perf(dash): improve readability and reduce number of loops in dash pa…

…rser (shaka-project#5768)

This change reduces the amount of loops in dash parser, and improves
code readability, as checking config and creating an array of sets is in
a separate method now.

---------

Co-authored-by: Ivan Kohut <ivan.kohut@lamin.ar>

lib/dash/dash_parser.js

@@ -850,40 +850,29 @@ shaka.dash.DashParser = class {
       }
     }
 
-    const audioSets = this.config_.disableAudio ? [] :
-        this.getSetsOfType_(normalAdaptationSets, ContentType.AUDIO);
-    const videoSets = this.config_.disableVideo ? [] :
-        this.getSetsOfType_(normalAdaptationSets, ContentType.VIDEO);
-    const textSets = this.config_.disableText ? [] :
-        this.getSetsOfType_(normalAdaptationSets, ContentType.TEXT);
-    const imageSets = this.config_.disableThumbnails ? [] :
-        this.getSetsOfType_(normalAdaptationSets, ContentType.IMAGE);
-
-    if (!videoSets.length && !audioSets.length) {
+    const audioStreams = this.getStreamsFromSets_(
+        this.config_.disableAudio,
+        normalAdaptationSets,
+        ContentType.AUDIO);
+    const videoStreams = this.getStreamsFromSets_(
+        this.config_.disableVideo,
+        normalAdaptationSets,
+        ContentType.VIDEO);
+    const textStreams = this.getStreamsFromSets_(
+        this.config_.disableText,
+        normalAdaptationSets,
+        ContentType.TEXT);
+    const imageStreams = this.getStreamsFromSets_(
+        this.config_.disableThumbnails,
+        normalAdaptationSets,
+        ContentType.IMAGE);
+
+    if (videoStreams.length === 0 && audioStreams.length === 0) {
       throw new shaka.util.Error(
           shaka.util.Error.Severity.CRITICAL,
           shaka.util.Error.Category.MANIFEST,
-          shaka.util.Error.Code.DASH_EMPTY_PERIOD);
-    }
-
-    const audioStreams = [];
-    for (const audioSet of audioSets) {
-      audioStreams.push(...audioSet.streams);
-    }
-
-    const videoStreams = [];
-    for (const videoSet of videoSets) {
-      videoStreams.push(...videoSet.streams);
-    }
-
-    const textStreams = [];
-    for (const textSet of textSets) {
-      textStreams.push(...textSet.streams);
-    }
-
-    const imageStreams = [];
-    for (const imageSet of imageSets) {
-      imageStreams.push(...imageSet.streams);
+          shaka.util.Error.Code.DASH_EMPTY_PERIOD,
+      );
     }
 
     return {
@@ -896,15 +885,26 @@ shaka.dash.DashParser = class {
   }
 
   /**
+   * Gets the streams from the given sets or returns an empty array if disabled
+   * or no streams are found.
+   * @param {boolean} disabled
    * @param {!Array.<!shaka.dash.DashParser.AdaptationInfo>} adaptationSets
-   * @param {string} type
-   * @return {!Array.<!shaka.dash.DashParser.AdaptationInfo>}
-   * @private
-   */
-  getSetsOfType_(adaptationSets, type) {
-    return adaptationSets.filter((as) => {
-      return as.contentType == type;
-    });
+   * @param {string} contentType
+   @private
+  */
+  getStreamsFromSets_(disabled, adaptationSets, contentType) {
+    if (disabled || !adaptationSets.length) {
+      return [];
+    }
+
+    return adaptationSets.reduce((all, part) => {
+      if (part.contentType != contentType) {
+        return all;
+      }
+
+      all.push(...part.streams);
+      return all;
+    }, []);
   }
 
   /**