function init(self)
    -- < 1 >
    self.player_one_health = 1.0
    self.player_two_health = 1.0
    self.game_boss_health = 1.0

    -- < 2 >
    timer.delay(1, true, function()
        -- < 3 >
        self.player_one_health = math.max(self.player_one_health - 0.1, 0)
        self.player_two_health = math.max(self.player_two_health - 0.1, 0)
        self.game_boss_health = math.max(self.game_boss_health - 0.1, 0)
        -- < 4 >
        msg.post("hud", "update_health", { health_name = "left_health", health_percentage = self.player_one_health })
        msg.post("hud", "update_health", { health_name = "right_health", health_percentage = self.player_two_health })
        msg.post("hud", "update_health", { health_name = "center_health", health_percentage = self.game_boss_health })
    end)
end

--[[
1. Set initial health percentage (1.0 = 100%, 0.0 = 0%).
2. Start a timer that will call every 1 second (first argument) repeateadly (second argument being true) a callback function (3rd argument)
3. Reduce each health percentage by 0.1 (10%), but no less than 0 (using math.max to select `0`, if `self.player_one_health - 0.1` is less than `0`).
4. Send messages to hud (gui component) to "updated_health" with health name and percentage to be set in GUI script.
]]

-- < 1 >
local min_size = 48
local max_size = 235 - min_size

-- < 2 >
local function set_healthbar(healthbar_node_name, health_percentage)
    local healthbar_node = gui.get_node(healthbar_node_name)    -- < 3 >
    local healthbar_size = gui.get_size(healthbar_node)            -- < 4 >
    healthbar_size.x = health_percentage * max_size + min_size    -- < 5 >
    gui.set_size(healthbar_node, healthbar_size)                -- < 6 >
end

function init(self)
    -- < 7 >
    set_healthbar("left_health", 1.0)
    set_healthbar("right_health", 1.0)
    set_healthbar("center_health", 1.0)
end

function on_message(self, message_id, message, sender)
    -- < 8 >
    if message_id == hash("update_health") then
        set_healthbar(message.health_name, message.health_percentage)
    end
end

--[[
1. Define minimum and maximum size of GUI healthbar (only width is changed).
2. Define a local helper function to update healthbar.
3. Get node of given name passed as "healthbar_node_name" and store it in local variable "healthbar_node".
4. Get size of this node and store it in local variable "healthbar_size".
5. Change size along X axis (width) of the node to given "health_percentage" scaled times "max_size" and added to "min_size", so that it can be no smaller than it.
6. Set the newly updated size of the node.
7. In init function, for each of three defined nodes set initial health_percentage to 1.0 (100%).
8. In on_message function, if the GUI component receives message "update_health" call helper function to update given health bar.
]]