ngraph.quadtreebh

ngraph.quadtreebh
=================

Implementation of Barnes-Hut N-Body simulation in Javascript.

Reference
---------
1. Fast Hierarchical Methods for the N-body Problem - one of the best explanations of Barnes-Hut method by James Demmel.
2. Wikipedia article - general introduction into the problem

Usage
=====
I'm still working on API and it might be changed in future.
`` js
var Body = require('ngraph.physics.primitives').Body;
// Create new bodies at (1, 0) and (2, 0):
var bodies = [];
bodies.push(new Body(1, 0));
bodies.push(new Body(2, 0));
/ ... create more as you need ... /

// build quad tree:
var createQuadTree = require('ngraph.quadtreebh');
var quadTree = createQuadTree();

// insert bodies into the quad tree
quadTree.insertBodies(bodies); // performance: O(n * log n)

// calculate forces acting on each body in the tree O(n * log n):
bodies.forEach(function(body) {
quadTree.updateBodyForce(body);
});
// At this point every body object has valid 2d force vecor
console.dir(bodies[0].force);
`

_Note:_ You don't necessary have to use ngraph.physics.primitives. That package merely defines an interface for a physical Body, which is expected by quad tree. As long as your Body object implements this interface (mass, pos and force) - you can use current quad tree structure to calculate forces.

Configuring quad tree
=====================
Quad tree allows to change two global options:

* gravity - Gravitational constant, used in force value calculation. Defaults to -1;
* theta - a threshold which determines when group of bodies is considered distant enough to be approximated as a single body. The value should be > 0, and usually little less than 1. It defaults to 0.8 if not suplied. This argument affects accuracy and speed of simulation. Please refer to [\[1\]](http://www.eecs.berkeley.edu/~demmel/cs267/lecture26/lecture26.html) for more details.

You can pass these setting to quad tree as follows:
` js
var createQuadTree = require('ngraph.quadtreebh');
var quadTree = createQuadTree({
theta: 1.2,
gravity: -10
});
`

To query current options of the tree use:
` js
var createQuadTree = require('ngraph.quadtreebh');
var quadTree = createQuadTree();
console.dir(quadTree.options()); // prints { theta: 0.8, gravity: -1};
`

To change options at run time:
` js
var createQuadTree = require('ngraph.quadtreebh');
var quadTree = createQuadTree();
quadTree.options({
theta: 0.5,
gravity: -42
});
`

Mass of each body affects overall result of computation. You can tweak it when creating new bodies:
` js
var Body = require('ngraph.physics.primitives').Body;
var earth = new Body(); earth.mass = 5.972;
var sun = new Body(); sun.mass = 1989000;
`

Install
=======
`
npm install ngraph.quadtreebh
``

License
=======
BSD 3-Clause