30 lines
854 B
Haskell
30 lines
854 B
Haskell
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module Main where
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import Control.Monad (when)
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import Data.Maybe (catMaybes, fromJust)
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import System.Exit (die)
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exampleExpectedOutput = 514579
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compute2020 :: Int -> Int -> Maybe Int
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compute2020 a b
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| a + b == 2020 = Just (a * b)
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| otherwise = Nothing
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compute :: String -> Int
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compute input = head . catMaybes $ processInput inputList
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where
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inputList :: [Int]
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inputList = map read $ lines input
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processInput :: [Int] -> [Maybe Int]
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processInput (_:[]) = [Nothing]
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processInput (x:xs) = map (compute2020 x) xs ++ processInput xs
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main :: IO ()
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main = do
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example <- readFile "example"
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let exampleOutput = compute example
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when (exampleOutput /= exampleExpectedOutput) (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
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input <- readFile "input"
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print $ compute input
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