-- The Fibonacci sequence is defined by the recurrence relation:
--
-- Fn = Fn−1 + Fn−2, where F1 = 1 and F2 = 1.
-- Hence the first 12 terms will be:
-- F1 = 1
-- F2 = 1
-- F3 = 2
-- F4 = 3
-- F5 = 5
-- F6 = 8
-- F7 = 13
-- F8 = 21
-- F9 = 34
-- F10 = 55
-- F11 = 89
-- F12 = 144
--
-- The 12th term, F12, is the first term to contain three digits.
--
-- What is the index of the first term in the Fibonacci sequence to contain 1000 digits?

fibs :: [Integer]
fibs = 0:1:zipWith (+) fibs (tail fibs)

thousandDigitFib = length $ takeWhile (\x -> length (show x) < 1000) fibs

main =  do
    let result = thousandDigitFib
    putStrLn $ "Project Euler, Problem 25\n"
        ++ "Answer: " ++ show result