Daniel Lemire's blog

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Are 64-bit random identifiers free from collision?

It is common in software system to map objects to unique identifiers. For example, you might map all web pages on the Internet to a unique identifier.

Often, these identifiers are integers. For example, many people like to use 64-bit integers. If you assign two 64-bit integers at random to distinct objects, the probability of a collision is very, very small. You can be confident that they will not collide.

However, what about the case where you have 300 million objects? Or maybe 7 billion objects? What is the probably that at least two of them collide?

This is just the Birthday’s paradox. Wikipedia gives us an approximation to the collision probability assuming that the number of objects r is much smaller than the number of possible values N: 1-exp(-r**2/(2N)). Because there are so many 64-bit integers, it should be a good approximation.

Number of objects Collision probability
500M 0.7%
1B 3%
1.5B 6%
2B 10%
4B 35%

Thus if you have a large system with many objects, it is quite conceivable that your randomly assigned 64-bit identifiers might collide. If a collision is a critical flaw, you probably should not use only 64 bits.

Computation (example):

>>> r=500000000
>>> N=2**64
>>> ratio = 2*N / r**2
>>> ratio
>>> 1-exp(-1/ratio)