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advent-of-code/2020/10/part1.hs

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Haskell
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#!/usr/bin/env runghc
import Day10
import Data.List
main = interact (show . solve . parse)
solve xs = ones * threes
where
sorted = 0:(sort xs)
differences = zipWith (flip (-)) sorted (tail sorted)
ones = count 1 differences
threes = count 3 differences + 1
count x = length . filter (==x)