Serhiy Storchaka added the comment:
The probability of a collision when generated n numbers from 0 to m-1 is
1 - factorial(m)/factorial(m-n)/m**n
When n << sqrt(m), it is approximately equal to n**2/(2*m). When m = 1000000,
we have problems for n about 1000. When increase m to 2**32, the probability of
collisions is decreased to 0.01%. When increase m to 2**64 as recommended in
[1], it is decreased to 2.7e-14. I.e. when generate 1000 IDs per second,
collisions happen one per a million years.
We could also take datetime with larger precision. E.g with 2 digits after the
dot, as recommended in [1].
When apply both changes, we could generate 100000 IDs per second with one
collision per 10000 years or 10000 IDs per second with one collision per
1000000 years.
[1] https://tools.ietf.org/html/draft-ietf-usefor-message-id-01
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nosy: +serhiy.storchaka
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