415055dcc2
For now this applies to Haskell only, and it may turn out to be tricky for the Rust implementation. In practice, the limitation hasn't turned out to be important, and we could even go the other way and use `Integer` everywhere. This does however at least help with debugging, as well as just being conceptually right. The `nil` and `(/\)` functions are intended to be overloaded to work for other list-like things in a later commit, and from there we will investigate using `OverloadedLists` and `RebindableSyntax` to recover standard list syntax, although there are probably limitations due to `(:)` being special.
46 lines
1.8 KiB
Haskell
46 lines
1.8 KiB
Haskell
module Puzzles.Day7 (puzzle) where
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import Pre
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import Data.IntMap qualified as IM
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import Data.IntSet qualified as IS
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puzzle :: Puzzle
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puzzle =
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Puzzle
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{ number = 7
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, parser = const do
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line1 <- some $ (single '.' $> False) <|> (single 'S' $> True)
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start <- maybe (fail "start not found") (pure . fst) $ find snd $ zip [0 ..] line1
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void newline
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rows <- (some $ (single '.' $> False) <|> (single '^' $> True)) `sepEndBy1` newline
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let splitters = map (IS.fromList . map fst . filter snd . zip [0 ..]) rows
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pure (start, splitters)
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, parts =
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( uncurry \start ->
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flip execState (0 :: Int)
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. foldlM
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( \beams splitters ->
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IS.fromList . concat <$> for (IS.toList beams) \x -> do
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let hit = x `IS.member` splitters
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when hit $ modify succ
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pure if hit then [x - 1, x + 1] else [x]
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)
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(IS.singleton start)
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)
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/\ ( uncurry \start ->
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sum
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. map snd
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. IM.toList
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. foldl
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( \beams splitters ->
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IM.fromListWith (+) . concat $ flip map (IM.toList beams) \(x, n) -> do
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let hit = x `IS.member` splitters
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zip (if hit then [x - 1, x + 1] else [x]) (repeat n)
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)
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(IM.singleton start (1 :: Int))
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)
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/\ nil
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, extraTests = mempty
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}
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