Reactive programming - value is greater than X for N seconds

Question

I have a stream randStream which emit random value every half second and boolStream which converts value from randStream into boolean.

let randStream = Kefir.fromPoll(500, () => Math.random())
let boolStream = Kefir.map((rand) => rand > 0.5)

I want to emit true when boolStream emits true for 5 seconds (in a row). Otherwise emit false.

I'm using Kefir.js library.

Do you have any ideas? Thanks.

Answer 1

With given conditions when you know exact rate at which randStream emit numbers, it pretty easy to achieve with .slidingWindow:

let result = boolStream
  .slidingWindow(10, 10)
  .map(items => _.every(items))
  .skipDuplicates();

If you want it to work with any rate of events, you can try something like:

let result = boolStream
  .scan(({mostRecentFalse, latestValue}, bool) => {
    return bool ? 
      {mostRecentFalse, latestValue: true} : 
      {mostRecentFalse: Date.now(), lastValue: false}
  }, {mostRecentFalse: Date.now()})
  .changes()
  .map(({mostRecentFalse, latestValue}) => 
    latestValue && (Date.now() - mostRecentFalse > 5000))
  .skipDuplicates();

Answer 2

Sorry I can't write ES6 yet, but... the idea is, if your original stream is sampling once each half-second, five seconds of true is eleven trues in a row, right?

// generate random numbers
var randStream = Kefir.fromPoll(500, function() {
  return Math.random();
});

// make into booleans
var boolStream = randStream.map(function(rand) {
  return rand > 0.5;
});

// count trues in a row
var trueStreakStream = boolStream.scan(function(numTrue, curr) {
  return curr ? numTrue + 1 : 0;
}, 0);

// see when there's exactly 11 of them
var elevenTruesStream = trueStreakStream.filter(function(numTrue) {
  return numTrue == 11;
});

// react
elevenTruesStream.onValue(function(numTrue) {
  console.log("five seconds of true!");
});

EDIT: I just read your question again; if you want a stream that will be true if all of your last 5 seconds were true, then use map instead of filter (and >= rather than ==):

var lastElevenAreTrueStream = trueStreakStream.map(function(numTrue) {
  return numTrue >= 11;
});