terrainlib: loop only once for all singular nodes at step 2 of flow routing

terrainlib_lua/rivermapper.lua

@@ -129,15 +129,16 @@ local function flow_routing(dem, dirs, lakes) -- 'dirs' and 'lakes' are optional
 		basin_id[i] = 0
 	end
 
-	for ib=1, nbasins do
-		-- Here we will recursively search upstream from the singular node to determine its drainage basin
-		local queue = {singular[ib]} -- Start with the singular node, then this queue will be filled with water donors neighbours
-		basin_links = {}
-		links[#links+1] = basin_links
-		local cur = 1
-		while cur > 0 do
-			local i = queue[cur]
+	local cur = nbasins
+	local ib = 0
+	while cur > 0 do
+			local i = singular[cur]
 			cur = cur - 1
+			if dirs[i] == 0 then
+				basin_links = {}
+				links[#links+1] = basin_links
+				ib = ib + 1
+			end
 			basin_id[i] = ib
 			local d = dirs2[i] -- Get the directions water is coming from
 
@@ -145,7 +146,7 @@ local function flow_routing(dem, dirs, lakes) -- 'dirs' and 'lakes' are optional
 			if d >= 8 then -- River coming from the East
 				d = d - 8
 				cur = cur + 1
-				queue[cur] = i+1
+				singular[cur] = i+1
 			-- If no river is coming from the East, we might be at the limit of two basins, thus we need to test adjacency.
 			elseif i%X > 0 then
 				add_link(i, i+1, ib, false)
@@ -156,7 +157,7 @@ local function flow_routing(dem, dirs, lakes) -- 'dirs' and 'lakes' are optional
 			if d >= 4 then -- River coming from the South
 				d = d - 4
 				cur = cur + 1
-				queue[cur] = i+X
+				singular[cur] = i+X
 			elseif i <= X*(Y-1) then
 				add_link(i, i+X, ib, true)
 			else
@@ -166,7 +167,7 @@ local function flow_routing(dem, dirs, lakes) -- 'dirs' and 'lakes' are optional
 			if d >= 2 then -- River coming from the West
 				d = d - 2
 				cur = cur + 1
-				queue[cur] = i-1
+				singular[cur] = i-1
 			elseif i%X ~= 1 then
 				add_link(i, i-1, ib, false)
 			else
@@ -175,13 +176,12 @@ local function flow_routing(dem, dirs, lakes) -- 'dirs' and 'lakes' are optional
 
 			if d >= 1 then -- River coming from the North
 				cur = cur + 1
-				queue[cur] = i-X
+				singular[cur] = i-X
 			elseif i > X then
 				add_link(i, i-X, ib, true)
 			else
 				add_link(i, 0, ib, true)
 			end
-		end
 	end
 	dirs2 = nil