terrainlib: compute current queue length instead of using '#' operator.

Important speedup.

terrainlib_lua/rivermapper.lua

@@ -134,15 +134,18 @@ local function flow_routing(dem, dirs, lakes) -- 'dirs' and 'lakes' are optional
 		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
-		while #queue > 0 do
+		local cur = 1
-			local i = tremove(queue)
+		while cur > 0 do
+			local i = queue[cur]
+			cur = cur - 1
 			basin_id[i] = ib
 			local d = dirs2[i] -- Get the directions water is coming from
 
 			-- Iterate through the 4 directions
 			if d >= 8 then -- River coming from the East
 				d = d - 8
-				queue[#queue+1] = i+1
+				cur = cur + 1
+				queue[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)
@@ -152,7 +155,8 @@ local function flow_routing(dem, dirs, lakes) -- 'dirs' and 'lakes' are optional
 
 			if d >= 4 then -- River coming from the South
 				d = d - 4
-				queue[#queue+1] = i+X
+				cur = cur + 1
+				queue[cur] = i+X
 			elseif i <= X*(Y-1) then
 				add_link(i, i+X, ib, true)
 			else
@@ -161,7 +165,8 @@ local function flow_routing(dem, dirs, lakes) -- 'dirs' and 'lakes' are optional
 
 			if d >= 2 then -- River coming from the West
 				d = d - 2
-				queue[#queue+1] = i-1
+				cur = cur + 1
+				queue[cur] = i-1
 			elseif i%X ~= 1 then
 				add_link(i, i-1, ib, false)
 			else
@@ -169,7 +174,8 @@ local function flow_routing(dem, dirs, lakes) -- 'dirs' and 'lakes' are optional
 			end
 
 			if d >= 1 then -- River coming from the North
-				queue[#queue+1] = i-X
+				cur = cur + 1
+				queue[cur] = i-X
 			elseif i > X then
 				add_link(i, i-X, ib, true)
 			else