Count Empty Chests

Count Empty Chests is a mod for Factorio.

It provides a combinator that outputs a signal indicating the number of
completely-empty storage chests in the logistic network it is placed in.

The primary intended use case is to set up an alarm for when the storage
is close to being full. Although Factorio provides a built-in alert for
when the storage is completely full, that is only triggered once every
slot is full, meaning that by then, many chests have been filled with
a random assortment of items, which is then a mess to clean up. With
this mod, you can set up an alert to trigger before the bots start
packing the chests to the gills with random trash.

Usage

The new entity, Empty Chest Combinator, is unlocked with the
Advanced combinators
research, the same one that unlocks
Selector combinator.
If your game already has that unlocked, then the new entity will be
available as soon as you load it with the mod enabled.

Place the combinator anywhere within a logistic network. It outputs
a signal called "E" that is the number of storage chests (the yellow
ones; other chest types are excluded) that are empty and do not have
a storage filter set.

Typically, one would then hook up an alarm to trigger when E is less
than some threshold like 5, meaning the robots are getting close to the
point where they will start putting more than one type of item into
each chest.

If you place it outside any logistic network, it just outputs E as 0,
without any indication that it is not in a network.

Performance considerations

This mod is intended to be used in factories that have large storage
networks.

Counting the number of empty chests must be done with a loop; the API
does not provide anything better. For a logistic network containing
4000 chests (regardless of whether they are empty), it takes around 10
ms to loop over them, and the time scales linearly with the number of
chests. 10 ms of time spent in a mod is roughly the point where frames
start getting delayed because it only leaves 6 ms for everything else.

Therefore, this mod does a few things to limit the performance impact:

• The combinator update code only runs once every 600 ticks (10 seconds)
  by default.

• Each update only looks at one combinator, cycling through them over
  time. So if you have N combinators in the world, then each combinator
  gets updated every 600*N ticks.

• The loop code itself has been profiled and optimized as best I can.

In terms of factory design, considering the above, it is better to use
just one combinator per network and route the one signal to where it
needs to go, rather than use multiple combinators if the information is
needed in multiple places.

Uninstallation

Simply remove the mod, as there should be no unexpected adverse
consequences:

• The mod adds one new entity, the combinator that counts empty chests.
  If you remove the mod, then instances of that entity will be deleted.

• The mod does not add any save-game state of its own.

Enhancement ideas

Some ideas for enhancements:

• Do something with filtered chests. In particular, I'm thinking it
  could output a signal for each distinct filtered item, where the
  signal value indicates the number of empty chests with that filter.

• Count the total number of chests, which might be useful for other
  types of capacity calculations. Does that include those with filters?

• Add an icon when the combinator is not in a logistic network, similar
  to how storage chests behave. (I looked into this and could not find
  an easy way, but may have overlooked something.)

Let me know if any of these seem important to you.

Related mods

In my search, the only vaguely related mod I found was Chest Slot
Reader, but that just
reads the number of empty slots in a single chest, whereas I want the
number of completely empty chests across an entire network.

Otherwise, I didn't find anything similar to this mod in my search. Let
me know if you know of another mod that does something like this, since
at very least I'd like to link to related alternatives.