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# dburner/lagrange.js

Forked from maccesch/lagrange.js
Created Jan 21, 2014
 /** * At least two points are needed to interpolate something. * @class Lagrange polynomial interpolation. * The computed interpolation polynomial will be reffered to as L(x). * @example * var l = new Lagrange(0, 0, 1, 1); * var index = l.addPoint(0.5, 0.8); * console.log(l.valueOf(0.1)); * * l.changePoint(index, 0.5, 0.1); * console.log(l.valueOf(0.1)); */ var Lagrange = function(x1, y1, x2, y2) { this.xs = [x1, x2]; this.ys = [y1, y2]; this.ws = []; this._updateWeights(); } /** * Adds a new point to the polynomial. L(x) = y * @return {Number} The index of the added point. Used for changing the point. See changePoint. */ Lagrange.prototype.addPoint = function(x, y) { this.xs.push(x); this.ys.push(y); this._updateWeights(); return this.xs.length-1; } /** * Changes a previously added point. */ Lagrange.prototype.changePoint = function(index, x, y) { this.xs[index] = x; this.ys[index] = y; this._updateWeights(); } /** * Recalculate barycentric weights. */ Lagrange.prototype._updateWeights = function() { var k = this.xs.length; var w; for (var j = 0; j < k; ++j) { w = 1; for (var i = 0; i < k; ++i) { if (i != j) { w *= this.xs[j] - this.xs[i]; } } this.ws[j] = 1/w; } } /** * Calculate L(x) */ Lagrange.prototype.valueOf = function(x) { var a = 0; var b = 0; var c = 0; for (var j = 0; j < this.xs.length; ++j) { if (x != this.xs[j]) { a = this.ws[j] / (x - this.xs[j]); b += a * this.ys[j]; c += a; } else { return this.ys[j]; } } return b / c; }

### AugLuk commented Feb 22, 2018

 I will use this in my project (and give you credit). I hope you don't mind :)