register_on_stack_change.lua

local INTERVAL = .1

local function tables_equal(t1, t2)
	if type(t1) ~= "table" or type(t2) ~= "table" then
		return false
	end
	for k, v1 in pairs(t1) do
		local v2 = t2[k]
		if type(v1) == "table" then
			if not tables_equal(v1, v2) then
				return false
			end
		elseif v1 ~= v2 then
			return false
		end
	end
	for k, v2 in pairs(t2) do
		local v1 = t1[k]
		if type(v2) == "table" then
			if not tables_equal(v2, v1) then
				return false
			end
		elseif v2 ~= v1 then
			return false
		end
	end
	return true
end

local function stacks_equal(s1, s2)
	if s1 == nil or s2 == nil then
		return s1 == s2
	end
	return tables_equal(s1:to_table(), s2:to_table())
end

local callbacks = { }

local function register_on_stack_change(callback)
	callbacks[#callbacks+1] = callback
end

local oldstacks = { }
local timer = 0

minetest.register_globalstep(function(dtime)
	timer = timer + dtime
	if timer < INTERVAL then return end
	timer = 0
	for _, p in ipairs(minetest.get_connected_players()) do
		local pn = p:get_player_name()
		local old = oldstacks[pn]
		local news = p:get_wielded_item()
		local newi = p:get_wield_index()
		if not old then
			old = { }
			oldstacks[pn] = old
		end
		if newi ~= old.index or not stacks_equal(news, old.stack) then
			-- Call in LIFO order.
			local olds, oldi = old.stack, old.index
			old.stack, old.index = news, newi
			for i = #callbacks, 1, -1 do
				callbacks[i](p, news, newi, olds, oldi)
			end
		end
	end
end)

register_on_stack_change(function(player,
		new_stack, new_index, old_stack, old_index)
	print("change:", player:get_player_name(),
			new_stack and new_stack:to_string(), new_index,
			old_stack and old_stack:to_string(), old_index)
end)