edges() method for counting edges

[MichaelChirico]

Hi,

This is arising from today's "Advent of Code" problem (Pt. 2):

https://adventofcode.com/2024/day/12

The problem basically gives a raster and asks to find the number of edges for each implied polygon, especially paying mind to holes.

Pt. 1 was particularly well-suited to using {terra} (especially with the new patches(values=TRUE)), since the perim() method immediately answers the "harder" part about the perimeter of the implied polygons.

I was left searching for something in the package to solve Pt. 2 and couldn't see anything (I looked at boundaries() and a few other things but couldn't see how to make it work).

I decided to peek at the source code for perim() and saw that it could easily be tweaked to solve the question of number of edges (as done here).

I leave the question of nomenclature/API up to you -- I chose edges() but am not married to that at all. I think you have much more context about the full API surface of the package, its documentation & use cases, etc, so I trust your judgment on how this should be named, or if perhaps it should just be a new argument to perim().

I left out the NEWS and documentation while this issue of API is pending.

One thing I didn't understand at all was the need to have -1 in the calculations, but I can report this gives the correct answer on the toy examples I've added to the tests, as well as the much more complicated input given by AoC.

Of course, if you think the use case here is too narrow / not likely to be needed in the package generally, feel free to close this PR.

[rhijmans]

With your example data from #1671

library(terra)
v <- matrix(c(5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,4,4,4,5,5,4,4,5,5,5,5,4,4,4,5,5,4,4,4,5,5,4,4,4,4,4,5,4,4,5,5,5,5,5,5,5,5,5,5,5,5,5), nrow=6, byrow=TRUE)
r <- rast(v, crs="local")
x <- patches(r, values=TRUE)
p <- as.polygons(x)

edges(p)
#[1] 20  8  8

With other functions you could do

sapply(1:nrow(p), \(i) nrow(disagg(p[i], segments=TRUE)))
#[1] 4 8 8
 
sapply(1:nrow(p), \(i) nrow(disagg(p[i], TRUE)) + nrow(disagg(fillHoles(p[i], TRUE), TRUE)))
#[1] 20  8  8

v <- as.lines(p)
sapply(1:nrow(v), \(i) nrow(disagg(v[i], TRUE)))
#[1] 20  8  8

The -1 in edges_geom is needed because you need (n+1) nodes (coordinates) to make (n) edges.

[rhijmans]

renamed edges to nseg (number of segments) to reserve edges for when I implement spatial graphs.