add mapgen prefill feature for water

init.lua

@@ -330,3 +330,39 @@ if springs then
 		end
 	})	
 end
+
+local mapgen_prefill = minetest.setting_getbool("dynamic_liquid_mapgen_prefill")
+mapgen_prefill = mapgen_prefill or mapgen_prefill == nil -- default true
+
+if mapgen_prefill then
+	local c_water = minetest.get_content_id("default:water_source")
+	local c_air = minetest.get_content_id("air")
+	local water_level = minetest.get_mapgen_params().water_level
+
+	minetest.register_on_generated(function(minp, maxp, seed)
+		if minp.y >= water_level then
+			return
+		end
+	
+		local vm, emin, emax = minetest.get_mapgen_object("voxelmanip")
+		local area = VoxelArea:new{MinEdge=emin, MaxEdge=emax}
+		vm:get_data(data)
+		
+		local filling = false
+		for z = minp.z, maxp.z do
+			for x = minp.x, maxp.x do
+				for y = maxp.y, minp.y, -1 do
+					local vi = area:index(x,y,z)
+					if data[vi] == c_water then
+						filling = true
+					elseif data[vi] == c_air and filling == true then
+						data[vi] = c_water
+					else filling = false end
+				end
+				filling = false
+			end
+		end
+		vm:set_data(data)
+		vm:write_to_map()
+	end)
+end

settingtypes.txt

@@ -24,6 +24,15 @@ dynamic_liquid_springs (Springs) bool true
 #"flow_through"
 dynamic_liquid_flow_through (Flow-through) bool true
 
+#During map generation, this mod can attempt to "pre-fill" underwater cave
+#entrances by replacing air directly below water with additional water blocks.
+#This process is only attempted below "sea level" and is not perfect - it will
+#only fill down to the bottom of the map block being generated and it won't
+#fill out openings to the sides. But it should hopefully reduce the magnitude
+#of the whirlpools that form over cave entrances and the load on the active
+#block modifier in these situations.
+dynamic_liquid_mapgen_prefill (Mapgen water prefill) bool true
+
 [Flow Rates]
 
 #Sets the probability of water flow per block per second.