Why is Int63n from Go's rand package called that?

Question

From the official Go docs:

Int63n returns, as an int64, a non-negative pseudo-random number in [0,n) from the default Source. It panics if n <= 0.

Why is it called Int63n when it returns a 64 bit integer?

Answer 1

Int63n returns, as an int64, a non-negative pseudo-random number in [0,n) from the default Source

int64 is a 64-bit signed integer type. That means it has 1 sign bit and 63 significant bits. That means that anything returning a non-negative int64 is producing 63 bits of data (the 64th bit, the sign bit, will always have the same value).