terrainlib_lua: Hardcode flow_local for performance

as it is unlikely that it will be changed one day.
This results in a drastic performance improvement (x4 speed for step 1)

terrainlib_lua/erosion.lua

@@ -134,7 +134,7 @@ local rivermapper = dofile(modpath .. "rivermapper.lua")
 local gaussian = dofile(modpath .. "gaussian.lua")
 
 local function flow(model)
-	model.dirs, model.lakes = rivermapper.flow_routing(model.dem, model.dirs, model.lakes, 'semirandom')
+	model.dirs, model.lakes = rivermapper.flow_routing(model.dem, model.dirs, model.lakes)
 	model.rivers = rivermapper.accumulate(model.dirs, model.rivers)
 end
 

terrainlib_lua/rivermapper.lua

@@ -10,50 +10,18 @@
 -- Big thanks to them for releasing this paper under a free license ! :)
 
 -- The algorithm here makes use of most of the paper's concepts, including the Planar Borůvka algorithm.
--- Only flow_local and accumulate_flow are custom algorithms.
 
 
-local function flow_local_semirandom(plist)
-	-- Determines how water should flow at 1 node scale.
-	-- The straightforward approach would be "Water will flow to the lowest of the 4 neighbours", but here water flows to one of the lower neighbours, chosen randomly, but probability depends on height difference.
-	-- This makes rivers better follow the curvature of the topography at large scale, and be less biased by pure N/E/S/W directions.
-	-- 'plist': array of downward height differences (0 if upward)
-	local sum = 0
-	for i=1, #plist do
-		sum = sum + plist[i] -- Sum of probabilities
-	end
-
-	if sum == 0 then
-		return 0
-	end
-	local r = math.random() * sum
-	for i=1, #plist do
-		local p = plist[i]
-		if r < p then
-			return i
-		end
-		r = r - p
-	end
-	return 0
-end
-
--- Maybe implement more flow methods in the future?
-local flow_methods = {
-	semirandom = flow_local_semirandom,
-}
-
 -- Applies all steps of the flow routing, to calculate flow direction for every node, and lake surface elevation.
 -- It's quite a hard piece of code, but we will go step by step and explain what's going on, so stay with me and... let's goooooooo!
-local function flow_routing(dem, dirs, lakes, method) -- 'dirs' and 'lakes' are optional tables to reuse for memory optimization, they may contain any data.
-	method = method or 'semirandom'
-	local flow_local = flow_methods[method] or flow_local_semirandom
-
+local function flow_routing(dem, dirs, lakes) -- 'dirs' and 'lakes' are optional tables to reuse for memory optimization, they may contain any data.
 	dirs = dirs or {}
 	lakes = lakes or {}
 
 	-- Localize for performance
 	local tremove = table.remove
 	local mmax = math.max
+	local mrand = math.random
 
 	local X, Y = dem.X, dem.Y
 	dirs.X = X
@@ -74,14 +42,29 @@ local function flow_routing(dem, dirs, lakes, method) -- 'dirs' and 'lakes' are
 	for y=1, Y do
 		for x=1, X do
 			local zi = dem[i]
-			local plist = { -- Get the height difference of the 4 neighbours (and 0 if uphill)
-				y<Y and mmax(zi-dem[i+X], 0) or 0, -- Southward
-				x<X and mmax(zi-dem[i+1], 0) or 0, -- Eastward
-				y>1 and mmax(zi-dem[i-X], 0) or 0, -- Northward
-				x>1 and mmax(zi-dem[i-1], 0) or 0, -- Westward
-			}
+			-- Determine how water should flow at 1 node scale.
+			-- The straightforward approach would be "Water will flow to the lowest of the 4 neighbours", but here water flows to one of the lower neighbours, chosen randomly, with probability depending on height difference.
+			-- This makes rivers better follow the curvature of the topography at large scale, and be less biased by pure N/E/S/W directions.
+			local pSouth = y<Y and mmax(zi-dem[i+X], 0) or 0
+			local pEast = x<X and mmax(zi-dem[i+1], 0) or 0
+			local pNorth = y>1 and mmax(zi-dem[i-X], 0) or 0
+			local pWest = x>1 and mmax(zi-dem[i-1], 0) or 0
+
+			local d = 0
+			local sum = pSouth + pEast + pNorth + pWest
+			local r = mrand() * sum
+			if sum > 0 then
+				if r < pSouth then
+					d = 1
+				elseif r-pSouth < pEast then
+					d = 2
+				elseif r-pSouth-pEast < pNorth then
+					d = 3
+				else
+					d = 4
+				end
+			end
 
-			local d = flow_local(plist)
 			-- 'dirs': Direction toward which water flow
 			-- 'dirs2': Directions from which water comes
 			dirs[i] = d
@@ -438,5 +421,4 @@ end
 return {
 	flow_routing = flow_routing,
 	accumulate = accumulate,
-	flow_methods = flow_methods,
 }