abr-controller: move magic numbers to hls config params

related to https://github.com/dailymotion/hls.js/pull/467
related to https://github.com/dailymotion/player/pull/1046

API.md

@@ -483,15 +483,27 @@ this helps playback continue in certain cases that might otherwise get stuck.
 
 parameter should be a boolean
 
-#### ```abrControllerBandwidthWeight```
+#### ```abrBandwidthWeight```
 (default : 1.0)
 
 The weight to apply to the current bandwidth measurement when calculating the Exponentially-Weighted Moving Average (EWMA) of the bandwidth in the ABR controller.
 
-If ```α := abrControllerBandwidthWeight```, then ```bandwidth average := (α * latest bandwidth measurement) + ((1 - α) * previous bandwidth average)```.
+If ```α := abrBandwidthWeight```, then ```bandwidth average := (α * latest bandwidth measurement) + ((1 - α) * previous bandwidth average)```.
 
 parameter should be a float in the range (0.0, 1.0]
 
+#### ```abrBandWidthFactor```
+(default : 0.8)
+
+scale factor to be applied against measured bandwidth average, to determine whether we can stay on current or lower quality level
+If ``` abrBandWidthFactor * bandwidth average < level.bitrate ``` then ABR can switch to that level providing that it is equal or less than current level
+
+#### ```abrBandWidthUpFactor```
+(default : 0.7)
+
+scale factor to be applied against measured bandwidth average, to determine whether  we can switch up to a higher quality level
+If ``` abrBandWidthUpFactor * bandwidth average < level.bitrate ``` then ABR can switch up to that quality level
+
 ## Video Binding/Unbinding API
 
 #### ```hls.attachMedia(videoElement)```

src/controller/abr-controller.js

@@ -45,7 +45,7 @@ class AbrController extends EventHandler {
       this.lastfetchduration = (performance.now() - stats.trequest) / 1000;
       let thisbw = (stats.loaded * 8) / this.lastfetchduration;
       if (this.lastbw) {
-        let w = this.hls.config.abrControllerBandwidthWeight;
+        let w = this.hls.config.abrBandwidthWeight;
         this.lastbw = (w * thisbw) + (1.0 - w) * this.lastbw;
       } else {
         this.lastbw = thisbw;
@@ -182,11 +182,11 @@ class AbrController extends EventHandler {
     // be even more conservative (70%) to avoid overestimating and immediately
     // switching back.
       if (i <= this.lastLoadedFragLevel) {
-        adjustedbw = 0.8 * lastbw;
+        adjustedbw = config.abrBandWidthFactor * lastbw;
       } else {
-        adjustedbw = 0.7 * lastbw;
+        adjustedbw = config.abrBandWidthUpFactor * lastbw;
       }
-      if (adjustedbw < hls.levels[i].bitrate) {
+      if (adjustedbw < levels[i].bitrate) {
         return Math.max(0, i - 1);
       }
     }

src/hls.js

@@ -91,7 +91,9 @@ class Hls {
           enableCEA708Captions: true,
           enableMP2TPassThrough : false,
           stretchShortVideoTrack: false,
-          abrControllerBandwidthWeight: 1.0
+          abrBandwidthWeight: 1.0,
+          abrBandWidthFactor : 0.8,
+          abrBandWidthUpFactor : 0.7
         };
     }
     return Hls.defaultConfig;