Fibonacci, fast

The problem

The Fibonacci numbers go 0, 1, 1, 2, 3, 5, 8, 13 ... Each one is the sum of the two before it. Write a function fibonacci(n) that gives back the n-th one, counting from 0: fibonacci(0) is 0, fibonacci(1) is 1, fibonacci(7) is 13. It must also work fast for n = 50.

You will practice: loops, keeping state in variables, why repeating work is slow