feat(operator): add groupBy

[benlesh]

closes #165

[trxcllnt]

Nitpicking here, but this logic seems ripe to put into a function on the GroupBySubscriber, the way we've done it in merge. This would also make it easier to subclass.

[benlesh]

Sounds reasonable

[benlesh]

There is a glaring bug in this one... if you select an object as a key. I'll need to rework this to add some sort of hash code or use an Array and lookup groups each time in the event that keys are objects.

[benlesh]

Made the desired changed for @trxcllnt ...

I'm not going to change the "bug" I mentioned above though, keys should be returned as strings or they will coerce as strings because it's using a hashtable under the hood for group storage. I'm sticking with this method for performance reasons. Native Map implementations and Map shims are much, much slower on the look up. Given that performance is a primary goal, and that creating a composite key in the keySelector should be trivial, I see no reason to slow things down to accommodate using "anything" as a key.

[benlesh]

Compromise

After speaking with both @trxcllnt and @mattpodwysocki on this... I've come up with what I think is a solid compromise...

• the underlying groups collection on the GroupBySubscriber starts off as null
• when the first key arrives, the type is examined (just once) to determine if it's a string or not.
  • if the key is a string: Use the FastMap, which is a Map-duck-quack-quack that simply wraps a Hash table.
  • if the key is not a string: Use native Map or a simple polyfill (copy pasta'ed from RxJS-of-yore).

My tests have determined that FastMap is twice as fast for getting and setting values than Map.

Caveats

• If the first key is a string, but other keys are Objects, you're going to have a bad time.

Future

I think this approach will enable us to easily remove the "FastMap" whenever ES6 Map is better optimized by more platforms.

[benlesh]

merged via rebase with 1e13aea

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