2023-06 in haskell

1 files changed, 55 insertions, 0 deletions

diff --git a/2023/06-Wait_For_It/second.hs b/2023/06-Wait_For_It/second.hs
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+++ b/2023/06-Wait_For_It/second.hs
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+-- requires cabal install --lib megaparsec parser-combinators
+module Main (main) where
+
+import Control.Applicative.Permutations
+import Control.Monad (void, when)
+import Data.Char qualified as C
+import Data.Either
+import Data.List qualified as L
+import Data.Map qualified as M
+import Data.Maybe
+import Data.Set qualified as S
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+
+import Debug.Trace
+
+exampleExpectedOutput = 71503
+
+data Input = Input [Int] [Int] deriving Show -- time distance
+
+type Parser = Parsec Void String
+
+parseNumber :: Parser Int
+parseNumber = read <$> some (digitChar <* many (char ' '))
+
+parseInput' :: Parser Input
+parseInput' = Input <$> (string "Time:" *> many (char ' ') *> some parseNumber <* eol)
+                    <*> (string "Distance:" *> many (char ' ') *> some parseNumber <* eol <* eof)
+
+parseInput :: String -> IO Input
+parseInput filename = do
+  input <- readFile filename
+  case runParser parseInput' filename input of
+    Left bundle -> error $ errorBundlePretty bundle
+    Right input' -> return input'
+
+race :: Int -> Int -> Int
+race limit record = limit + 1 - 2 * threshold
+  where
+    threshold = minTimeToBeat 0
+    minTimeToBeat :: Int -> Int
+    minTimeToBeat t | (limit - t) * t > record = t
+                    | otherwise = minTimeToBeat (t+1)
+
+compute :: Input -> Int
+compute (Input times distances) = product $ zipWith race times distances
+
+main :: IO ()
+main = do
+  example <- parseInput "example"
+  let exampleOutput = compute example
+  when  (exampleOutput /= exampleExpectedOutput)  (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  input <- parseInput "input"
+  print $ compute input