Question

I’m making an anonymous commenting system for my blog. I need the users to have a randomly picked username from an array I have made, it has 600 usernames. I can’t just make it random because then people wouldn’t know if it was the same person posting a reply, so I have given each post a randomly generated key between 1-9999, using the key and the users ID I want to do some sort of calculation so that number will stay consistent through that particular post. The result has to be within 1-600.

something like:

user_id x foo(1-9999) = bar(1-600)

Thanks.

Answer 1

What you’re probably looking for is a hash function. To quote Wikipedia:

A hash function is any algorithm or subroutine that maps large data sets of variable length, called keys, to smaller data sets of a fixed length.

So you can use a standard hash function, plus modular arithmetic to further map the output of that hash function to your username range, like so:

function anonymise($username, $post_key) {
    $hash = hash("adler32", "$username/$post_key");
    $hash_decimal = base_convert($hash, 16, 10);
    $anonymised_id = $hash_decimal % 600;

    return $usernames[$anonymised_id];
}