From cffd7e04a6171bfa633d6769416d0f2b268fa9f3 Mon Sep 17 00:00:00 2001
From: Daniel Lin <ephemient@gmail.com>
Date: Sat, 7 Dec 2024 01:34:30 -0500
Subject: [PATCH] It's faster in reverse

---
 hs/src/Day7.hs | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/hs/src/Day7.hs b/hs/src/Day7.hs
index 7a8aebec..d4845adf 100644
--- a/hs/src/Day7.hs
+++ b/hs/src/Day7.hs
@@ -8,7 +8,8 @@ module Day7 (part1, part2) where
 import Common (readEntire, readSome)
 import Control.Monad (ap)
 import Control.Parallel.Strategies (parMap, rseq)
-import Data.List.NonEmpty (NonEmpty ((:|)))
+import Data.List.NonEmpty (NonEmpty ((:|)), nonEmpty)
+import Data.List.NonEmpty qualified as NE (reverse)
 import Data.Text (Text)
 import Data.Text qualified as T (lines, stripPrefix)
 import Data.Text.Read (Reader)
@@ -18,7 +19,7 @@ parseLine :: (Integral a) => Reader (a, NonEmpty a)
 parseLine line = do
   (lhs, line') <- T.decimal line
   (rhs, line'') <- maybe (Left "") (readSome T.decimal) $ T.stripPrefix ": " line'
-  pure ((lhs, rhs), line'')
+  pure ((lhs, NE.reverse rhs), line'')
 
 parFilter :: (a -> Bool) -> [a] -> [a]
 parFilter f = fmap fst . filter snd . (zip `ap` parMap rseq f)
@@ -26,11 +27,22 @@ parFilter f = fmap fst . filter snd . (zip `ap` parMap rseq f)
 solve :: (Integral a) => (a -> a -> [a]) -> Text -> Either String a
 solve op input = sum . map fst . parFilter f <$> mapM (readEntire parseLine) (T.lines input)
   where
-    f (lhs, r0 :| []) = lhs == r0
-    f (lhs, r0 :| r1 : rhs) = or [r2 <= lhs && f (lhs, r2 :| rhs) | r2 <- r0 `op` r1]
+    f (x, y :| rest)
+      | Just rest' <- nonEmpty rest = or [f (z, rest') | z <- x `op` y]
+      | otherwise = x == y
+
+op1, op2 :: (Integral a) => a -> a -> [a]
+op1 x y = [x - y | x >= y] <> case x `divMod` y of (z, 0) -> [z]; _ -> []
+op2 x y =
+  op1 x y
+    <> [ z
+       | d <- take 1 $ dropWhile (<= y) $ iterate (* 10) 10,
+         let (z, r) = x `divMod` d,
+         z /= 0 && r == y
+       ]
 
 part1 :: Text -> Either String Int
-part1 = solve $ \a b -> [a + b, a * b]
+part1 = solve op1
 
 part2 :: Text -> Either String Int
-part2 = solve $ \a b -> [a + b, a * b, read $ show a ++ show b]
+part2 = solve op2