index.js

var Readable = require('readable-stream').Readable
var util = require('util')

module.exports = RandomSignal

function RandomSignal (opt) {

  if (!opt) opt = {}

  if (!(this instanceof RandomSignal))
    return new RandomSignal(opt);


  Readable.call(this, opt)

  var self = this
  self._destroyed = false
  self.iv = null

  var tdelta = opt.tdelta || 50 // milliseconds
    , hz = opt.hz || 1000 / (20 * tdelta) // seconds
    , amp = opt.amp || 1
    , noiseHz = opt.noiseHz || 4 * hz
    , noiseAmp = opt.noiseAmp || 0.3 * amp
    , trendIV = opt.trendIV || (1000 * 1/hz) // milliseconds
    , lowtrend = opt.lowtrend || -amp
    , hightrend = opt.hightrend || amp
    , sep = (typeof(opt.sep) === 'string') ? opt.sep : "\n"
    , timeFormatter = opt.timeFormatter || getTimeString

  hz *= 2 * Math.PI // compute once here for later ts computations
  noiseHz *= 2 * Math.PI

  var randTrendGen = randGenerator(lowtrend, hightrend)
    , randNoiseAmp = randGenerator(0, noiseAmp)

  if (self.trendIV < 2*self.tdelta)
    throw new Error("trendIV must be greater than 2*tdelta")



  startSignal()

  function startSignal () {
    var last = 0
      , trend = 0
      , start = Date.now()

    self.iv = setInterval( function () {
      var now = new Date()
        , ts = now.getTime()
      if ((ts - last) > trendIV) {
        trend = randTrendGen()
        last = ts
      }
      var x = timeFormatter(now)
      var y = generateSignal((ts - start) / 1000, trend)

      var data = {x:x, y:y}

      if (!opt.objectMode) {
        data = JSON.stringify(data) + sep
        data = new Buffer(data, 'utf8');
      }
      self.push(data);

    }, tdelta)
  }

  function generateSignal(ts, trend) {
    var y = amp*Math.sin(hz*ts)
      , noise = randNoiseAmp() * Math.sin(noiseHz*ts)
    return y + noise + trend
  }

}

util.inherits(RandomSignal, Readable)

RandomSignal.prototype._read = function () {
  if (this._destroyed) {
    this.push(null)
  }
}

RandomSignal.prototype.destroy = function () {
  clearInterval(this.iv)
  this._destroyed = true
  this.emit("end")
}

/*
 * Helper functions
 */
function getTimeString (now) {
  var year = "" + now.getFullYear();
  var month = "" + (now.getMonth() + 1); if (month.length == 1) { month = "0" + month }
  var day = "" + now.getDate(); if (day.length == 1) { day = "0" + day }
  var hour = "" + now.getHours(); if (hour.length == 1) { hour = "0" + hour }
  var minute = "" + now.getMinutes(); if (minute.length == 1) { minute = "0" + minute }
  var second = "" + now.getSeconds(); if (second.length == 1) { second = "0" + second }
  var ms = "" + now.getMilliseconds(); while (ms.length < 3) { ms = "0"+ms }
  return year + "-" + month + "-" + day + " " + hour + ":" + minute + ":" + second + "." + ms;
}


function randGenerator(min, max) {
  if (isNaN(min)) min = -1
  if (isNaN(max)) max = 1
  return function () {
    return Math.random() * (max - min + 1) + min
  }
}