‘Hard to specify’

I read “two strings” literally. The strings are the only case-specific inputs. A fact read from a database, sensor, or anywhere else is another input, whether or not it appears in the function signature.

No function of those two strings can always provide such a ranking.

The bounty says the code must accept two strings as input. It doesn’t say only or at most or exactly two. But I see now that this may not have been clear – if you think it’ll help, I can edit the bounty to say ‘at least two’.

Also, when I wrote the bounty, I meant input as in ‘part of the function signature’. If the function body later reads from a database or prompts the user for input, that doesn’t in and of itself disqualify the submission.