Propose isParallel as new module

Propose @turf/boolean-parallel module

To complement the boolean themed modules, this would only input (Multi)LineString geometry types and should be able to detect if the two inputs are parallel or not.

The majority of the code is already built in @turf/line-offset in isParallel() which was inspired by node-intersection.

The only trick with this module would be to support LineString inputs with more than 2 vertices, @turf/line-segment can help iterate over both inputs.

[stebogit]

@dpmcmlxxvi

For a segment using a constant distance seems like the easiest to implement using the available turf functions. I try to avoid angles and bearings if I can get away with distances.

What function would you use to calculate the distance? I can’t think of one Why would you avoid angles, what kind of issues do you think might arise?

Additional thought, would two parallel rhumb-lines have always the same (rhumb, I guess) distance?

What about Great-Circle lines? Maybe these can’t be parallel…?

Would it be sequential segments that are parallel?

I’d say yes.

Does each sequential pair have to be of the same length?

To me this seems to be maybe too restrictive; two multi-segment LineString would be parallel only in case of a translation, as you pointed out. At this point comparing just the bearings makes it very convenient in my opinion.

For MultiLineString …

I would leave to the user how to define how to compare complex geometries; I feel like it would be kind of a guessing and most of the time a waist of resources in trying a lot of different combinations. However we can implement this part at any later time if necessary.

[dpmcmlxxvi]

I’d recommend working up a definition for “parallel” for the various cases first:

• For a segment using a constant distance seems like the easiest to implement using the available turf functions. I try to avoid angles and bearings if I can get away with distances.
• For a LineString it’s a little vague how the parallel postulate would be applied. Would it be sequential segments that are parallel? Does each sequential pair have to be of the same length? If they do, are you really looking for an equal geometry that has just been shifted?
• For MultiLineString you could define parallel as being that every LineString in MultiLineString #1 has a unique LineString in MultiLineString #2 to which it is parallel, and vice versa.

Lots of things to think through. Would be interesting to see if there is an agreed upon definition for these types of geometries out there.