-- Input and output functions for grid maps

local worldpath = mapgen_rivers.world_data_path

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,
}

function mapgen_rivers.load_file(filename, bytes, signed, size, converter)
	local file = io.open(worldpath .. 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

-- Writing files
function mapgen_rivers.write_file(filename, data, bytes)
	local file = io.open(worldpath .. filename, 'wb')

	local bytelist = {}
	for j=1, bytes do
		bytelist[j] = 0
	end

	for i=1, #data do
		local n = floor(data[i])
		data[i] = n
		for j=bytes, 2, -1 do
			bytelist[j] = n % 256
			n = floor(n / 256)
		end
		bytelist[1] = n % 256

		file:write(schar(unpk(bytelist)))
	end

	file:close()
end