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Day09.hs
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Day09.hs
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{-# LANGUAGE DuplicateRecordFields #-}
{-# LANGUAGE OverloadedRecordDot #-}
module Day09 where
import Data.Maybe
import Text.ParserCombinators.ReadP
import Harness
import ParseHelper
import Data.List qualified as L
main :: IO ()
main = getInputAndSolve (parseInput parseValues) sumOfNextPredictions sumOfPrevPredictions
-- SOLVE
sumOfNextPredictions :: [Values] -> Int
sumOfNextPredictions = sum . map (predictValue $ \vals diff -> last vals + diff)
sumOfPrevPredictions :: [Values] -> Int
sumOfPrevPredictions = sum . map (predictValue $ \vals diff -> head vals - diff)
-- HELPERS
predictValue :: ([Int] -> Int -> Int) -> Values -> Int
predictValue applyDiff vs =
if all (== 0) vs.vals
then 0
else
let diffs =
catMaybes . snd $
L.mapAccumL
(\s v -> (Just v, (v -) <$> s))
Nothing
vs.vals
nextDiff = predictValue applyDiff $ Values diffs
in applyDiff vs.vals nextDiff
-- PARSE
newtype Values = Values
{ vals :: [Int]
}
deriving (Show)
parseValues :: ReadP Values
parseValues = Values <$> sepBy1 parseInt (char ' ')