R/ah2sf.R

In rangeBuilder: Occurrence Filtering, Geographic Standardization and Generation of Species Range Polygons

# Function written by Andrew Bevan, found on R-sig-Geo, and modified by Pascal Title
# modified to support sf objects 17 Nov 2022

ah2sf <- function(x, increment = 360, rnd = 10, crs = 4326, tol = 1e-4) {
	if (!inherits(x, "ahull")) {
		stop("x needs to be an ahull class object")
	}
	# Extract the edges from the ahull object as a dataframe
	xdf <- as.data.frame(x$arcs)
 	 
	#correct for possible arc order strangeness (Pascal Title addition 29 Nov 2013)
	k <- 1
	xdf <- cbind(xdf, flip = rep(FALSE, nrow(xdf)))
	repeat {
		if (is.na(xdf[k+1, 'end1'])) {
			break
		}
		#cat(k, '\n')
		if (xdf[k,'end2'] == xdf[k+1,'end1']) {
		#cat('case 1\n')
			k <- k + 1
		} else if (xdf[k,'end2'] != xdf[k+1,'end1'] & !xdf[k,'end2'] %in% xdf$end1[k+1:nrow(xdf)] & !xdf[k,'end2'] %in% xdf$end2[k+1:nrow(xdf)]) {
			#cat('case 2\n')
		 		 k <- k + 1
		} else if (xdf[k,'end2'] != xdf[k+1,'end1'] & xdf[k,'end2'] %in% xdf$end1[k+1:nrow(xdf)] & !xdf[k,'end2'] %in% xdf$end2[k+1:nrow(xdf)]) {
			#cat('case 3\n')
			m <- which(xdf$end1[k+1:nrow(xdf)] == xdf[k,'end2']) + k
			xdf <- rbind(xdf[1:k,],xdf[m,],xdf[setdiff((k+1):nrow(xdf),m),])
		} else if (xdf[k,'end2'] != xdf[k+1,'end1'] & !xdf[k,'end2'] %in% xdf$end1[k+1:nrow(xdf)] & xdf[k,'end2'] %in% xdf$end2[k+1:nrow(xdf)]) {
			#cat('case 4\n')
			m <- which(xdf$end2[k+1:nrow(xdf)] == xdf[k,'end2']) + k
			tmp1 <- xdf[m,'end1']
			tmp2 <- xdf[m,'end2']
			xdf[m,'end1'] <- tmp2
			xdf[m,'end2'] <- tmp1
			xdf[m,'flip'] <- TRUE
			xdf <- rbind(xdf[1:k,], xdf[m,], xdf[setdiff((k+1):nrow(xdf), m),])
		} else {
			k <- k + 1
		}
	}	


	# Remove all cases where the coordinates are all the same			
	xdf <- subset(xdf, xdf$r > 0)
	res <- NULL
	if (nrow(xdf) > 0) {
		# Convert each arc to a line segment
		# linesj <- list()
		linesj <- sf::st_sf(id = 1:nrow(xdf), geometry = sf::st_sfc(lapply(1:nrow(xdf), function(x) sf::st_multilinestring())), crs = 4326)
		prevx <- NULL
		prevy <- NULL
		j <- 1
		for (i in 1:nrow(xdf)) {
			rowi <- xdf[i,]
			v <- c(rowi$v.x, rowi$v.y)
			theta <- rowi$theta
			r <- rowi$r
			cc <- c(rowi$c1, rowi$c2)
			# Arcs need to be redefined as strings of points. Work out the number of points to allocate in this arc segment.
			ipoints <- 2 + round(increment * (rowi$theta / 2), 0)
			# Calculate coordinates from arc() description for ipoints along the arc.
			angles <- alphahull::anglesArc(v, theta)
			if (rowi['flip'] == TRUE) angles <- rev(angles)
			seqang <- seq(angles[1], angles[2], length = ipoints)
			x <- round(cc[1] + r * cos(seqang), rnd)
			y <- round(cc[2] + r * sin(seqang), rnd)
			# Check for line segments that should be joined up and combine their coordinates
			if (is.null(prevx)) {
				prevx <- x
				prevy <- y
			# added numerical precision fix (Pascal Title Dec 9 2013)
			} else if ((x[1] == round(prevx[length(prevx)],rnd) | abs(x[1] - prevx[length(prevx)]) < tol) && (y[1] == round(prevy[length(prevy)],rnd) | abs(y[1] - prevy[length(prevy)]) < tol)) {
				if (i == nrow(xdf)) {
				#We have got to the end of the dataset
					prevx <- append(prevx ,x[2:ipoints])
					prevy <- append(prevy, y[2:ipoints])
					prevx[length(prevx)] <- prevx[1]
					prevy[length(prevy)] <- prevy[1]
					coordsj <- cbind(prevx,prevy)
					colnames(coordsj) <- NULL
					# Build as Line and then Lines class
					# linej <- Line(coordsj)
					# linesj[[j]] <- Lines(linej, ID = as.character(j))
					linej <- sf::st_linestring(coordsj)
					linesj$geometry[j] <- linej

