lib/rotjs-library/src/rng.js

/**
 * @namespace
 * This code is an implementation of Alea algorithm; (C) 2010 Johannes Baagøe.
 * Alea is licensed according to the http://en.wikipedia.org/wiki/MIT_License.
 */
ROT.RNG = {
	/**
	 * @returns {number} 
	 */
	getSeed: function() {
		return this._seed;
	},

	/**
	 * @param {number} seed Seed the number generator
	 */
	setSeed: function(seed) {
		seed = (seed < 1 ? 1/seed : seed);

		this._seed = seed;
		this._s0 = (seed >>> 0) * this._frac;

		seed = (seed*69069 + 1) >>> 0;
		this._s1 = seed * this._frac;

		seed = (seed*69069 + 1) >>> 0;
		this._s2 = seed * this._frac;

		this._c = 1;
		return this;
	},

	/**
	 * @returns {float} Pseudorandom value [0,1), uniformly distributed
	 */
	getUniform: function() {
		var t = 2091639 * this._s0 + this._c * this._frac;
		this._s0 = this._s1;
		this._s1 = this._s2;
		this._c = t | 0;
		this._s2 = t - this._c;
		return this._s2;
	},

	/**
	 * @param {float} [mean=0] Mean value
	 * @param {float} [stddev=1] Standard deviation. ~95% of the absolute values will be lower than 2*stddev.
	 * @returns {float} A normally distributed pseudorandom value
	 */
	getNormal: function(mean, stddev) {
		do {
			var u = 2*this.getUniform()-1;
			var v = 2*this.getUniform()-1;
			var r = u*u + v*v;
		} while (r > 1 || r == 0);

		var gauss = u * Math.sqrt(-2*Math.log(r)/r);
		return (mean || 0) + gauss*(stddev || 1);
	},

	/**
	 * @returns {int} Pseudorandom value [1,100] inclusive, uniformly distributed
	 */
	getPercentage: function() {
		return 1 + Math.floor(this.getUniform()*100);
	},
	
	/**
	 * @param {object} data key=whatever, value=weight (relative probability)
	 * @returns {string} whatever
	 */
	getWeightedValue: function(data) {
		var avail = [];
		var total = 0;
		
		for (var id in data) {
			total += data[id];
		}
		var random = Math.floor(this.getUniform()*total);
		
		var part = 0;
		for (var id in data) {
			part += data[id];
			if (random < part) { return id; }
		}
		
		return null;
	},

	/**
	 * Get RNG state. Useful for storing the state and re-setting it via setState.
	 * @returns {?} Internal state
	 */
	getState: function() {
		return [this._s0, this._s1, this._s2, this._c];
	},

	/**
	 * Set a previously retrieved state.
	 * @param {?} state
	 */
	setState: function(state) {
		this._s0 = state[0];
		this._s1 = state[1];
		this._s2 = state[2];
		this._c  = state[3];
		return this;
	},

	_s0: 0,
	_s1: 0,
	_s2: 0,
	_c: 0,
	_frac: 2.3283064365386963e-10 /* 2^-32 */
}

ROT.RNG.setSeed(Date.now());