Some applications of random number generators. games, for instance, require that the random sequence be available to run in reverse.

This is easy to do with a simple linear congruential random number generator, which is characterized by the formula next = a * prev + c (mod m). With a little bit of algebra, that formula can be written prev = a−1 * (next - c) (mod m), where a−1 is the inverse of a (mod m); that modular inverse always exists because the linear congruential generator requires that a and m are coprime.

Your task is to write a reversible random number generator. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.


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