DistanceConstraint.js

module.exports = DistanceConstraint;

var Constraint = require('./Constraint');
var ContactEquation = require('../equations/ContactEquation');

/**
 * Constrains two bodies to be at a constant distance from each others center of mass.
 * @class DistanceConstraint
 * @constructor
 * @author schteppe
 * @param {Body} bodyA
 * @param {Body} bodyB
 * @param {Number} [distance] The distance to keep. If undefined, it will be set to the current distance between bodyA and bodyB
 * @param {Number} [maxForce=1e6]
 * @extends Constraint
 */
function DistanceConstraint(bodyA,bodyB,distance,maxForce){
    Constraint.call(this,bodyA,bodyB);

    if(typeof(distance)==="undefined") {
        distance = bodyA.position.distanceTo(bodyB.position);
    }

    if(typeof(maxForce)==="undefined") {
        maxForce = 1e6;
    }

    /**
     * @property {number} distance
     */
    this.distance = distance;

    /**
     * @property {ContactEquation} distanceEquation
     */
    var eq = this.distanceEquation = new ContactEquation(bodyA, bodyB);
    this.equations.push(eq);

    // Make it bidirectional
    eq.minForce = -maxForce;
    eq.maxForce =  maxForce;
}
DistanceConstraint.prototype = new Constraint();

DistanceConstraint.prototype.update = function(){
    var bodyA = this.bodyA;
    var bodyB = this.bodyB;
    var eq = this.distanceEquation;
    var halfDist = this.distance * 0.5;
    var normal = eq.ni;

    bodyB.position.vsub(bodyA.position, normal);
    normal.normalize();
    normal.mult(halfDist, eq.ri);
    normal.mult(-halfDist, eq.rj);
};