Hi, I'm trying to calculate the Euler Characteristic of my mesh. I need to figure the Genus of my mesh, to check if it is a torus or disk.

To to this I just need to count the number of vertexs (points), faces (primitives) and edges..

Points and prims are easy, but how can I count the number of edges?

https://en.wikipedia.org/wiki/Euler_characteristic [en.wikipedia.org]

but it will be slow for high res models.

Python is indeed very slow when you have to deal with millions of polygons.

But there is another way using vex

You run an attrib wrangle over primitives and for each primitive you look at its half edges.
hedge_equivcount() will give you the number of half edges that are equivalent to your current half edge.
If it has only one equivalent, then it's an unshared edge.
If it has more, then you know it's a shared edge and it contributes by ( 1 / hedge_equivcount ) to the total count.
An edge between two primitives thus equals 1, because each prim's half edge contributes 0.5.
Last but not least you promote the count to a detail attribute to get the total sum.

This might sound more complicated, but it is faster because most of it is running multithreaded in the wrangle sop.

Not very elegant, but you could set carve node to
U = 0 and V = 1
Breakpoints -> Cut Breakpoints

and count prims.

Konstantin Magnus
Not very elegant, but you could set carve node to
U = 0 and V = 1
Breakpoints -> Cut Breakpoints

and count prims.

Doesn't carve return multiple lines along shared edges ? That would give you a wrong number.

There's also a hedge_isprimary() vex function which only returns true for one in every set of shared half edges, you can make a wrangle to “Run Over: Detail” and iterate over the half edges testing for primary:

EDIT: although I think there's more magic to @blackpixel's solution

nice one friedasparagus! It's a bit slower than running over prims, but still miles ahead of python