Steve Dunham
d6156ebc79
- Holes are no longer allowed when building executables - Stack overflow in mapMaybe (Day15)
111 lines
3.5 KiB
Agda
111 lines
3.5 KiB
Agda
module Day20
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import Prelude
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import Node
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import Aoc
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import SortedMap
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gridPoints : String → List (Char × Int × Int)
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gridPoints text = go 0 0 (unpack text) Nil
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where
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-- might as well be tail recursive
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go : Int → Int → List Char → List (Char × Int × Int) → List (Char × Int × Int)
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go row col Nil points = points
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go row col ('\n' :: cs) points = go (row + 1) 0 cs points
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go row col (c :: cs) points = go row (col + 1) cs ((c,row,col) :: points)
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State Grid Next Cheat : U
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Grid = SortedMap Point Char
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State = SortedMap Point Int
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Next = (Point × Int)
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Cheat = (Point × Point)
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data Dir : U where North East South West : Dir
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dirs : List Dir
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dirs = (North :: South :: East :: West :: Nil)
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move : Point → Dir → Point
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move (r, c) North = (r - 1, c)
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move (r, c) East = (r, c + 1)
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move (r, c) South = (r + 1, c)
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move (r, c) West = (r, c - 1)
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distances : SortedMap Point Char → Point → State
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distances grid start = go EmptyMap ((start,0) :: Nil) Lin
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where
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valid : State → Point → Bool
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valid st pt = case lookupMap pt grid of
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Just (_, '.') => case lookupMap pt st of
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Just _ => False
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Nothing => True
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_ => False
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go : State → List (Point × Int) → SnocList (Point × Int) → State
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go st Nil Lin = st
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go st xs Lin = go st Nil (Lin <>< xs)
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go st xs (ys :< (pt, l)) =
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let next = map (flip _,_ $ l + 1) $ filter (valid st) $ map (move pt) dirs
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st = foldMap const st next
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in go st (next ++ xs) ys
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manh : Point → Point → Int
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manh (r,c) (r',c') =
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let dr = if r < r' then r' - r else r - r'
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dc = if c < c' then c' - c else c - c'
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in dr + dc
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candidates : Point → Int → List Point
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candidates pt@(sr,sc) dist = go (sr - dist) (sc - dist) Nil
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where
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go : Int → Int → List Point → List Point
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go r' c' acc =
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if sr + dist < r' then acc
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else if sc + dist < c' then go (r' + 1) (sc - dist) acc
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else if manh pt (r',c') <= dist
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then go r' (c' + 1) ((r', c') :: acc)
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else go r' (c' + 1) acc
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calcCheats : State → State → Int → Int → Int
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calcCheats start dists fuel threshold = go (toList start) 0
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where
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-- only '.' have dist
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tryCand : Int → Point → Point → Int
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tryCand l pt cand = case lookupMap cand dists of
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Just (_, dist) => if l + manh pt cand + dist <= threshold then 1 else 0
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Nothing => 0
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go : List (Point × Int) → Int → Int
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go Nil acc = acc
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go (x@(pt,l) :: xs) acc =
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let acc = foldl _+_ acc $ map (tryCand l pt) $ candidates pt fuel
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in go xs acc
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run : String → Int → IO Unit
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run fn time = do
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putStrLn fn
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text <- readFile fn
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let pts = map swap $ gridPoints text
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let grid = foldMap const EmptyMap pts
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let (Just (start,_)) = find (_==_ 'S' ∘ snd) pts | _ => putStrLn "no start"
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let (Just (end,_)) = find (_==_ 'E' ∘ snd) pts | _ => putStrLn "no end"
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let grid' = updateMap start '.' $ updateMap end '.' grid
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let dists = distances grid' end
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-- this bit threw me off for a while
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let dists = updateMap end 0 dists
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let dists' = distances grid' start
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let dists' = updateMap start 0 dists'
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let (Just (_,base)) = lookupMap start dists | _ => putStrLn "no start in dists"
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let p1 = calcCheats dists dists' 2 (base - time)
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putStrLn $ "base " ++ show base
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putStrLn $ "part1 " ++ show p1
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let p2 = calcCheats dists dists' 20 (base - time)
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putStrLn $ "part2 " ++ show p2
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pure MkUnit
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main : IO Unit
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main = do
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run "aoc2024/day20/eg.txt" 50
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run "aoc2024/day20/input.txt" 100
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