bayes network

bayes.js

var Bayes = (function(){
  function constructor(positives, negatives) {
    this.positives = positives;
    this.negatives = negatives || reverse(positives);
  }

  function compute(probabilities) {
    return probabilities.reduce(function(previous, current){
      return previous * current;
    }, 1);
  }

  function reverse(probabilities) {
    return probabilities.map(function(value){
      return 1 - value;
    });
  }

  constructor.calculate = function(positives, negatives){
    return new this(positives, negatives).calculate();
  };

  constructor.prototype.calculate = function(){
    var p = compute(this.positives)
      , r = compute(this.negatives);

    return p / (p + r);
  };

  return constructor;
}());

if (typeof(module) !== "undefined") { module.exports = Bayes; }

bayes_test.js

var assert = require("assert")
  , Bayes = require("./bayes");

describe("Bayes", function(){
  it("calculates the independent probabilities", function(){
    assert_calc([0.6, 0.72], undefined, 0.794118);
    assert_calc([0.01, 0.9, 0.9, 0.2], undefined, 0.169811);
  });

  it("calculates the dependent probabilitiess", function(){
    assert_calc([0.01, 0.9, 0.9], [0.99, 0.2, 0.2], 0.169811);
    assert_calc([0.01, 0.9, 0.1], [0.99, 0.2, 0.8], 0.00564972);
  });

  function assert_calc(positives, negatives, expectation){
    assert.equal(
      Bayes.calculate(positives, negatives).toFixed(5),
      expectation.toFixed(5)
    );
  }
});