HOW to map both ways, in a one to one relation, (5-digit-integer <=> string) without losing any information?
CREATE TABLE pin_to_url (
pin int PRIMARY KEY,
SELECT v.*, pu.url FROM visitors v
JOIN pin_to_url pu ON pu.pin = v.pin
ORDER BY whatever;
Just store the filename alongside the pin into the table and it'll work pretty well.
The only other way of mapping string to pin -- apart from the lookup table I've just typed out -- is enumerating over the possible filenames and hashing each in turn, comparing the result to the pin. There's no way around it with the constraints you stubbornly add.
Oh, by the way, with 100 files and 10k available (random-ish) identification numbers, you might hit the birthday paradox. About 60% probable, I think. You don't want a hashing function -- you want an integer sequence. Databases provide those.