Cubic extrapolator

Extrapolator.js

  
/*
 * A cubic extrapolator using the polynomial
 *  P(T) = aT^2 + bT + c
 */
function Extrapolator() {
  this.s = [];
  this.t = [];
  this.sol = [];
};

/*
 * Add a sample to the extrapolator
 * @param {Number} s Sample
 * @param {Number} t Time 
 */
Extrapolator.prototype.addSample = function(s, t) {
  this.s.push(s);
  this.t.push(t);
  
  //We only handle 3 samples, so shift if we get more
  if(this.s.length > 3) {
    this.s.shift();
    this.t.shift();
  }
  
  //Only compute solution if we have 3 samples
  if(this.s.length > 2) {
    this.sol = Extrapolator._solve(this.s,this.t);
  }
};

/*
 * Return a predicted value from the extrapolator.
 * @param {Number} t Time to predict value for.
 * @Return {Number} P(T) of polynomial.
 */
Extrapolator.prototype.getValue = function(t) {  
  var s = this.sol;
  return s[0] * t * t + s[1] * t + s[2];
};

/*
 * Solves a 3x3 simultaneous equation system 
 * using cramers rule.
 */
Extrapolator._solve = function(p, t) {
 
 //cramers rule.
 var t0 = t[0], t1 = t[1], t2 = t[2],
     p0 = p[0], p1 = p[1], p2 = p[2],
     
     //Set up system
     a  = t0 * t0, b = t0, c = 1, d = p0,
     e  = t1 * t1, f = t1, g = 1, h = p1,
     i  = t2 * t2, j = t2, k = 1, l = p2,
     
     //Delta determinant
     delta = (a * f * k) + (b * g * i) +
             (c * e * j) - (c * f * i) -
             (a * g * j) - (b * e * k),
     
     //x answer(a);
     xnum  = (d * f * k) + (b * g * l) +
             (c * h * j) - (c * f * l) -
             (d * g * j) - (b * h * k),
     xans = xnum/delta,
     
     //y answer(b)
     ynum  = (a * h * k) + (d * g * i) +
             (c * e * l) - (c * h * i) -
             (a * g * l) - (d * e * k),
     yans = ynum/delta,
     
     //z answer(c)
     znum  = (a * f * l) + (b * h * i) +
             (d * e * j) - (d * f * i) -
             (a * h * j) - (b * e * l),
     zans = znum/delta;
     
     return [xans, yans, zans];
};

/*
 * A 2D Vector extrapolator using
 * one extrapolator for each component
 */
function Vector2Extrapolator() {
  this.ex = new Extrapolator();
  this.ey = new Extrapolator();
};

/*
 * Add a sample to the extrapolator
 * @param {Number} s Sample
 * @param {Number} t Time 
 */
Vector2Extrapolator.prototype.addSample = function(v,t) {
  this.ex.addSample(v.x, t);
  this.ey.addSample(v.y, t);
};

/*
 * Return a predicted value from the extrapolator.
 * @param {Number} t Time to predict value for.
 * @Return {Number} P(T) of polynomial.
 */
Vector2Extrapolator.prototype.getValue = function(t) {
  var x = this.ex.getValue(t);
  var y = this.ey.getValue(t);
  return new Vector(x,y);
};