advent07/Main.hs

   1 -- Writeup at https://work.njae.me.uk/2021/12/07/advent-of-code-2021-day-7/
   2
   3 import Data.List.Split
   4
   5 main :: IO ()
   6 main =
   7   do  text <- readFile "data/advent07.txt"
   8       let subs = map (read @Int) $ splitOn "," text
   9       print $ part1 subs
  10       print $ part2 subs
  11
  12 part1 = bestFuel loss1
  13 part2 = bestFuel loss2
  14
  15 bestFuel :: ([Int] -> Int -> Int) -> [Int] -> Int
  16 bestFuel loss subs = loss subs best
  17   where meanSub = (sum subs) `div` (length subs)
  18         best = gradientDescend loss subs meanSub
  19
  20 loss1 :: [Int] -> Int -> Int
  21 loss1 subs target = sum $ map diff subs
  22   where diff s = abs (target - s)
  23
  24 loss2 :: [Int] -> Int -> Int
  25 loss2 subs target = sum $ map triangleDiff subs
  26   where diff s = abs (target - s)
  27         triangleDiff s = ((diff s) * ((diff s) + 1)) `div` 2
  28
  29 gradientDescend :: ([Int] -> Int -> Int) -> [Int] -> Int -> Int
  30 gradientDescend loss subs guess =
  31   if | lossLower  < lossHere -> gradientDescend loss subs (guess - 1)
  32      | lossHigher < lossHere -> gradientDescend loss subs (guess + 1)
  33      | otherwise -> guess
  34   where lossHere   = loss subs guess
  35         lossLower  = loss subs (guess - 1)
  36         lossHigher = loss subs (guess + 1)