Re-organize grid management code for less dependance between files

Remove gridio.lua and move its function to appropriate files

gridmanager.lua

@@ -1,6 +1,6 @@
 -- Manages grid loading, writing and generation
 
-local world_data = mapgen_rivers.world_data_path
+local datapath = mapgen_rivers.world_data_path
 
 local registered_on_grid_loaded = {}
 function mapgen_rivers.register_on_grid_loaded(func)
@@ -21,31 +21,64 @@ local function offset_conv(o)
 	return (o + 0.5) * (1/256)
 end
 
-local grid_maps_list = {
-	dem = {bytes=2, signed=true},
-	lakes = {bytes=2, signed=true},
-	dirs = {bytes=1, signed=false},
-	rivers = {bytes=4, signed=false},
+local floor = math.floor
+local sbyte, schar = string.byte, string.char
+local unpk = unpack
 
-	offset_x = {bytes=1, signed=true, conv=offset_conv},
-	offset_y = {bytes=1, signed=true, conv=offset_conv},
+-- Loading files
+
+-- Never load the full map during mapgen. Instead, create an empty lookup table
+-- and read the file on-the-fly when an element is requested for the first time,
+-- using __index metamethod.
+local loader_mt = {
+	__index = function(loader, i) -- Called when accessing a missing key
+		local file = loader.file
+		local bytes = loader.bytes
+		file:seek('set', (i-1)*bytes)
+		local strnum = file:read(bytes)
+
+		local n = sbyte(strnum, 1)
+		if loader.signed and n >= 128 then
+			n = n - 256
+		end
+
+		for j=2, bytes do
+			n = n*256 + sbyte(strnum, j)
+		end
+
+		if loader.conv then
+			n = loader.conv(n)
+		end
+		-- Cache key for next use
+		loader[i] = n
+		return n
+	end,
 }
 
-function mapgen_rivers.try_load_grid(grid)
+local function load_file(filename, bytes, signed, size, converter)
+	local file = io.open(datapath .. filename, 'rb')
+	if file then
+		converter = converter or false
+		return setmetatable({file=file, bytes=bytes, signed=signed, size=size, conv=converter}, loader_mt)
+	end
+end
+
+function mapgen_rivers.load_or_generate_grid()
 	-- First, check whether a grid is already loaded
 	if mapgen_rivers.grid then
 		return true
-	-- Then, check the provided argument is a valid grid
-	elseif grid then
-		mapgen_rivers.grid = grid
-		on_grid_loaded_callback(grid)
-		return true
 	end
 
-	-- Fall back to loading the grid from the files
-	local sfile = io.open(world_data .. 'size', 'r')
+	-- If not, try to load the grid from the files
+	local sfile = io.open(datapath .. 'size', 'r')
 	if not sfile then
-		return false
+		dofile(mapgen_rivers.modpath .. "/pregenerate.lua")
+		collectgarbage()
+
+		sfile = io.open(datapath .. 'size', 'r')
+		if not sfile then
+			return false
+		end
 	end
 
 	local x, z = sfile:read('*n'), sfile:read('*n')
@@ -55,31 +88,18 @@ function mapgen_rivers.try_load_grid(grid)
 
 	minetest.log("action", '[mapgen_rivers] Loading grid')
 
-	grid = {
+	local grid = {
 		size = {x=x, y=z},
+		dem = load_file('dem', 2, true, x*z),
+		lakes = load_file('lakes', 2, true, x*z),
+		dirs = load_file('dirs', 1, false, x*z),
+		rivers = load_file('rivers', 4, false, x*z),
+		offset_x = load_file('offset_x', 1, true, x*z, offset_conv),
+		offset_y = load_file('offset_y', 1, true, x*z, offset_conv),
 	}
 
-	for map, params in pairs(grid_maps_list) do
-		grid[map] = mapgen_rivers.load_file(map, params.bytes, params.signed, x*z, params.conv)
-	end
-
 	mapgen_rivers.grid = grid
 	on_grid_loaded_callback(grid)
 
 	return true
 end
-
-function mapgen_rivers.write_grid(grid)
-	minetest.mkdir(world_data)
-
-	for map, params in pairs(grid_maps_list) do
-		mapgen_rivers.write_file(map, grid[map], params.bytes)
-	end
-
-	local sfile = io.open(world_data .. 'size', "w")
-	sfile:write(grid.size.x..'\n'..grid.size.y)
-	sfile:close()
-
-	return true
-end
-