Issue 6916 - Avoid Set object for filters with only one subscription

lib/filterClasses.js

Index: lib/filterClasses.js
===================================================================
--- a/lib/filterClasses.js
+++ b/lib/filterClasses.js
@@ -38,20 +38,20 @@
  * @constructor
  */
 function Filter(text)
 {
   this.text = text;
 
   /**
    * Subscriptions to which this filter belongs.
-   * @type {Set.<Subscription>}
+   * @type {(Subscription|Set.<Subscription>)?}
    * @private
    */
-  this._subscriptions = new Set();
+  this._subscriptions = null;
 }
 exports.Filter = Filter;
 
 Filter.prototype =
 {
   /**
    * String representation of the filter
    * @type {string}
@@ -68,46 +68,83 @@
   },
 
   /**
    * Yields subscriptions to which the filter belongs.
    * @yields {Subscription}

[Jon Sonesen, 2018/09/02 17:22:50]

Should this be {(Subscription|Set.<Subscription>)?}

[Manish Jethani, 2018/09/03 19:03:42]

No, the function only ever yields a single Subscription object. Note that we're
using `yield*` if it's a set, so each entry in the set is yielded individually.
This way the caller doesn't need to know about the internal optimization and
always gets a consistent value.

    */
   *subscriptions()
   {
-    yield* this._subscriptions;
+    if (this._subscriptions)
+    {
+      if (this._subscriptions instanceof Set)
+        yield* this._subscriptions;
+      else
+        yield this._subscriptions;
+    }
   },
 
   /**
    * The number of subscriptions to which the filter belongs.
    * @type {number}
    */
   get subscriptionCount()
   {
-    return this._subscriptions.size;
+    if (this._subscriptions instanceof Set)
+      return this._subscriptions.size;
+
+    return this._subscriptions ? 1 : 0;
   },
 
   /**
    * Adds a subscription to the set of subscriptions to which the filter
    * belongs.
    * @param {Subscription} subscription
    */
   addSubscription(subscription)
   {
-    this._subscriptions.add(subscription);
+    // Since we use truthy checks in our logic, we must avoid adding a
+    // subscription that isn't a non-null object.

[Jon Sonesen, 2018/09/02 17:22:50]

the double negative in the comment is sort of confusing, perhaps 

"Since we use truthy checks in our logic, we must avoid adding a subscription
that is null or not an object"

is a bit more clear?

[Manish Jethani, 2018/09/03 19:03:42]

The non-null part is a bit of a distraction: the problem is that `typeof null`
is also "object" in JavaScript, otherwise we could have just had the typeof
check. I can remove the "non-null" in that sentence if it makes it more
readable, it's obvious from the line that follows that we're checking for
`null`. What do you think?

+    if (subscription === null || typeof subscription != "object")
+      return;
+
+    if (this._subscriptions)
+    {
+      if (this._subscriptions instanceof Set)
+        this._subscriptions.add(subscription);
+      else if (subscription != this._subscriptions)
+        this._subscriptions = new Set([this._subscriptions, subscription]);
+    }
+    else
+    {
+      this._subscriptions = subscription;
+    }
   },
 
   /**
    * Removes a subscription from the set of subscriptions to which the filter
    * belongs.
    * @param {Subscription} subscription
    */
   removeSubscription(subscription)
   {
-    this._subscriptions.delete(subscription);
+    if (this._subscriptions)
+    {
+      if (this._subscriptions instanceof Set)
+      {
+        this._subscriptions.delete(subscription);
+
+        if (this._subscriptions.size == 1)
+          this._subscriptions = [...this._subscriptions][0];

[Jon Sonesen, 2018/09/02 17:22:50]

Just asking, not so much  a comment but this line changes _subscriptions to
whichever the remaining member of the set is?

[Manish Jethani, 2018/09/03 19:03:42]

Yes, that's exactly what it does. We can't do `this._subscriptions[0]` because
it's not an array, so basically we convert it to an array first using the spread
operator.

+      }
+      else if (subscription == this._subscriptions)
+      {
+        this._subscriptions = null;
+      }
+    }
   },
 
   /**
    * Serializes the filter to an array of strings for writing out on the disk.
    * @param {string[]} buffer  buffer to push the serialization results into
    */
   serialize(buffer)
   {