refactor(2023.06-haskell): use the quadratic formula

yurrriq · Dec 22, 2023 · 65e7ae9 · 65e7ae9
1 parent 4a27606
commit 65e7ae9
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Showing 2 changed files with 8 additions and 4 deletions.
diff --git a/VERSION b/VERSION
@@ -1 +1 @@
-2023.6.2.2
+2023.6.2.3
diff --git a/src/AdventOfCode/Year2023/Day06.hs b/src/AdventOfCode/Year2023/Day06.hs
@@ -2,7 +2,6 @@ module AdventOfCode.Year2023.Day06 where
 
 import AdventOfCode.Input (parseInput, parseString)
 import AdventOfCode.TH (defaultMain, inputFilePath)
-import AdventOfCode.Util (count)
 import Data.List.Extra (productOn')
 import Text.Trifecta (Parser, natural, some, symbol)
 
@@ -19,8 +18,13 @@ partTwo races = waysToWin (time, distance)
     distance = read $ concatMap (show . snd) races
 
 waysToWin :: (Int, Int) -> Int
-waysToWin (time, distance) =
-  count (\speed -> (time - speed) * speed > distance) [1 .. time - 1]
+waysToWin (time, distance) = maxTime - minTime + 1
+  where
+    maxTime = floor ((time' + delta) / 2)
+    minTime = ceiling ((time' - delta) / 2)
+    delta, time' :: Double
+    delta = sqrt $ time' ** 2 - 4 * fromIntegral distance
+    time' = fromIntegral time
 
 getInput :: IO [(Int, Int)]
 getInput = parseInput document $(inputFilePath)