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-- Even Fibonacci numbers
-- Problem 2
-- Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:
-- 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
-- By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.
main = do
print (sum (filter even (fib_until 4000000)))
fib_until threshold = fib_from_index_until 0 threshold
fib_from_index_until :: Int -> Int -> [Int]
fib_from_index_until i threshold
| fib_i < threshold = fib_i:(fib_from_index_until (i + 1) threshold)
| otherwise = []
where fib_i = fibonacci i
fibonacci :: Int -> Int
fibonacci 0 = 0
fibonacci 1 = 1
fibonacci 2 = 1
fibonacci x = fibonacci (x - 1) + fibonacci (x - 2)
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