23 lines
616 B
Haskell
23 lines
616 B
Haskell
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#!/usr/bin/env runghc
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import qualified Data.Set as S
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import Data.List (subsequences)
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import Data.Either (fromLeft, lefts)
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import Day9
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main = interact (show . solve . parse)
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solve = head . lefts . verify 25
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verify :: Int -> [Int] -> [Either Int Int]
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verify len xs
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| length xs < len + 1 = []
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| otherwise = verified:(verify len $ tail xs)
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where
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(preamble, [x]) = splitAt len $ take (len + 1) xs
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pairs = combinations 2 preamble
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sums = S.fromList $ map sum pairs
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verified = if S.member x sums then (Right x) else (Left x)
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combinations n xs = filter ((n==) . length) $ subsequences xs
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