This is a calculator for a Magic: the Gathering rules variant I have daydreamed about called "Birthmark." In Birthmark, every player has a unique card pool from which it is legal to build a deck. That card pool is determined by hashing the name of each Modern-Legal card with some unique unchanging identifier. Each person's card pool grows by about 15 cards each with each set. Not all Birthmark players are born equal. If you are unlucky you'll get weak cards with poor synergy.... too bad. This is your life now!
You can read the script source too see how boringly simple it is. The very first function is where the magic happens. This page is offline-only. I won't steal your data. You can save this page to your computer.
I chose MTGO usernames for the identifier, tentatively, but a person's phone number, birthdate, etc, may be a better thing to use for paper play. Of course you can cheat by re-registering on MTGO with a name that hashes better... but this sort of thing comes down to honor system no matter what.
You may use this page to figure out what you're card pool would be. The number of cards you get varies. It is almost always between 650 and 800 (stdev = 25).