ValidationUtils.isNonnegativeInteger returns true for -0

[thedavidmeister]

Just something I came across while working on #46 - i'm not sure if this is desired behaviour or not

[tonyofbyteball]

I didn't know about signed 0 until today. Where can we meet it?

[thedavidmeister]

@tonyofbyteball https://stackoverflow.com/questions/7223359/are-0-and-0-the-same

It's part of IEEE 754. Most of the time you can ignore it because there is no difference in strict equality checks or addition/subtraction, etc. -0 === 0 and 1+ -0 === 1 + 0 etc.

The main difference in practical terms is when dividing or multiplying by zero as:

1/-0 === -Infinity and 1/0 === Infinity

1 * -0; // -0 and 1 * 0; // 0

Of course, dividing anything by zero is problematic for other reasons anyway...

JS also returns false for Object.is(-0,0).

The reason it is coming up at all is because JS doesn't actually have an explicit integer number type, as it uses doubles for everything. Even after ES6 introduced Number.isInteger it still uses doubles for all "integers" - this breaks down a bit at 0 and outside Number.MIN_SAFE_INTEGER and Number.MAX_SAFE_INTEGER.

As a comparison, here is the output from my REPL in clojure (using the JVM which has both floats and integer number types):

boot.user=> (/ 1 0)
java.lang.ArithmeticException: Divide by zero
boot.user=> (/ 1 -0)
java.lang.ArithmeticException: Divide by zero
boot.user=> (/ 1 0.0)
Infinity
boot.user=> (/ 1 -0.0)
-Infinity

You can see there is a difference between signed 0 in doubles but dividing by integer 0 is always an exception as there isn't an integer negative zero AFAIK.

IMO the JVM behaviour of integers is usually what you want, dividing by the integer 0 is undefined in normal arithmetic and throwing an error is more helpful than returning +/-Infinity to be propagated through the rest of your code :/

[thedavidmeister]

So yeah... tbh it could go either way...

Argument for:

There is no difference between the integer represented by both -0 and 0 as there is only one integer 0 so therefore -0 is a negative double but not a negative integer, so therefore isNonnegativeInteger should return true.

Argument against:

JavaScript does not have any integers in reality, so following IEEE 754 there is a difference between -0 and 0 that will be apparent as soon as you multiply or divide anything by either of them. Even though we are willing to take ES6's lead on designating a subset of doubles as "integers" as per Number.isInteger, and -0 is definitely one of these integers, it is also a negative value, so we cannot say that it is "non negative". Therefore isNonnegativeInteger should return false.