0 is non-negative but also... non-positive?

The difference is a classic off-by-one error that can lead to a security exploit.

The integer numbers can be divided into "negative" (less than zero), zero, and "positive" (greater than zero). When one says "non-negative", they are excluding only the negative numbers, so zero is included; when one says "positive", zero is not included.

Positive numbers are greater than zero[1], while non-negative numbers are greater or equal to zero.

[1]: https://en.wikipedia.org/wiki/List_of_types_of_numbers#Signe...