47 lines
1.3 KiB
Haskell
47 lines
1.3 KiB
Haskell
import Data.List
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import Data.List.Split
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main :: IO ()
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main = do
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input <- readFile "7/input.txt"
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let x = sort $ parse input
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-- Problem 1
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print $ f x
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-- Problem 2
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print $ g x
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where
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f = solve distance
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g = solve triangle
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-- | Split on comma and interpret each element as an integer
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parse :: String -> [Int]
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parse = map read . wordsBy (== ',')
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-- | Solve the problem by finding the minimum of a list of integers, where
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-- | each element is the total amount of fuel needed for each crab to reach
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-- | that position.
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solve :: (Int -> Int -> Int) -- ^ Fuel calculation to get between two points
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-> [Int] -- ^ List of initial positions
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-> Int
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solve f x = minimum $ map g (narrow x)
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where
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g t = sum $ map (f t) x
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-- | Get a slice of numbers around the mean in order to narrow the
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-- | range of possible best positions to search.
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narrow :: [Int] -> [Int]
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narrow x = [ m - n .. m + n ]
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where m = sum x `div` length x
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n = 5
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-- | Calculate the fuel usage based on the triangle number of the distance
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-- | between the two numbers.
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triangle :: Int -> Int -> Int
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triangle a b = tri $ distance a b
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where
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tri n = n * (n + 1) `div` 2 -- Calculate nth triangle number
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-- | Calculate the absolute distance between two numbers
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distance :: Int -> Int -> Int
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distance a b = abs (a - b)
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