Tuesday, September 29, 2015

Do you feel lucky today? (aka random Android notification ID)

Recently created an Android app that receives Google Cloud Messaging (GCM) messages.  Upon receiving certain messages, notifications will be shown on devices. Due to nature of the data, it is expected that a device will receive short bursts of multiple messages from time to time.

Now, to show a notification on Android, we need to assign an ID that is unique within the app. The "right way" to do it is to keep a counter as ID for each notification.

Well, I am too lazy to store that ID in the app. I have seen on stacktrace that some people tried to use the current timestamp in millisecond (long int)  as ID. But since the ID is an int, they divide the long by 1000 (ie reduce the resolution to second instead of millisecond) and cast the number to int. That is going to fail in my case as my app will receive multiple messages within a second.

There is an easy (and risky?) way to generate a "random" ID on the fly without keeping track of it: get system nano second in long and hash it.


The (potential) downside is:
- nanoTime in signed long will repeat every 292 years (roughly). But that is ok for my app
- hashing long into int may result in collision, but the chance should be small if I only need a few numbers in each burst of incoming notifications

So, do I feel lucky today?

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