Find 4-Neighbour (Rook) Adjacency Matrix for a Spatial Grid

Builds a sparse adjacency matrix \(A\) (patches x patches) where \(A[p, q] = 1\) if patches \(p\) and \(q\) share an edge (rook adjacency), and 0 otherwise. An optional water mask restricts edges so that land patches are excluded from the adjacency structure.

Usage

find_neighbors(resolution, water_mask)

Arguments

resolution

Integer vector of length 2: c(X, Y), where X is the number of columns (x-direction) and Y the number of rows (y-direction).

water

Logical vector of length X * Y in patch order (TRUE = water/open, FALSE = land/closed). Pass rep(TRUE, X * Y) for a fully open grid.

Value

A symmetric sparse matrix (class "dgCMatrix") of dimensions \(P \times P\) (where \(P = X \times Y\)) with dimnames "1", "2", ..., "P".

Details

Patch indexing follows the marlin convention: $$p = (x - 1) \times Y + y \quad \text{(y varies fastest)}$$ which matches the ordering produced by tidyr::expand_grid(x = 1:X, y = 1:Y) |> mutate(patch = 1:n()).

Used internally during movement-matrix construction in simmar to identify which patches are adjacent when updating habitat taxis.

See also

prep_movement, simmar