mainmenu_csm/builtin/game/statbars.lua

-- cache setting
local enable_damage = core.settings:get_bool("enable_damage")

local health_bar_definition = {
	hud_elem_type = "statbar",
	position = { x=0.5, y=1 },
	text = "heart.png",
	number = core.PLAYER_MAX_HP_DEFAULT,
	direction = 0,
	size = { x=24, y=24 },
	offset = { x=(-10*24)-25, y=-(48+24+16)},
}

local breath_bar_definition = {
	hud_elem_type = "statbar",
	position = { x=0.5, y=1 },
	text = "bubble.png",
	number = core.PLAYER_MAX_BREATH_DEFAULT,
	direction = 0,
	size = { x=24, y=24 },
	offset = {x=25,y=-(48+24+16)},
}

local hud_ids = {}

local function scaleToDefault(player, field)
	-- Scale "hp" or "breath" to the default dimensions
	local current = player["get_" .. field](player)
	local nominal = core["PLAYER_MAX_".. field:upper() .. "_DEFAULT"]
	local max_display = math.max(nominal,
		math.max(player:get_properties()[field .. "_max"], current))
	return current / max_display * nominal
end

local function update_builtin_statbars(player)
	local name = player:get_player_name()

	if name == "" then
		return
	end

	local flags = player:hud_get_flags()
	if not hud_ids[name] then
		hud_ids[name] = {}
		-- flags are not transmitted to client on connect, we need to make sure
		-- our current flags are transmitted by sending them actively
		player:hud_set_flags(flags)
	end
	local hud = hud_ids[name]

	local immortal = player:get_armor_groups().immortal == 1
	if flags.healthbar and enable_damage and not immortal then
		local number = scaleToDefault(player, "hp")
		if hud.id_healthbar == nil then
			local hud_def = table.copy(health_bar_definition)
			hud_def.number = number
			hud.id_healthbar = player:hud_add(hud_def)
		else
			player:hud_change(hud.id_healthbar, "number", number)
		end
	elseif hud.id_healthbar then
		player:hud_remove(hud.id_healthbar)
		hud.id_healthbar = nil
	end

	local breath_max = player:get_properties().breath_max
	if flags.breathbar and enable_damage and not immortal and
			player:get_breath() < breath_max then
		local number = 2 * scaleToDefault(player, "breath")
		if hud.id_breathbar == nil then
			local hud_def = table.copy(breath_bar_definition)
			hud_def.number = number
			hud.id_breathbar = player:hud_add(hud_def)
		else
			player:hud_change(hud.id_breathbar, "number", number)
		end
	elseif hud.id_breathbar then
		player:hud_remove(hud.id_breathbar)
		hud.id_breathbar = nil
	end
end

local function cleanup_builtin_statbars(player)
	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 == "" or not hud_ids[name] then
		return
	end

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

		if hud_ids[name].id_healthbar then
			return true
		end
	end

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

		if hud_ids[name].id_breathbar then
			return true
		end
	end

	if eventname == "hud_changed" or eventname == "properties_changed" then
		update_builtin_statbars(player)
		return true
	end

	return false
end

function core.hud_replace_builtin(hud_name, definition)

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

	if hud_name == "health" then
		health_bar_definition = definition

		for name, ids in pairs(hud_ids) do
			local player = core.get_player_by_name(name)
			if player and ids.id_healthbar then
				player:hud_remove(ids.id_healthbar)
				ids.id_healthbar = nil
				update_builtin_statbars(player)
			end
		end
		return true
	end

	if hud_name == "breath" then
		breath_bar_definition = definition

		for name, ids in pairs(hud_ids) do
			local player = core.get_player_by_name(name)
			if player and ids.id_breathbar then
				player:hud_remove(ids.id_breathbar)
				ids.id_breathbar = nil
				update_builtin_statbars(player)
			end
		end
		return true
	end

	return false
end

-- Append "update_builtin_statbars" as late as possible
-- This ensures that the HUD is hidden when the flags are updated in this callback
core.register_on_mods_loaded(function()
	core.register_on_joinplayer(update_builtin_statbars)
end)
core.register_on_leaveplayer(cleanup_builtin_statbars)
core.register_playerevent(player_event_handler)