Find arcs (maximal segment sequences between nodes)

Find arcs (maximal segment sequences between nodes)

Usage

find_arcs(x, quotient = FALSE)

Arguments

x

A wkpool (ideally after merge_coincident)

quotient

Walk the quotient graph: duplicated shared boundaries collapse to a single undirected edge (first occurrence keeps its direction), so a boundary shared by two features becomes ONE arc rather than one per feature, and its interior vertices are degree-2 pass-throughs. This is the TopoJSON-style arc decomposition of a polygon layer. Default FALSE walks every segment.

Value

A list of integer vectors, each containing .vx IDs forming an arc

Details

Arcs are maximal paths through degree-2 vertices. They start and end at nodes (degree != 2) or form closed loops through degree-2 vertices.

Examples

x <- wk::as_wkb(c(
  "POLYGON ((0 0, 1 0, 1 1, 0 1, 0 0))",
  "POLYGON ((1 0, 2 0, 2 1, 1 1, 1 0))"
))
pool <- establish_topology(x)
merged <- merge_coincident(pool)
find_arcs(merged)
#> [[1]]
#> [1] 2 1 4 3
#> 
#> [[2]]
#> [1] 2 3
#> 
#> [[3]]
#> [1] 2 5 6 3
#> 
#> [[4]]
#> [1] 2 3
#>