Avoid ewma sampling from failing on 0 duration segments. (#583)

This fixes a divide-by-zero that caused the estimate to become NaN.

Fixes #582

lib/abr/ewma.js

@@ -54,8 +54,12 @@ shaka.abr.Ewma = function(halfLife) {
  */
 shaka.abr.Ewma.prototype.sample = function(weight, value) {
   var adjAlpha = Math.pow(this.alpha_, weight);
-  this.estimate_ = value * (1 - adjAlpha) + adjAlpha * this.estimate_;
-  this.totalWeight_ += weight;
+  var newEstimate = value * (1 - adjAlpha) + adjAlpha * this.estimate_;
+
+  if (!isNaN(newEstimate)) {
+    this.estimate_ = newEstimate;
+    this.totalWeight_ += weight;
+  }
 };
 
 

test/abr/simple_abr_manager_unit.js

@@ -182,6 +182,29 @@ describe('SimpleAbrManager', function() {
     });
   });
 
+  it('can handle 0 duration segments', function() {
+    var audioBandwidth = 5e5;
+    var videoBandwidth = 2e6;
+    var bytesPerSecond =
+        sufficientBWMultiplier * (audioBandwidth + videoBandwidth) / 8.0;
+
+    abrManager.chooseStreams(streamSetsByType);
+
+    // 0 duration segment shouldn't cause us to get stuck on the lowest variant
+    abrManager.segmentDownloaded(1000, 1000, bytesPerSecond);
+    abrManager.segmentDownloaded(2000, 3000, bytesPerSecond);
+
+    abrManager.enable();
+
+    abrManager.segmentDownloaded(4000, 5000, bytesPerSecond);
+
+    expect(switchCallback).toHaveBeenCalled();
+    expect(switchCallback.calls.argsFor(0)[0]).toEqual({
+      'audio': jasmine.objectContaining({bandwidth: audioBandwidth}),
+      'video': jasmine.objectContaining({bandwidth: videoBandwidth})
+    });
+  });
+
   it('picks lowest audio Stream when there is insufficient bandwidth',
       function() {
         // The lowest audio track will only be chosen if needed to fit the