				} else {
					prevx <- append(prevx, x[2:ipoints])
					prevy <- append(prevy, y[2:ipoints])
				}
			} else {
		# We have got to the end of a set of lines, and there are several such sets, so convert the whole of this one to a line segment and reset.
				prevx[length(prevx)] <- prevx[1]
				prevy[length(prevy)] <- prevy[1]
				coordsj <- cbind(prevx,prevy)
				colnames(coordsj) <- NULL
		# Build as Line and then Lines class
				# linej <- Line(coordsj)
				# linesj[[j]] <- Lines(linej, ID = as.character(j))
				linej <- sf::st_linestring(coordsj)
				linesj$geometry[j] <- linej
				j <- j + 1
				prevx <- NULL
				prevy <- NULL
			}
		}
		
		# drop empty line geometries
		linesj <- linesj[which(sf::st_is_empty(linesj) == FALSE),]
		
		res <- sf::st_geometry(sf::st_cast(linesj, 'POLYGON'))
		
		#Drop lines that will not produce adequate polygons (Pascal Title addition 9 Dec 2013)
		# badLines <- vector()
		# for (i in 1:length(linesj)){
			# if (nrow(linesj[[i]]@Lines[[1]]@coords) < 4){
				# badLines <- c(badLines,i)
			# }
		# }
		# if (length(badLines) > 0) linesj <- linesj[-badLines] 
		
		# # Promote to SpatialLines
		# lspl <- SpatialLines(linesj)
		# # Convert lines to polygons
		# # Pull out Lines slot and check which lines have start and end points that are the same
		# lns <- slot(lspl, "lines")
		# polys <- sapply(lns, function(x) { 
			# crds <- slot(slot(x, "Lines")[[1]], "coords")
			# identical(crds[1, ], crds[nrow(crds), ])
		# }) 
		# # Select those that do and convert to SpatialPolygons
		# polyssl <- lspl[polys]
		# list_of_Lines <- slot(polyssl, "lines")
		# sppolys <- SpatialPolygons(list(Polygons(lapply(list_of_Lines, function(x) { Polygon(slot(slot(x, "Lines")[[1]], "coords")) }), ID = "1")), proj4string=proj4string)
		# # Create a set of ids in a dataframe, then promote to SpatialPolygonsDataFrame
		# hid <- sapply(slot(sppolys, "polygons"), function(x) slot(x, "ID"))
		# areas <- sapply(slot(sppolys, "polygons"), function(x) slot(x, "area"))
		# df <- data.frame(hid,areas)
		# names(df) <- c("HID","Area")
		# rownames(df) <- df$HID
		# res <- SpatialPolygonsDataFrame(sppolys, data=df)
		# res <- res[which(res@data$Area > 0),]
	}	
	return(res)
}