Simple Pi calculation library for JavaScript

spigot.js

/**
 * spigot.js
 * version 1.1.0
 * (C) 2012 shin <s2pch.luck at gmail.com>
 * Licensed under the Apache License 2.0
 * http://opensource.org/licenses/apache2.0
 * 
 * The Pi calculation is based on the spigot (streaming) algorithm[1].
 * This library depends on jsbn[2] (jsbn.js and jsbn2.js),
 * one of BigInteger library written by pure JavaScript.
 *
 * Referecne:
 * [1] Unbounded Spigot Algorithms for the Digits of Pi
 *     (http://www.cs.ox.ac.uk/jeremy.gibbons/publications/spigot.pdf)
 * [2] RSA and ECC in JavaScript
 *     (http://www-cs-students.stanford.edu/~tjw/jsbn/)
 */
(function() {

	var bi = function(n, b) {
		return new BigInteger(n.toString(), b ? b : 10);
	};
	var bi2int = function(bi, b) {
		return parseInt(bi.toString(b ? b : 10));
	};

	var Pi = function(fn) {
		this.fn_ = fn || function(v) { console.log(v); };
		this.q_ = bi(1);
		this.r_ = bi(180);
		this.t_ = bi(60);
		this.i_ = bi(2);
	};

	Pi.prototype.next_ = function() {
		var q = this.q_, r = this.r_, t = this.t_, i = this.i_;
		var tmp = i.multiply(bi(3));
		var u = tmp.add(bi(1)).multiply(bi(3)).multiply(tmp.add(bi(2)));
		var y = i.multiply(bi(27)).subtract(bi(12)).multiply(q).add(r.multiply(bi(5))).divide(t.multiply(bi(5)));
		this.q_ = q.multiply(bi(10)).multiply(i).multiply(i.multiply(bi(2)).subtract(bi(1)));
		tmp = q.multiply(i.multiply(bi(5)).subtract(bi(2))).add(r).subtract(t.multiply(y));
		this.r_ = u.multiply(bi(10)).multiply(tmp);
		this.t_ = t.multiply(u);
		this.i_ = i.add(bi(1));
		this.fn_(bi2int(y));
	};

	Pi.prototype.step = function(n) {
		if (n > 0)
			for (var i = 0; i < n; ++i)
				this.next_();
		else
			this.next_();
	};

	Pi.prototype.setCallback = function(fn) {
		this.fn_ = fn;
	};

	Pi.prototype.digits = function() {
		var t = parseInt(this.i_.toString());
		return t - 3;
	};

	Pi.prototype.save = function() {
		return {
			q: this.q_.toString(16),
			r: this.r_.toString(16),
			t: this.t_.toString(16),
			i: this.i_.toString(16)
		};
	};

	Pi.prototype.load = function(d) {
		this.q_ = bi(d.q, 16);
		this.r_ = bi(d.r, 16);
		this.t_ = bi(d.t, 16);
		this.i_ = bi(d.i, 16);
	};

	//-----------------------------------

	if (typeof exports !== "undefined")
		module.exports = Pi; // Node.js
	else
		this.SpigotPi = Pi;

}).call(this);