Combine mapgen callbacks for efficiency (#7)

1. Two read/write pairs to the VM have been replaced with one.
2. If no changes are made to the data, it is not written back to the VM.

init.lua

@@ -320,29 +320,7 @@ if not springs then
 end
 minetest.register_node("dynamic_liquid:clay", clay_def)
 
-local data = {}
-
 if springs then	
-	local c_clay = minetest.get_content_id("default:clay")
-	local c_spring_clay = minetest.get_content_id("dynamic_liquid:clay")
-
-	-- Turn mapgen clay into spring clay
-	minetest.register_on_generated(function(minp, maxp, seed)
-		if minp.y >= water_level or maxp.y <= -15 then
-			return
-		end
-		local vm, emin, emax = minetest.get_mapgen_object("voxelmanip")
-		vm:get_data(data)
-		
-		for voxelpos, voxeldata in pairs(data) do
-			if voxeldata == c_clay then
-				data[voxelpos] = c_spring_clay
-			end
-		end
-		vm:set_data(data)
-		vm:write_to_map()
-	end)
-	
 	minetest.register_abm({
 		label = "dynamic_liquid damp clay spring",
 		nodenames = {"dynamic_liquid:clay"},
@@ -409,34 +387,38 @@ end
 
 local mapgen_prefill = minetest.settings:get_bool("dynamic_liquid_mapgen_prefill", true)
 
-local waternodes
+if springs or mapgen_prefill then
+	local data = {}
 
-if mapgen_prefill then
+	local c_clay = minetest.get_content_id("default:clay")
+	local c_spring_clay = minetest.get_content_id("dynamic_liquid:clay")
 	local c_water = minetest.get_content_id("default:water_source")
 	local c_air = minetest.get_content_id("air")
-	waternodes = {}
+	local waternodes = {}
 
 	local fill_to = function (vi, data, area)
 		if area:containsi(vi) and area:position(vi).y <= water_level then
 			if data[vi] == c_air then
 				data[vi] = c_water
 				table.insert(waternodes, vi)
+				return true
 			end
 		end
+		return false
 	end
 
 --	local count = 0
 	local drop_liquid = function(vi, data, area, min_y)
 		if data[vi] ~= c_water then
 			-- we only care about water.
-			return
+			return false
 		end
 		local start = vi -- remember the water node we started from
 		local ystride = area.ystride
 		vi = vi - ystride
 		if data[vi] ~= c_air then
 			-- if there's no air below this water node, give up immediately.
-			return
+			return false
 		end
 		vi = vi - ystride -- There's air below the water, so move down one.
 		while data[vi] == c_air and area:position(vi).y > min_y do
@@ -452,19 +434,34 @@ if mapgen_prefill then
 --		if count % 100 == 0 then
 --			minetest.chat_send_all("dropped water " .. (start-vi)/ystride .. " at " .. minetest.pos_to_string(area:position(vi)))
 --		end
+		return true
 	end
 	
 	minetest.register_on_generated(function(minp, maxp, seed)
-		if minp.y > water_level then
-			-- we're in the sky.
-			return
-		end
-	
-		local vm, emin, emax = minetest.get_mapgen_object("voxelmanip")
-		local area = VoxelArea:new{MinEdge=emin, MaxEdge=emax}
-		vm:get_data(data)
 		local maxp_y = maxp.y
 		local minp_y = minp.y
+		local vm, emin, emax
+		local dirty = false
+		local update_liquids = false
+
+		if springs and minp_y < water_level and maxp_y > -15 then
+			vm, emin, emax = minetest.get_mapgen_object("voxelmanip")
+			vm:get_data(data)
+
+			for voxelpos = 1, #data do
+				if data[voxelpos] == c_clay then
+					data[voxelpos] = c_spring_clay
+					dirty = true
+				end
+			end
+		end
+
+		if mapgen_prefill and minp_y <= water_level then
+			if not vm then
+				vm, emin, emax = minetest.get_mapgen_object("voxelmanip")
+				vm:get_data(data)
+			end
+			local area = VoxelArea:new{MinEdge=emin, MaxEdge=emax}
 
 			if maxp_y > -70 then
 				local top = vector.new(maxp.x, math.min(maxp_y, water_level), maxp.z) -- prevents flood fill from affecting any water above sea level
@@ -482,11 +479,11 @@ if mapgen_prefill then
 					local front = vi - 1
 					local back = vi + 1
 					
-				fill_to(below, data, area)
+					dirty = fill_to(below, data, area) or dirty
-				fill_to(left, data, area)
+					dirty = fill_to(left, data, area) or dirty
-				fill_to(right, data, area)
+					dirty = fill_to(right, data, area) or dirty
-				fill_to(front, data, area)
+					dirty = fill_to(front, data, area) or dirty
-				fill_to(back, data, area)
+					dirty = fill_to(back, data, area) or dirty
 				end
 			else
 				-- Caves sometimes generate with liquid nodes hovering in mid air.
@@ -497,13 +494,20 @@ if mapgen_prefill then
 					-- We could possibly be a bit more efficient by remembering how far we dropped then
 					-- last liquid node in a column and moving stuff down that far,
 					-- but for now let's keep it simple.
-				drop_liquid(vi, data, area, minp_y)
+					dirty = drop_liquid(vi, data, area, minp_y) or dirty
 				end
 			end
 
+			update_liquids = dirty
+		end
+
+		if dirty then
 			vm:set_data(data)
 			vm:write_to_map()
+			if update_liquids then
 				vm:update_liquids()
+			end
+		end
 	end)
 end