Random Numbers Included Mar 31, 2025

I’ve recently worked on a PRNG API for TigerBeetle, and made a surprising discovery! While most APIs work best with “half-open” intervals, for(int i = 0; i < n; i++), it seems that random numbers really work best with closed intervals, ≤n.

First, closed interval means that you can actually generate the highest-possible number:

prng.range_inclusive(
    u32,
    math.intMax(u32) - 9,
    math.intMax(u32),
);

This call generates one of the ten largest u32s. With exclusive ranges, you’d have to generate u64 and downcast it.

Second, close interval removes a possibility of a subtle crash. It is impossible to generate a random number less than zero, so exclusive APIs are panicky. This can crash!: rng.random_range(..n)

Third, as a flip-side of the previous bullet point, by pushing the -1 to the call-site, you make it immediately obvious that there’s non-zero pre-condition:

const replica = prng.int_inclusive(u8, replica_count - 1);