"It's deliberately unrunnable, as you don't have the code for f1() or f2() or any sample data. Does that make it too hard?"

Yes. I'd like to hack on this, but without the functions and sample data, my brain swims.

Dragonchild and hv came up with good ideas, but for those of us with less experience, having dummy f1/2 and some sample data would make it a lot easier to see if our ideas were legitimate.

I think you could have coded some dummyup up with a lot less effort than it would take to do unit tests, as dws suggested

Just my 2c!