Table Persistence

wiki

TablePersistence is a small code snippet that allows storing and loading of lua variables containing primitive types. It is licensed under the MIT license, use it how ever is needed. A more detailed description and complete source can be downloaded on http://the-color-black.net/blog/article/LuaTablePersistence. A fork has been created on github that included lunatest unit tests: https://github.com/hipe/lua-table-persistence

Shortcomings/Limitations:

Does not export udata
Does not export threads
Only exports a small subset of functions (pure lua without upvalue)

local write, writeIndent, writers, refCount;



persistence =

{

	store = function (path, ...)

		local file, e = io.open(path, "w");

		if not file then

			return error(e);

		end

		local n = select("#", ...);

		-- Count references

		local objRefCount = {}; -- Stores reference that will be exported

		for i = 1, n do

			refCount(objRefCount, (select(i,...)));

		end;

		-- Export Objects with more than one ref and assign name

		-- First, create empty tables for each

		local objRefNames = {};

		local objRefIdx = 0;

		file:write("-- Persistent Data\n");

		file:write("local multiRefObjects = {\n");

		for obj, count in pairs(objRefCount) do

			if count > 1 then

				objRefIdx = objRefIdx + 1;

				objRefNames[obj] = objRefIdx;

				file:write("{};"); -- table objRefIdx

			end;

		end;

		file:write("\n} -- multiRefObjects\n");

		-- Then fill them (this requires all empty multiRefObjects to exist)

		for obj, idx in pairs(objRefNames) do

			for k, v in pairs(obj) do

				file:write("multiRefObjects["..idx.."][");

				write(file, k, 0, objRefNames);

				file:write("] = ");

				write(file, v, 0, objRefNames);

				file:write(";\n");

			end;

		end;

		-- Create the remaining objects

		for i = 1, n do

			file:write("local ".."obj"..i.." = ");

			write(file, (select(i,...)), 0, objRefNames);

			file:write("\n");

		end

		-- Return them

		if n > 0 then

			file:write("return obj1");

			for i = 2, n do

				file:write(" ,obj"..i);

			end;

			file:write("\n");

		else

			file:write("return\n");

		end;

		if type(path) == "string" then

			file:close();

		end;

	end;



	load = function (path)

		if type(path) == "string" then

			local f, e = loadfile(path);

		else

			local f, e = path:read('*a')

		end

		if f then

			return f();

		else

			return nil, e;

		end;

	end;

}



-- Private methods



-- write thing (dispatcher)

write = function (file, item, level, objRefNames)

	writers[type(item)](file, item, level, objRefNames);

end;



-- write indent

writeIndent = function (file, level)

	for i = 1, level do

		file:write("\t");

	end;

end;



-- recursively count references

refCount = function (objRefCount, item)

	-- only count reference types (tables)

	if type(item) == "table" then

		-- Increase ref count

		if objRefCount[item] then

			objRefCount[item] = objRefCount[item] + 1;

		else

			objRefCount[item] = 1;

			-- If first encounter, traverse

			for k, v in pairs(item) do

				refCount(objRefCount, k);

				refCount(objRefCount, v);

			end;

		end;

	end;

end;



-- Format items for the purpose of restoring

writers = {

	["nil"] = function (file, item)

			file:write("nil");

		end;

	["number"] = function (file, item)

			file:write(tostring(item));

		end;

	["string"] = function (file, item)

			file:write(string.format("%q", item));

		end;

	["boolean"] = function (file, item)

			if item then

				file:write("true");

			else

				file:write("false");

			end

		end;

	["table"] = function (file, item, level, objRefNames)

			local refIdx = objRefNames[item];

			if refIdx then

				-- Table with multiple references

				file:write("multiRefObjects["..refIdx.."]");

			else

				-- Single use table

				file:write("{\n");

				for k, v in pairs(item) do

					writeIndent(file, level+1);

					file:write("[");

					write(file, k, level+1, objRefNames);

					file:write("] = ");

					write(file, v, level+1, objRefNames);

					file:write(";\n");

				end

				writeIndent(file, level);

				file:write("}");

			end;

		end;

	["function"] = function (file, item)

			-- Does only work for "normal" functions, not those

			-- with upvalues or c functions

			local dInfo = debug.getinfo(item, "uS");

			if dInfo.nups > 0 then

				file:write("nil --[[functions with upvalue not supported]]");

			elseif dInfo.what ~= "Lua" then

				file:write("nil --[[non-lua function not supported]]");

			else

				local r, s = pcall(string.dump,item);

				if r then

					file:write(string.format("loadstring(%q)", s));

				else

					file:write("nil --[[function could not be dumped]]");

				end

			end

		end;

	["thread"] = function (file, item)

			file:write("nil --[[thread]]\n");

		end;

	["userdata"] = function (file, item)

			file:write("nil --[[userdata]]\n");

		end;

}

It is used like this:

t_original = {1, 2, ["a"] = "string", b = "test", {"subtable", [4] = 2}};

persistence.store("storage.lua", t_original);

t_restored = persistence.load("storage.lua");

Example output (storage.lua):

-- Persistent Data

return {

	[1] = 1;

	[2] = 2;

	[3] = {

		[1] = "subtable";

		[4] = 2;

	};

	["a"] = "string";

	["b"] = "test";

}

Table Persistence

See Also