galaxygen/prng.js

const MersenneTwister = require('./twister');

function PRNG(seed) {
    this.mt = new MersenneTwister(seed);
    this.seed = seed;
    return this;
};

PRNG.prototype.seed = 0;

PRNG.prototype.value = function() {
  return this.mt.random()
};

PRNG.prototype.range = function(min, max) {
    return Math.floor(this.value() * (max - min + 1) + min);
};

PRNG.prototype.realRange = function(min, max, skew_function) {
    var v = this.value();
    if (typeof skew_function === 'function') {
        v = skew_function(v);
    }
    return v * (max - min) + min;
};

PRNG.prototype.gaussrandom = function(dev) {
    var X;
    var context = this.gaussrandom.context;
    if (context === undefined) {
        context = this.gaussrandom.context = {
            phase: 0
        };
    };
    if (context.phase === 0) {
        do {
            context.V1 = this.realRange(-1, 1);
            context.V2 = this.realRange(-1, 1);
            context.S = context.V1 * context.V1 + context.V2 * context.V2;
        } while (context.S >= 1 || context.S == 0);
        X = context.V1 * Math.sqrt(-2 * Math.log(context.S) / context.S);
    } else {
        X = context.V2 * Math.sqrt(-2 * Math.log(context.S) / context.S);
    }
    context.phase = 1 - context.phase;
    if (dev === undefined) {
        dev = 1.0;
    }
    return X * dev;
};

PRNG.prototype.pick = function(a, weights) {
    var s = 0,
        idx;
    if (weights !== undefined) {
        for (idx = 0; idx < weights.length; idx++) {
            s += weights[idx];
        }
        s = this.value() * s;
        for (idx = 0; idx < weights.length; idx++) {
            s -= weights[idx];
            if (s < 0) {
                break;
            }
        }
    } else {
      idx = this.range(0, a.length - 1);
    }

    return a[idx];
};

module.exports = PRNG;