48 lines
1.4 KiB
Haskell
48 lines
1.4 KiB
Haskell
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import Data.List
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main :: IO ()
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main = do
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input <- readFile "6/input.txt"
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let x = 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 80
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g = solve 256
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parse :: String -> [Int]
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parse = map read . words . map f
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where f ',' = ' '
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f x = x
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-- Solve the problem by creating an association list mapping a
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-- value for the internal timer to a number of fishies as the seed,
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-- and then simulating `n` iterations of growth.
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solve :: Int -- ^ n: The number of generations to simulate
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-> [Int] -- ^ The initial population
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-> Int -- ^ The total population at the end of n generations
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solve n = sum . map snd . simulate n . map assoc . group . sort
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where assoc x = (head x, length x)
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-- Simulate `n` iterations of growth and return the state
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-- after that many iterations.
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simulate :: Int -> [(Int, Int)] -> [(Int, Int)]
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simulate 0 st = st
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simulate n st = simulate (n - 1) (update st)
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-- Update the simulation state by one iteration.
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update :: [(Int, Int)] -> [(Int, Int)]
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update = merge . (>>= f)
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where f (0, n) = [(6, n), (8, n)]
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f (t, n) = [(t - 1, n)]
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-- Merge the values of duplicate key entries and produce a new list
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-- where each key is mapped to the sum of all values associated with
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-- that key.
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merge :: [(Int, Int)] -> [(Int, Int)]
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merge [] = []
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merge ((t,n) : ls) = (t, n + sum (map snd x)) : merge r
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where (x,r) = partition ((== t) . fst) ls
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