Output redstone single for first empty barrel

Question

I'm looking to build a system, where given a row of barrels, and an input line, I want to get the output of the first (or last if that's easier) empty barrel only. I can easily enough get an output for every empty barrel, but struggling to get it for the first only. It needs to be tileable if possible.

If tried various things with locking repeaters, observer chains and pistons to break the chain, but they've all had issues and not worked.

Has anyone does anything like this before or have any pointers on what to try?

Screenshot is just an example layout to hopefully help clarify a little, doesn't have to fit into those dimensions

Answer 1

Fabian's comment helped me come up with the solution. Pressing the button will cause 1 of the redstone torches on the top to light up, and only the first one, which first what I wanted exactly!

Answer 2

If you mean first as in the first one to get emptied (regardless of position), the multi-input NOR is the answer:

Tiles up to 16 with output anywhere, up to 31 with output in the middle. More will require a non-backfeeding repeater trickery to remain 1-tileable.

If you meant "barrel in position 1", the trivial solution is not to do anything with the rest of the barrels.

Also, if instead of continuous signal "One or more barrels is empty" you want a pulse "First barrel emptied" you can add a simple monostable:

The observer will send a pulse after being moved by the sticky piston, but only if the piston extends (first barrel just emptied) it will output into the redstone line. Retraction as all barrels get some contents will send the pulse into empty air.