Item Repair Recipe

I'd like to be able to have an item repaired in the crafting grid with something other than an identical item — currently, I'm making an uncraftable item that's equivalent to an iron sword, and I want it to be able to be repaired by combining it with one, but I can see other potential uses too. Seems like it should be an easy thing to do, but the best I can manage is to make a shapeless recipe that always outputs a fully repaired item. Not ideal. Preferably, I'd like to respect the durability formula used for normal repair recipes. Doesn't bother me if it takes coding to do, but does anyone know a way to do this already?