Nothing

```
# optional arguments are passed to poly2nb
polygonAdjacency <- function(x, v='MUSYM', ...) {
# sanity check: package requirements
if(!requireNamespace('spdep'))
stop('please install the `spdep` package', call. = FALSE)
# compute neighbor list
nb <- spdep::poly2nb(x, ...)
# init empty list to store common attribute polygon IDs
# list index references the original polygon number
# each list item contains indexes to neighbors with the same map unit symbol
common.polys <- list()
# init empy list to store adjacency information via attribute `v`
edge.list <- list()
# iterate over polygons
for(i in 1:nrow(x)) {
# get the neighbors for polygon i
n.idx <- nb[[i]]
# keep track of the current polygon's attribute and its neighbors'
this.attr <- x[[v]][i]
this.nb <- x[[v]][n.idx]
# get an index to those neighbors that share the same symbol
common.symbol.idx <- which(this.attr == this.nb)
# store index to polygons with the same symbol
common.polys[[i]] <- n.idx[common.symbol.idx]
# store edge list information
# if there is no matching neighbor, then pad with NA
if(length(this.nb) == 0)
this.nb <- NA
edge.list[[i]] <- cbind(this.attr, this.nb)
}
# get a unique set of polygon indices to investigate
polys.to.investigate <- na.omit(unique(unlist(common.polys)))
# reduce edge list to single matrix of edges
edge.list <- do.call('rbind', edge.list)
edge.list <- na.omit(edge.list)
# init igraph object: note that there will be many duplicate edges
g <- graph.edgelist(edge.list, directed=FALSE)
# keep track of duplicate edges as weight, then remove
E(g)$weight <- log(count.multiple(g))
g <- simplify(g)
# save as weighted adjacancy matrix for plotting with sharpshootR functions
a <- get.adjacency(g, attr='weight')
# done
return(list(commonLines=polys.to.investigate, adjMat=a))
}
```

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