{-

- Solution to Project Euler problem 225

- Copyright (c) Project Nayuki. All rights reserved.

-

- https://www.nayuki.io/page/project-euler-solutions

- https://github.com/nayuki/Project-Euler-solutions

-}

index = 124 -- 1-based

main = putStrLn (show ans)

ans = (filter hasNoTribonacciMultiple [1,3..]) !! (index - 1)

hasNoTribonacciMultiple :: Int -> Bool

hasNoTribonacciMultiple n = let

tribonacci (a, b, c) = (b, c, mod (a + b + c) n)

func init slow@(a, _, _) fast

| mod a n == 0 = False

| not init && slow == fast = True

| otherwise = func False (tribonacci slow) (tribonacci (tribonacci fast))

in func True (1,1,1) (1,1,1)