Check a Point lies in a Line segment

[Zachm]

I assumed (and later was proved correct) that the is_between function checks that x >= x1 (-tolerance) and that x <= x2 (+tolerance). This implies that x2 must be greater than x1, so x2-x1 is a positive number.

Regards,
Zachm

Ok, so maybe I wasn't proved to be correct in the end
You are correct again. The above code may return unwanted results due to the specific implementation of the is_between function and it is solved by using fabs as you suggested.
It seems you have 100% understanding of the code above and a keen eye to see that bug in the code, so you don't have to feel confused so much

Regards,
Zachm