Coding classes in Gloucestershire

A linear congruential generator is one line of code: take the last number, multiply it by a constant a, add another constant c, and keep the remainder after dividing by 65,536. The output jumps around convincingly. The learner starts with a = 25,173 and c = 13,849, which we picked, and runs three tests. The first asks how long before the numbers repeat. For that they use Floyd's method, a tortoise that moves one step and a hare that moves two, which finds the loop without storing a single earlier number.

The second test looks at single bits. With the good constants, the lowest bit of the output goes 1, 0, 1, 0, 1, 0 for all twenty numbers we printed, and the next bit repeats every four. The numbers look random as a whole, but their last binary digit is as predictable as a ticking clock. Anyone who used that bit to decide a coin toss would lose every time to someone who knew.

The third test is the attack. Suppose you know the formula's shape and the 65,536, but not a or c. Watch three outputs in a row. The difference between the second and third must be a times the difference between the first and second, which pins down a; then c follows. In our run, exactly one pair of constants fits three observed numbers, and it predicts the next 17 outputs, 17 out of 17. A generator like this is fine for a dice game. For a password or a key, it is a disaster.

So a Gloucestershire teenager should learn to code in 2026, in the county of the Doughnut, to be able to tell random-looking from unpredictable. The longer argument is in why teenagers should still learn to code in 2026.