-- Minetest: builtin/serialize.lua

-- https://github.com/fab13n/metalua/blob/no-dll/src/lib/serialize.lua
-- Copyright (c) 2006-2997 Fabien Fleutot <metalua@gmail.com>
-- License: MIT
--------------------------------------------------------------------------------
-- Serialize an object into a source code string. This string, when passed as
-- an argument to deserialize(), returns an object structurally identical
-- to the original one. The following are currently supported:
-- * strings, numbers, booleans, nil
-- * tables thereof. Tables can have shared part, but can't be recursive yet.
-- Caveat: metatables and environments aren't saved.
--------------------------------------------------------------------------------

local no_identity = { number=1, boolean=1, string=1, ['nil']=1 }

function minetest.serialize(x)

	local gensym_max   =  0  -- index of the gensym() symbol generator
	local seen_once    = { } -- element->true set of elements seen exactly once in the table
	local multiple     = { } -- element->varname set of elements seen more than once
	local nested       = { } -- transient, set of elements currently being traversed
	local nest_points  = { }
	local nest_patches = { }
	
	local function gensym()
		gensym_max = gensym_max + 1 ;  return gensym_max
	end

	-----------------------------------------------------------------------------
	-- nest_points are places where a table appears within itself, directly or not.
	-- for instance, all of these chunks create nest points in table x:
	-- "x = { }; x[x] = 1", "x = { }; x[1] = x", "x = { }; x[1] = { y = { x } }".
	-- To handle those, two tables are created by mark_nest_point:
	-- * nest_points [parent] associates all keys and values in table parent which
	--   create a nest_point with boolean `true'
	-- * nest_patches contain a list of { parent, key, value } tuples creating
	--   a nest point. They're all dumped after all the other table operations
	--   have been performed.
	--
	-- mark_nest_point (p, k, v) fills tables nest_points and nest_patches with
	-- informations required to remember that key/value (k,v) create a nest point
	-- in table parent. It also marks `parent' as occuring multiple times, since
	-- several references to it will be required in order to patch the nest
	-- points.
	-----------------------------------------------------------------------------
	local function mark_nest_point (parent, k, v)
		local nk, nv = nested[k], nested[v]
		assert (not nk or seen_once[k] or multiple[k])
		assert (not nv or seen_once[v] or multiple[v])
		local mode = (nk and nv and "kv") or (nk and "k") or ("v")
		local parent_np = nest_points [parent]
		local pair = { k, v }
		if not parent_np then parent_np = { }; nest_points [parent] = parent_np end
		parent_np [k], parent_np [v] = nk, nv
		table.insert (nest_patches, { parent, k, v })
		seen_once [parent], multiple [parent]  = nil, true
	end

	-----------------------------------------------------------------------------
	-- First pass, list the tables and functions which appear more than once in x
	-----------------------------------------------------------------------------
	local function mark_multiple_occurences (x)
		if no_identity [type(x)] then return end
		if     seen_once [x]     then seen_once [x], multiple [x] = nil, true
		elseif multiple  [x]     then -- pass
		else   seen_once [x] = true end
		
		if type (x) == 'table' then
			nested [x] = true
			for k, v in pairs (x) do
				if nested[k] or nested[v] then mark_nest_point (x, k, v) else
					mark_multiple_occurences (k)
					mark_multiple_occurences (v)
				end
			end
			nested [x] = nil
		end
	end

	local dumped    = { } -- multiply occuring values already dumped in localdefs
	local localdefs = { } -- already dumped local definitions as source code lines

	-- mutually recursive functions:
	local dump_val, dump_or_ref_val

	--------------------------------------------------------------------
	-- if x occurs multiple times, dump the local var rather than the
	-- value. If it's the first time it's dumped, also dump the content