My aim is a procedure that will take two integer arguments:
A number representing a position in a stack of numbers
A random seed
And output the position of the first number, after the shuffling has occurred.

I'd just shuffle the stack manually, except the procedure has to work under conditions where stack is many billions of numbers high. Because of the massive potential size of the stack and the frequency of checks on various positions, I need to calculate the position mathematically. So, for example, I might want to know where position 500032 will end up after the shuffling occurs. The returned position needs to be the same each time it shuffles as long as the random seed is the same and the number of items in the stack are the sane, naturally.

Asking people that I know about this problem, two terms have been used: Hash table and shuffling algorithm. Can anyone point me to a code example or resouce that can help me to write the procedure?