Type safety for indices

[wtholliday]

Manifold uses ints for indices which shouldn't be assigned to each other, such as vertex indices and half-edge indices. This proposal adds custom index support to VecView.

This is a WIP, and I will attempt the rest of the changes with an OK from @elalish :)

[pca006132]

LGTM

[codecov]

Codecov Report

Attention: Patch coverage is 54.05405% with 17 lines in your changes missing coverage. Please review.

Project coverage is 88.09%. Comparing base (d437097) to head (a3563fc).
Report is 104 commits behind head on master.

Files with missing lines	Patch %	Lines
src/manifold/src/shared.h	40.00%	15 Missing ⚠️
src/manifold/src/quickhull.cpp	0.00%	1 Missing ⚠️
src/utilities/include/manifold/vec_view.h	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #958      +/-   ##
==========================================
- Coverage   91.84%   88.09%   -3.75%     
==========================================
  Files          37       62      +25     
  Lines        4976     8685    +3709     
  Branches        0     1052    +1052     
==========================================
+ Hits         4570     7651    +3081     
- Misses        406     1034     +628     

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[elalish]

I think this is probably a good idea, but can you show a little bit more, specifically how using these new indices looks? My main concern is code readability, so this will be okay so long as it's not adding lots of characters.

Also, should GetIndex be a member function instead of a free function?

[wtholliday]

I don't think it will make code readability worse.

I think perhaps some policy template could keep GetIndex from being a free function. The issue is that VecView still needs to work for size_t indices. I'll look into it.

[elalish]

We do a lot of index addition/multiplication, so I just want to make sure that's as automatic with HalfedgeIdx as it is with size_t.

[wtholliday]

Gotcha. Well, we see how it looks :)

[wtholliday]

I think this is just too hard a change to make, even if the result would be nice.

[elalish]

Yeah, I'm not too surprised; thanks anyway.