Intro

There's a lot of confusion over smart splitters. How they work is not completely obvious, and the reality is a lot 'dumber' than you would expect. So I decided to make a post to go into detail about how they actually work and how a lot of us think they should work.

How They Work

First off, Smart Splitters should really be called something more descriptive like 'Single-sided Filter-by-item Splitter' as they are very limited in usefulness (right now, hopefully they will get some fixes). They currently obey a handful of main rules.

1. Filters can be set for each output side
   - Press E while facing the splitter and you can choose an item, ANY, NONE for all three output sides.
2. Direction Priority: CENTER > LEFT > RIGHT
   - This means that if you set all three sides to the same item, they will choose a single output side according to the above priority. It looks at center first, if no match/any, it looks left, then right.
3. Only 1 side per item allowed. WILL NOT overflow even if all sides set to same item
   - So if you set all sides to the same item, it will only go out one output side, and will not go out any others like a normal splitter would. This also means if that one side gets blocked, the line will halt and no more items of any type will get through.
4. Specified 'ITEM1' side takes priority over 'ANY' side for 'ITEM1' output
   - Pretty straightforward. If you set CENTER to 'ANY', and RIGHT to 'ITEM1' specifically, it will always go down the right side.

Testing Results

I did some basic testing to prove the above results. It's not particularly interesting, but I figured I would include it for provenance sake. Note: All 'sides' are while standing on top facing the 'output' center, with the 'input' behind you.

1. All three outputs set to ITEM: goes down center by default
   - If center disconnected/blocked: NOTHING GOES THROUGH
2. All three outputs set to ANY: goes down center by default
   - If center disconnected/blocked: NOTHING GOES THROUGH
3. Center set to ANY, left/right set to ITEM: goes down LEFT by default
4. Center set to ANY, left/right set to ITEM. LEFT output blocked/disconnected: NOTHING GOES THROUGH
5. Center set to ITEM1, left/right set to ANY: ITEM1 goes down center, ITEM2 goes down left
   - If ITEM1 or ITEM2 lines become blocked, NO OVERFLOW happens, thus the whole line shuts down

Needed Changes

Smart Spitters really need only a single minor fix to be useful: Split single item types to multiple sides. Just like normal splitters, but ruled by your filter settings. So if you are sending down Ingots, they default to the current rules, with specified outputs given priority. However, if that line becomes blocked, it should check the other lines for an ANY route and then start routing them that way. Allowing a full line to overflow to an additional side (or two) would be amazing, and allow many additional uses for smart splitters.