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-- 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?
main = print (length $ takeWhile (\x -> x `div` 10 ^ 999 == 0) fibs)
-- thanks to you, random german video
-- found this: https://wiki.haskell.org/The_Fibonacci_sequence
fibs :: [Integer]
fibs = 0 : 1 : zipWith (+) fibs (tail fibs)
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