ngraph.graph

ngraph.graph

[Graph](http://en.wikipedia.org/wiki/Graph_(mathematics\)) data structure for ngraph.*. Library implements API to modify graph structure and supports event-driven notifications when graph changes.

Creating a graph

Create a graph with no edges and no nodes:

var createGraph = require('ngraph.graph');
var g = createGraph();

Growing a graph

The graph g can be grown in two ways. You can add one node at a time:

g.addNode('hello'); 
g.addNode('world'); 

Now graph g contains two nodes: hello and world. You can also use addLink() method to grow a graph. Calling this method with nodes which are not present in the graph creates them:

g.addLink('space', 'bar'); // now graph 'g' has two new nodes: 'space' and 'bar'

If nodes already present in the graph 'addLink()' makes them connected:

g.addLink('hello', 'world'); // Only a link between 'hello' and 'bar' is created. No new nodes.

What to use as nodes and edges?

The most common and convenient choices are numbers and strings. You can associate arbitrary data with node via optional second argument of addNode() method:

g.addNode('world', 'custom data'); // Now node 'world' is associated with a string object 'custom data'

You can also associate arbitrary object with a link using third optional argument of addLink() method:

g.addLink(1, 2, x); // A link between nodes '1' and '2' is now associated with object 'x'

Enumerating nodes and links

After you created a graph one of the most common things to do is to enumerate its nodes/links to perform an operation.

g.forEachNode(function(node){
    console.log(node.id, node.data);
});

The function takes callback which accepts current node. Node object may contain internal information. node.id and node.data represent parameters passed to the g.addNode(id, data) method and they are guaranteed to be present in future versions of the library.

To enumerate all links in the graph use forEachLink() method:

g.forEachLink(function(link) {
    console.dir(link);
});

To enumerate all links for a specific node use forEachLinkedNode() method:

g.forEachLinkedNode('hello', function(linkedNode, link){
    console.log("Connected node: ", linkedNode.id, linkedNode.data); 
    console.dir(link); // link object itself
});

This method always enumerates both inbound and outbound links. If you want to get only outbound links, pass third optional argument:

g.forEachLinkedNode('hello',
    function(linkedNode, link) { /* ... */ },
    true // enumerate only outbound links
  );

To get a particular node object use getNode() method. E.g.:

var world = g.getNode('world'); // returns 'world' node
console.log(world.id, world.data);

Finally to remove a node or a link from a graph use removeNode() or removeLink() correspondingly:

g.removeNode('space');
// Removing link is a bit harder, since method requires actual link object:
g.forEachLinkedNode('hello', function(linkedNode, link){
  g.removeLink(link); 
});

Listening to Events

Whenever someone changes your graph you can listen to notifications:

g.on('changed', function(changes) {
  console.dir(changes); // prints array of change records
});

g.add(42); // this will trigger 'changed event'

Each change record holds information:

ChangeRecord = {
  changeType: add|remove|update - describes type of this change
  node: - only present when this record reflects a node change, represents actual node
  link: - only present when this record reflects a link change, represents actual link
}

Sometimes it is desirable to react only on bulk changes. ngraph.graph supports this via beginUpdate()/endUpdate() methods:

g.beginUpdate();
for(var i = 0; i < 100; ++i) {
  g.addLink(i, i + 1); // no events are triggered here
}
g.endUpdate(); // this triggers all listners of 'changed' event

If you want to stop listen to events use off() method:

g.off('changed', yourHandler); // no longer interested in changes from graph

For more information about events, please follow to ngraph.events

Install

With npm do:

npm install ngraph.graph

License

BSD 3-clause