`2305843009213693951`

is `2^61 - 1`

. It's the largest Mersenne prime that fits into 64 bits.

If you have to make a hash just by taking the value mod some number, then a large Mersenne prime is a good choice -- it's easy to compute and ensures an even distribution of possibilities. (Although I personally would never make a hash this way)

It's especially convenient to compute the modulus for floating point numbers. They have an exponential component that multiplies the whole number by `2^x`

. Since `2^61 = 1 mod 2^61-1`

, you only need to consider the `(exponent) mod 61`

.