Refactor grid loading and reorganize code

Move grid management functions out of polygons.lua, in a new file called gridmanager.lua
Explicitly call other files and grid management functions from init.lua
to make workflow more apparent
Move mapgen loop out of init.lua, into mapgen.lua

gridmanager.lua

@@ -0,0 +1,146 @@
+local world_data = mapgen_rivers.world_data_path
+
+local registered_on_grid_loaded = {}
+function mapgen_rivers.register_on_grid_loaded(func)
+	if type(func) == "function" then
+		registered_on_grid_loaded[#registered_on_grid_loaded+1] = func
+	else
+		minetest.log("error", "[mapgen_rivers] register_on_grid_loaded can only register functions!")
+	end
+end
+
+local function on_grid_loaded_callback(grid)
+	for _, func in ipairs(registered_on_grid_loaded) do
+		func(grid)
+	end
+end
+
+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},
+
+	offset_x = {bytes=1, signed=true, conv=offset_conv},
+	offset_y = {bytes=1, signed=true, conv=offset_conv},
+}
+
+local function apply_grid_conversion(grid)
+	if grid.load_method ~= "full" then
+		minetest.log("warning", ("Could not apply data conversion for load method %s"):format(grid.load_method))
+		return false
+	end
+	if grid.conv_applied then
+		return true
+	end
+
+	local size = grid.size.x * grid.size.y
+	for mapname, params in pairs(grid_maps_list) do
+		local conv = params.conv
+		if conv then
+			local map = grid[mapname]
+			for i=1, size do
+				map[i] = conv(map[i])
+			end
+		end
+	end
+	grid.conv_applied = true
+
+	return true
+end
+
+function mapgen_rivers.try_load_grid(grid)
+	local load_method = mapgen_rivers.settings.load_method
+
+	-- First, check whether a grid is already loaded with the appropriate method
+	if mapgen_rivers.grid and mapgen_rivers.grid.load_method == load_method then
+		if not mapgen_rivers.grid.conv_applied then
+			apply_grid_conversion(mapgen_rivers.grid)
+		end
+		return true
+	-- Then, check the provided argument is a valid grid
+	elseif grid and grid.load_method == load_method then
+		if not mapgen_rivers.grid.conv_applied then
+			apply_grid_conversion(grid)
+		end
+		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 sfile then
+		return false
+	end
+
+	local x, z = sfile:read('*n'), sfile:read('*n')
+	if not x or not z then
+		return false
+	end
+
+	if load_method == "full" then
+		minetest.log("action", '[mapgen_rivers] Loading full grid')
+	elseif load_method == "interactive" then
+		minetest.log("action", '[mapgen_rivers] Loading grid as interactive loaders')
+	end
+
+	grid = {
+		load_method = load_method,
+		size = {x=x, y=z},
+	}
+
+	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
+	grid.conv_applied = true
+
+	mapgen_rivers.grid = grid
+	on_grid_loaded_callback(grid)
+
+	return true
+end
+
+function mapgen_rivers.generate_grid()
+	minetest.log("action", '[mapgen_rivers] Generating grid, this may take a while...')
+	local grid = {}
+
+	local blocksize = mapgen_rivers.settings.blocksize
+	local xsize = math.floor(mapgen_rivers.settings.map_x_size / blocksize)
+	local zsize = math.floor(mapgen_rivers.settings.map_z_size / blocksize)
+	grid.size = {x=xsize, y=zsize}
+	grid.conv_applied = false
+
+	if not mapgen_rivers.pregenerate then
+		minetest.log("error", "[mapgen_rivers] Pre-generation function is not available.")
+		return false
+	end
+
+	mapgen_rivers.pregenerate(grid)
+
+	return grid
+end
+
+function mapgen_rivers.write_grid(grid)
+	minetest.mkdir(world_data)
+
+	if grid.conv_applied then
+		minetest.log("error", '[mapgen_rivers] Could not write grid if data conversion is already done')
+		return false
+	end
+
+	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
+