https://algospot.com/judge/problem/read/STARFORCE

starforce.hs

module Starforce where

import Data.Bits
import Data.List

answers :: Int -> [Int]
answers = \bitnum -> concat $ map (bit_permutation bitnum) $ reverse [1..bitnum]

bit_permutation :: Int -> Int -> [Int]
bit_permutation bitnum onebit
  | zerobit == 0 = [foldr (.|.) 0 $ map bit [0..(onebit-1)]]
  | onebit == 0 = [0]
  | otherwise = (bit_permutation (bitnum-1) onebit)
                ++ (map ((bit (bitnum-1)) .|.) (bit_permutation (bitnum-1) (onebit-1)))
  where zerobit = bitnum - onebit

possible :: [Int] -> Int -> Int -> Bool
possible = possible' 0

possible' :: Int -> [Int] -> Int -> Int -> Bool
possible' _ _ 0 _ = True
possible' _ [] _ _ = False
possible' c (n:ns) m a
  | (partial .&. a) == a = possible' 0 ns (m-1) a
  | otherwise = possible' partial ns m a
  where partial = c .|. n

starforce :: [Int] -> Int -> Int
starforce ns m = case (find (possible ns m) (answers 16)) of Just n -> popCount n
                                                             Nothing -> 0