Maximum Global Read, Minimum Write Access

When you here people say never use global variable, there is a lot of extra information they're excluding that is a software way to enforce good practice.

As A General Rule

Any variable defined in a string, integer, float, array, or hash should be defined inside of a constant if defined outside of a module or class. This includes immortal symbols.

Any variable defined locally should have maximum read access in its own dominion, but no ability to read variables outside its own function, with the exception of global variables.

Global variables are initialized by constants, but you never modify the constant directly. You can think of this like a bucket and pond, you can never deplete the pond ( in most cases, except on an industrial scale ). These can be used across the program, as you're never directly interacting with the constant. These are "moderate read, moderate write".

Here is an example ruby program based on variable access priviledge.

$INTIALIZE_GLOBAL_DAY_COUNT = 0

$TRUE_HP_LEVEL_1 = 15
$TRUE_HP_LEVEL_2 = 32
$TRUE_HP_LEVEL_3 = 76

module Game
  class Environment
    def self.initialize
      @current_day = $INITIALIZE_GLOBAL_DAY_COUNT
      
      @possible_hp_resets = [$TRUE_HP_LEVEL_1,
                             $TRUE_HP_LEVEL_2,
                             $TRUE_HP_LEVEL_3]
                              
      @player_level = 0
    end
    
    
    def self.operation
    
      if @current_day == 30
        @current_day = $INITIALIZE_GLOBAL_DAY_COUNT
        
        puts "The lunar calender has reset to zero."
      else
        if    @player_level == 0
          current_hp_reset = @possible_hp_resets[0]
          
          @current_day = @current_day + 1
        elsif @player_level == 1
          current_hp_reset = @possible_hp_resets[1]
          
          @current_day = @current_day + 1
        elsif @player_level == 2
          current_hp_reset = @possible_hp_resets[2]
          
          @current_day = @current_day + 1
        else
          current_hp_reset = @possible_hp_resets[0]
          
          @current_day = @current_day + 1
        end
      end
      
    end
  end
end