Why Use Prime Numbers for Better Hash Code Distribution?

Why Use Prime Numbers in HashCode Methods?

Prime numbers are widely used in hashCode() methods to optimize the distribution of hash values among hash buckets. This choice is particularly advantageous when handling data with potential patterns or biases.

When input data exhibits random and evenly distributed patterns, the choice of hash code modulus becomes less critical. However, real-world data often presents inherent biases, such as alignment constraints or predictable address ranges.

Consider the example of 32-bit integers, which are typically aligned to addresses divisible by 4. Using a prime modulus, such as 7, results in better distribution compared to a non-prime modulus, such as 8:

Input	Modulo 8	Modulo 7
0	0	0
4	4	4
8	0	1
12	4	5
16	0	2
20	4	6
24	0	3
28	4	0

As evident, the distribution using a prime modulus is much more uniform, preventing collisions or uneven distribution.

Therefore, when dealing with data that may have patterns or biases, using a prime number as the hash code modulus can significantly enhance the distribution of hash values, reducing the likelihood of hash collisions and improving the overall performance of the hashing mechanism.

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