mapgen_rivers/load_grid.lua

local datapath = minetest.get_worldpath() .. "/river_data/"

local floor = math.floor
local sbyte = string.byte
local unpk = unpack

local load_map
local use_interactive_loader
if minetest.settings:has("mapgen_rivers_use_interactive_loader") then
	use_interactive_loader = minetest.settings:get_bool("mapgen_rivers_use_interactive_loader")
else
	use_interactive_loader = not minetest.settings:get_bool("mapgen_rivers_load_all")
end

if use_interactive_loader then
	local loader_mt = {
		__index = function(loader, i)
			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
			loader[i] = n
			return n
		end,
	}

	load_map = function(filename, bytes, signed, size, converter)
		local file = io.open(datapath .. filename, 'rb')
		if file then
			minetest.register_on_shutdown(function()
				file:close()
			end)
			converter = converter or false
			return setmetatable({file=file, bytes=bytes, signed=signed, size=size, conv=converter}, loader_mt)
		end
	end

else

	load_map = function(filename, bytes, signed, size, converter)
		local file = io.open(datapath .. filename, 'rb')
		local data = file:read('*all')
		if #data < bytes*size then
			data = minetest.decompress(data)
		end

		local map = {}

		for i=1, size do
			local i0 = (i-1)*bytes+1
			local i1 = i*bytes
			local elements = {data:byte(i0, i1)}
			local n = sbyte(data, i0)
			if signed and n >= 128 then
				n = n - 256
			end

			for j=1, bytes-1 do
				n = n*256 + sbyte(data, i0+j)
			end

			map[i] = n
		end
		file:close()

		if converter then
			for i=1, size do
				map[i] = converter(map[i])
			end
		end

		return map
	end
end

local sfile = io.open(datapath..'size', 'r')
assert(sfile)
local X, Z = tonumber(sfile:read('*l')), tonumber(sfile:read('*l'))
sfile:close()

local function offset_converter(o)
	return (o + 0.5) * (1/256)
end

mapgen_rivers.grid = {
	size = {x=X, y=Z},
	dem = load_map('dem', 2, true, X*Z),
	lakes = load_map('lakes', 2, true, X*Z),
	dirs = load_map('dirs', 1, false, X*Z),
	rivers = load_map('rivers', 4, false, X*Z),

	offset_x = load_map('offset_x', 1, true, X*Z, offset_converter),
	offset_y = load_map('offset_y', 1, true, X*Z, offset_converter);
}