The code wouldn't be special cased... it would run through the first 10,000 or so rows to find out the dataset to generate the state transition table. If any invalid entry is found later, you'll end up in an invalid state, and can detect that case and fall back to slow code (or even theoretically update the state transition table and return to the fast path).
Even simpler. Just have a Dictionary mapping place name to ID. The slow path is "I haven't seen this name before and I need to store it, making sure that I'm not racing any other thread doing the same."
You'd have an upper limit on how many place names you can have. But no hard coding of what they are.
