add directed argument to st_line_merge()

closes #2264

NEWS.md

@@ -1,5 +1,7 @@
 # version 1.0-15
 
+* add `directed` argument to `st_line_merge()`; #2264
+
 * `st_union.sfc()` with `x` and `y` given works consistent across geodetic and projected objects; #2262
 
 * `st_union.sf()` with `x` and `y` given unions pairwise if `by_feature=TRUE`; #2259

R/geom-transformers.R

@@ -514,26 +514,30 @@ st_polygonize.sf = function(x) {
 
 #' @name geos_unary
 #' @export
+#' @param directed logical; if \code{TRUE}, lines with opposite directions will not be merged
 #' @details \code{st_line_merge} merges lines. In case of \code{st_line_merge}, \code{x} must be an object of class \code{MULTILINESTRING}, or an \code{sfc} geometry list-column object containing these
 #' @examples
 #' mls = st_multilinestring(list(rbind(c(0,0), c(1,1)), rbind(c(2,0), c(1,1))))
 #' st_line_merge(st_sfc(mls))
-st_line_merge = function(x)
+st_line_merge = function(x, ..., directed = FALSE)
 	UseMethod("st_line_merge")
 
 #' @export
-st_line_merge.sfg = function(x)
-	get_first_sfg(st_line_merge(st_sfc(x)))
+st_line_merge.sfg = function(x, ..., directed = FALSE)
+	get_first_sfg(st_line_merge(st_sfc(x), directed = directed, ...))
 
 #' @export
-st_line_merge.sfc = function(x) {
+st_line_merge.sfc = function(x, ..., directed = FALSE) {
 	stopifnot(inherits(x, "sfc_MULTILINESTRING"))
-	st_sfc(CPL_geos_op("linemerge", x, numeric(0), integer(0), numeric(0), logical(0)))
+	if (directed)
+		st_sfc(CPL_geos_op("linemergedirected", x, numeric(0), integer(0), numeric(0), logical(0)))
+	else
+		st_sfc(CPL_geos_op("linemerge", x, numeric(0), integer(0), numeric(0), logical(0)))
 }
 
 #' @export
-st_line_merge.sf = function(x) {
-	st_set_geometry(x, st_line_merge(st_geometry(x)))
+st_line_merge.sf = function(x, ..., directed = FALSE) {
+	st_set_geometry(x, st_line_merge(st_geometry(x), directed = directed, ...))
 }
 
 #' @name geos_unary

src/geos.cpp

@@ -835,6 +835,13 @@ Rcpp::List CPL_geos_op(std::string op, Rcpp::List sfc,
 	} else if (op == "linemerge") {
 		for (size_t i = 0; i < g.size(); i++)
 			out[i] = geos_ptr(chkNULL(GEOSLineMerge_r(hGEOSCtxt, g[i].get())), hGEOSCtxt);
+	} else if (op == "linemergedirected") {
+#ifdef HAVE311
+		for (size_t i = 0; i < g.size(); i++)
+			out[i] = geos_ptr(chkNULL(GEOSLineMergeDirected_r(hGEOSCtxt, g[i].get())), hGEOSCtxt);
+#else
+		Rcpp::stop("directed line merge requires GEOS >= 3.11");
+#endif
 	} else if (op == "polygonize") {
 		for (size_t i = 0; i < g.size(); i++) {
 			const GEOSGeometry* gi = g[i].get();