main.hs

solveWithRepa :: (R.Array R.U R.DIM1 Int) -> IO (Int, Int, Int)
solveWithRepa stockPrices = do
  let indices = R.fromListUnboxed (R.Z R.:. (8::Int)) [0..(S.size $ R.extent stockPrices)-1] :: R.Array R.U R.DIM1 Int
  temp <- (R.computeP $ R.zipWith (\a b -> (a,b)) indices indices) :: IO(R.Array R.U R.DIM1 (Int, Int))
  mapped <- (R.computeP $ R.zipWith (\(a,b) c -> (a,b,c)) temp indices) :: IO(R.Array R.U R.DIM1 (Int, Int, Int))

  -- let temp = R.foldS foldF (0::Int, 0::Int, 0::Int) mapped
  temp <- R.foldP foldF (0::Int, 0::Int, 0::Int) mapped
  let (buyIdx, sellIdx, minIdx) = head $ R.toList temp
  let buyAmount = stockPrices R.! (R.Z R.:. buyIdx)
  let sellAmount = stockPrices R.! (R.Z R.:. sellIdx)

  return (buyIdx, sellIdx, sellAmount - buyAmount)
  where
    foldF :: (Profit, Int) -> (Profit, Int) -> (Profit, Int)
    foldF ((b1, s1), min) ((b2, s2), min2) = (maxProfit, )
      where
        amount idx = stockPrices R.! (R.Z R.:. idx)
        minIdx     = min (amount min) (amount min2)

         --  YOU WERE HERE, DETERMINE HOW TO GET MAX PROFIT

        maxProfit  = max lProfit $ max mProfit rProfit
        lProfit    = (amount s1) - (amount b1)
        rProfit    = (amount s2) - (amount b2)
        mProfit    = max ((amount s2) - (amount b1)) ((amount s2) - (amount min))