How to make a tiered light indicator.

Factorio indicator for tanks

# A tiered light indicator is pretty easy to build and this will show you how to build scalable indicators using (2n + 1) decoders 
# and one arithmetic. I made mine with three lights, which is enough to understand at a glance what's going on.
# 
# You can also put the logic on the lights and use fewer deciders but I prefer this setup because it's easier to modify.
# 
# Inputs: 
#   - R is a constant "ratio". It should be set to the integer value of (tank size - 1 / # of lights).
#   - resource types
#
# Variables:
#   - Colors are used to control the lights and have a value of 1
#   - [0], [1], [2], .. are used to toggle light 0, 1, 2, and so on. Each light gets a decider. This allows you to make changes easily
#                       for example you might want to make an error state have a blinking light.
#   - P is resource / R and is used to make logic independent of tank and light size.

def calc_P(R, resource):
    return resource / R # this is an arith combinator + a constant combinator.

def calc_color(P):
    if P == 0:
        # decoder combinator
        return red
    if P == 1:
        # decider combinator
        return yellow
    if P == 2:
        # decider combinator
        return green
    if P >= 3:
        # decider combinator
        return red for error or blue if this is ok (tank is full)
    # just line up four decider combinators and connect all the inputs. on a different line connect all the outputs. This group of 
    # four should always have exactly one color coming out!

def calc_numbers(P):
    # Numbers are used to control the lights. If [1] is emitted then light 1 is on. More than one number will be emitted at the 
    # same time. I will use a function called "emit" to mean the output of a combinator
    if P >= 0:
        emit([0], 1)
    if P >= 1:
        emit([1], 1)
    if P >= 2:
        emit([2], 1)

# then all you need to do is wire up three lights:

light 0:
* Enable Disable ([0] = 1)
* Use Colors

light 1:
* Enable Disable ([1] = 1)
* Use Colors

light 2:
* Enable Disable ([2] = 1)
* Use Colors

# That's it! You can reduce the combinators by moving the number logic into the lights if you prefer a compact design.