-- Manages grid loading, writing and generation

local datapath = 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 floor = math.floor
local sbyte, schar = string.byte, string.char
local unpk = unpack

-- 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,
}

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
	end

	-- If not, try to load the grid from the files
	local sfile = io.open(datapath .. 'size', 'r')
	if not sfile then
		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')
	if not x or not z then
		return false
	end

	minetest.log("action", '[mapgen_rivers] Loading 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),
	}

	mapgen_rivers.grid = grid
	on_grid_loaded_callback(grid)

	return true
end