Guidance on how to handle IllegalStateException from TimeBasedEpochGenerator about java-uuid-generator HOT 5 CLOSED

Actually, it is partially dependent on randomness.
I was able to trigger the exception consistently by providing my own (adversarial) Random subclass, whose nextBytes() method writes mostly 0xff values.

When _lastEntropy is set to 0xff ff ff ff ff ff ff ff ff ff, you get the exception if you call generate() 2 times within the same millisecond.
For 0xff ff ff ff ff ff ff ff ff 00, you get the exception if you call generate() 257 times within the same millisecond.

So in general (without the adversarial Random :)), there's a 1/2^80 probability to get the exception from the 2nd call during the same millisecond, and 1/2^72 probability to get the exception during the first 257 calls within the same millisecond.

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That would result in generating values "too fast": I forget what is the specific granularity (implementation came as contribution). The challenge is that for same millisecond-based timestamp implementation does use secondary counter, but its capacity is limited -- and I suspect that if you generated UUIDs in a busy loop, a single cpu can overflow it.

So failure would NOT be due to heavy load (quite the opposite actually) but on trying to generate LOTS of UUIDs (for regular time-based ones it's 10 million per second; for epoch maybe less).
I doubt typical applications could reach that rate for normal usage, but if that did happen, yes, waiting for 1 millisecond would be something simple to do. Plain old retry would keep on failing until System.currentTimeMilllis() returns the next new value.

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Yes, that is compatible with my recollection of how things work.
The choice of random sequence (or, probability) changes number of "overlapping" uuids that can be generated for given time value.
And yes, with bad luck it could be a very low number.
I don't remember what specification says about choosing entropy values only up to half of max size: that would reduce likelihood of overrun, but also reduce entropy wrt security.

I am open to improvements but do not have enough time to spend on digging deep at this point, due to daytime work.

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Thank you, sir. You were very helpful.

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Thank you for your help here @gabrielbalan !

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