Question

I’m looking for a hash algorithm, that when given a 64 bit sequential value, will hash to 8 bits uniformly.

I don’t want to just use the least-significant byte as the hash value.

Answer 1

As a general case of Oli Charlesworth’s solution, you can select coprime with 256 and pre-multiply each byte from your input by that value and then XOR all values together. You’ll still get uniform distribution, but for sequential inputs you’ll get non-sequential output, for instance:

byte result = 0;
int q = 33149;
foreach (byte b in BitConverter.GetBytes(input)) result += (byte)(b * q);

For 1, 2, 3, 4, 5, 6, … it will get you 125, 250, 119, 244, 113, 238, etc