Programming Praxis – Fibonacci Primes

Today’s Programming Praxis exercise comes from the same challenge as the longest palindrome exercise from two weeks ago. The goal is to write a function that calculates the sum of the factors of 1 plus the first prime fibonacci number higher than the input number. Let’s get started, shall we?

To generate the fibonacci numbers, we use the well-known one-liner.

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

Getting the factors of a number is done via simple trial division. Unlike the Scheme solution, we don’t repeat factors, since the original challenge says not to.

factors :: Integer -> [Integer]
factors = f 2 where
    f d n | n < d        = []
          | mod n d == 0 = d : f (d + 1) (div n d)
          | otherwise    = f (d + 1) n

We could write some fancy algorithm to check primality, but since we already have a function to calculate factors, why bother? Since the only factor of a prime number will be itself, we can just use that as a check. Initally I just used the isPrime function from Data.Numbers.Primes for this, but looking through the source code I realized I could replace it with this version. Obviously this won’t work too well on large numbers, but for our test case it’s fast enough.

isPrime :: Integer -> Bool
isPrime n = factors n == [n]

The top-level function does what it says on the tin: look through the fibonacci numbers to find one that’s greater than n and prime, add one and return the sum of the factors.

greplin :: Integer -> Integer
greplin n = head [sum $ factors (f + 1) | f <- fibs, f > n, isPrime f]

All that’s left is to apply it to the test number:

main :: IO ()
main = print $ greplin 227000

As expected, we get 352 as a result. And, in the spirit of the contest, which is to do these challenges quickly, I think this one took me about 20 minutes.

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