ProjectEuler/haskell/src/0023.hs

{-
A perfect number is a number for which the sum of its proper divisors is exactly equal to the number. For example, the sum of the proper divisors of 28 would be 1 + 2 + 4 + 7 + 14 = 28, which means that 28 is a perfect number.
A number n is called deficient if the sum of its proper divisors is less than n and it is called abundant if this sum exceeds n.

As 12 is the smallest abundant number, 1 + 2 + 3 + 4 + 6 = 16, the smallest number that can be written as the sum of two abundant numbers is 24. By mathematical analysis, it can be shown that all integers greater than 28123 can be written as the sum of two abundant numbers. However, this upper limit cannot be reduced any further by analysis even though it is known that the greatest number that cannot be expressed as the sum of two abundant numbers is less than this limit.
Find the sum of all the positive integers which cannot be written as the sum of two abundant numbers.
-}

import Utils

import Data.List

--import Data.Map (Map)
--import qualified Data.Map as Map

import Data.IntSet (IntSet)
import qualified Data.IntSet as IntSet

import Debug.Trace

isAbundant :: Int -> Bool
isAbundant x = (divs x |> sum) > x

abundants :: [Int]
abundants = iterate (+1) 1 |> take 28124 |> filter isAbundant
abundantsSet :: IntSet
abundantsSet = IntSet.fromList abundants

f :: Int -> Bool
f n = go' abundants
  where
    go' [] = False
    go' [x] = IntSet.member (n-x) abundantsSet
    go' (x:xs)
      | x >= n                           = False
      | IntSet.member (n-x) abundantsSet = True
      | otherwise                        = go' xs

solution :: Int
solution = iterate (+1) 1 |> take 28123 |> filter (\x -> (f x) == False) |> sum

main :: IO ()
main = putStrLn ("Solution: " ++ show solution)