minetest/builtin/game/statbars.lua

local health_bar_definition =
{
	hud_elem_type = "statbar",
	position = { x=0.5, y=1 },
	text = "heart.png",
	number = 20,
	direction = 0,
	size = { x=24, y=24 },
	offset = { x=(-10*24)-25, y=-(48+24+10)},
}

local breath_bar_definition =
{
	hud_elem_type = "statbar",
	position = { x=0.5, y=1 },
	text = "bubble.png",
	number = 20,
	direction = 0,
	size = { x=24, y=24 },
	offset = {x=25,y=-(48+24+10)},
}

local hud_ids = {}

local function initialize_builtin_statbars(player)

	if not player:is_player() then
		return
	end

	local name = player:get_player_name()

	if name == "" then
		return
	end

	if (hud_ids[name] == nil) then
		hud_ids[name] = {}
	end

	if player:hud_get_flags().healthbar then
		if hud_ids[name].id_healthbar == nil then
			health_bar_definition.number = player:get_hp()
			hud_ids[name].id_healthbar  = player:hud_add(health_bar_definition)
		end
	else
		if hud_ids[name].id_healthbar ~= nil then
			player:hud_remove(hud_ids[name].id_healthbar)
			hud_ids[name].id_healthbar = nil
		end
	end

	if (player:get_breath() < 11) then
		if player:hud_get_flags().breathbar then
			if hud_ids[name].id_breathbar == nil then
				hud_ids[name].id_breathbar = player:hud_add(breath_bar_definition)
			end
		else
			if hud_ids[name].id_breathbar ~= nil then
				player:hud_remove(hud_ids[name].id_breathbar)
				hud_ids[name].id_breathbar = nil
			end
		end
	elseif hud_ids[name].id_breathbar ~= nil then
		player:hud_remove(hud_ids[name].id_breathbar)
		hud_ids[name].id_breathbar = nil
	end
end

local function cleanup_builtin_statbars(player)

	if not player:is_player() then
		return
	end

	local name = player:get_player_name()

	if name == "" then
		return
	end

	hud_ids[name] = nil
end

local function player_event_handler(player,eventname)
	assert(player:is_player())

	local name = player:get_player_name()

	if name == "" then
		return
	end

	if eventname == "health_changed" then
		initialize_builtin_statbars(player)

		if hud_ids[name].id_healthbar ~= nil then
			player:hud_change(hud_ids[name].id_healthbar,"number",player:get_hp())
			return true
		end
	end

	if eventname == "breath_changed" then
		initialize_builtin_statbars(player)

		if hud_ids[name].id_breathbar ~= nil then
			player:hud_change(hud_ids[name].id_breathbar,"number",player:get_breath()*2)
			return true
		end
	end

	if eventname == "hud_changed" then
		initialize_builtin_statbars(player)
		return true
	end

	return false
end

function minetest.hud_replace_builtin(name, definition)

	if definition == nil or
		type(definition) ~= "table" or
		definition.hud_elem_type ~= "statbar" then
		return false
	end

	if name == "health" then
		health_bar_definition = definition

		for name,ids in pairs(hud_ids) do
			local player = minetest.get_player_by_name(name)
			if  player and hud_ids[name].id_healthbar then
				player:hud_remove(hud_ids[name].id_healthbar)
				initialize_builtin_statbars(player)
			end
		end
		return true
	end

	if name == "breath" then
		breath_bar_definition = definition

		for name,ids in pairs(hud_ids) do
			local player = minetest.get_player_by_name(name)
			if  player and hud_ids[name].id_breathbar then
				player:hud_remove(hud_ids[name].id_breathbar)
				initialize_builtin_statbars(player)
			end
		end
		return true
	end

	return false
end

minetest.register_on_joinplayer(initialize_builtin_statbars)
minetest.register_on_leaveplayer(cleanup_builtin_statbars)
minetest.register_playerevent(player_event_handler)