered at the center node. A center/root can be selected automatically by the layout algorithm.

The node separation parameter controls the distance between nodes. Increasing the value of this parameter will cause the distance between nodes to increase. Similarly, decreasing the value of this parameter will cause the distance between nodes to decrease.

## Credits and License

&rocs;

Program Copyright:

- Copyright 2008 Ugo Sangiori (ugorox AT gmail.com)

- Copyright 2008-2012 Tomaz Canabrava (tcanabrava AT kde.org)

- Copyright 2008-2012 Wagner Reck (wagner.reck AT gmail.com)

- Copyright 2011-2015 Andreas Cord-Landwehr (cordlandwehr AT kde.org)

Documentation Copyright:

- Documentation copyright 2009 &Anne-Marie.Mahfouf; &Anne-Marie.Mahfouf.mail;

- Documentation copyright 2009 Tomaz Canabrava (tcanabrava AT kde.org)

- Documentation copyright 2011-2015 Andreas Cord-Landwehr (cordlandwehr AT kde.org)

&underFDL; &underGPL;
&documentation.index;