Add "nth bit is set" to circuit signal comparisons

[kbios]

This is a proposal to add "nth bit is set" (1 to 32) to the list of comparisons between two signals or between a signal and a constant (the current list is more than, less than, equal, more or equal, less or equal, not equal).

With the comparison operations available right now the only way to check for a specific bit is to check for every possible value, for example to check if the first bit is set you need to say "equal to 1 or equal to 3 or equal to 5 or equal to 7" and so on which is obviously infeasible.

I think this would greatly expand the possibilities of signals in combination with circuit-based train interrupts.

• Imagine being able to transmit a whole dynamic schedule to a train using a single signal, where every set bit corresponds to a stop.
• Imagine being able to interpret some bits as flags, for example to trigger slightly different versions of an interrupt (recursive versus not-recursive, or with different waiting times etc).

It would also be useful in other situations besides trains, because it would basically offer 32 times the amount of signals with no complications for those who don't want or need them.

Note that this is not the same as extracting the wanted bit using a network of combinators, because that still requires to use many signals for the "final leg" of the transmission and thus does not increase the expressive power of the system.

[secelt]

you don’t need this at all, since what you’re describing can be done with only one or two arithmetic combinators and some judicious bitwise operators:

• to check if a bit is set (to get its value), you just need to calculate the bitwise AND of the input and a constant with just that bit set (here, bit 5, the sixth bit):

  

  Screenshot_20241222_200336.png (82.23 KiB) Viewed 243 times

  (to check bit 0, you can even just calculate the input modulo 2, modulo being represented by the “%” operator.)
• to check if any of a set of bits is set (to filter out the other bits), just get the bitwise AND of the input and a constant with just those bits set to 1 (here, bits 0, 1 and 5):

  

  Screenshot_20241222_200847.png (82.93 KiB) Viewed 243 times

• to set some bits, do the input OR the bits you want to set (here, bit 3):

  

  Screenshot_20241222_202243.png (84.28 KiB) Viewed 243 times

• to unset some bits, AND the input with the bitwise NOT of the bits you don’t want (here, bit 2):

  

  Screenshot_20241222_203409.png (82.38 KiB) Viewed 243 times

  remember that Factorio signals are 32‑bit two’s‐complement signed integers!

you can look on your own how to use XOR to compare bits, “<<” and “>>” to move bits around and much more!

also, your one‐bit‐per‐train system falls apart when there are more than 32 stops.

also also, you don’t really need this in most situations since wires already carry every signal at once (signals with a value of 0 are hidden), so you aren’t likely to run out of them; you can absolutely use a whole signal for each individual flag and not run out until you need like 70 or something (for virtual signals).

[kbios]

Hi, thank you for your detailed response.

However I think there is a misunderstanding here: I'm fully aware of how to set and check bits using combinators.

The problem lies in the communications between the circuit network and the trains themselves, where you cannot do that because trains cannot check individual bits, and thus you are forced to "waste" whole signals (hence my proposal).

Also I'd like to point out that my system wouldn't fall apart with more than 32 stops, because you can have 32 stops per signal (so a few thousands).

As for running out of signals, well that depends: depending on how detailed you want to be when controlling stuff the 70 or so signals may not be enough