diff --git a/common/src/Advent/Search.hs b/common/src/Advent/Search.hs
index 94eb2fe..0792bb0 100644
--- a/common/src/Advent/Search.hs
+++ b/common/src/Advent/Search.hs
@@ -19,7 +19,10 @@ module Advent.Search (
 
   -- * A* search
   AStep(..),
-  astar, astarN, astarOn, astarOnN
+  astar, astarN, astarOn, astarOnN,
+
+  -- * Reachable exploration
+  fill, fillN, fillInt, fillNInt,
 
   ) where
 
@@ -198,3 +201,37 @@ data AStep a = AStep {
   astepCost      :: !Int, -- ^ cost of edge
   astepHeuristic :: !Int  -- ^ heuristic cost to goal from this new node
   } deriving Show
+
+-- | Generate a set of all the values reachable from a starting
+-- state and a step function.
+fill :: Ord a => (a -> [a]) -> a -> Set.Set a
+fill step x = fillN step [x]
+{-# INLINE fill #-}
+
+-- | Generate a set of all the values reachable from a list
+-- of starting states and a step function.
+fillN :: Ord a => (a -> [a]) -> [a] -> Set.Set a
+fillN step = foldl' go Set.empty
+  where
+    go seen x
+      | x `Set.member` seen = seen
+      | otherwise = foldl' go (Set.insert x seen) (step x)
+{-# INLINE fillN #-}
+
+-- | Generate a set of all the values reachable from a starting
+-- state and a step function. Specialized version of 'fill' when
+-- working with 'Int'.
+fillInt :: (Int -> [Int]) -> Int -> IntSet.IntSet
+fillInt step x = fillNInt step [x]
+{-# INLINE fillInt #-}
+
+-- | Generate a set of all the values reachable from a list
+-- of starting states and a step function. Specialized version
+-- of 'fillN' when working with 'Int'.
+fillNInt :: (Int -> [Int]) -> [Int] -> IntSet.IntSet
+fillNInt step = foldl' go IntSet.empty
+  where
+    go seen x
+      | x `IntSet.member` seen = seen
+      | otherwise = foldl' go (IntSet.insert x seen) (step x)
+{-# INLINE fillNInt #-}
diff --git a/solutions/src/2018/06.hs b/solutions/src/2018/06.hs
index 851b7ef..a6eb89e 100644
--- a/solutions/src/2018/06.hs
+++ b/solutions/src/2018/06.hs
@@ -15,7 +15,7 @@ module Main (main) where
 import Advent        (format, counts)
 import Advent.Coord (Coord(C), cardinal, coordCol, coordRow,
                      above, below, right, left, manhattan, boundingBox)
-import Advent.Search (dfs)
+import Advent.Search (fill)
 import Data.List (groupBy, sort, sortBy)
 import Data.Function (on)
 import Data.Ix (range)
@@ -78,7 +78,7 @@ part1 input
 -- is in bounds. Once we're unable to grow the region any further we return its
 -- size.
 part2 :: [Coord] -> Int
-part2 input = length (dfs step startingPoint)
+part2 input = length (fill step startingPoint)
   where
     distances :: Coord -> Int
     distances c = sum (map (manhattan c) input)
diff --git a/solutions/src/2022/17.hs b/solutions/src/2022/17.hs
index 5c58f4c..142f885 100644
--- a/solutions/src/2022/17.hs
+++ b/solutions/src/2022/17.hs
@@ -26,7 +26,7 @@ import Data.Set qualified as Set
 
 import Advent (format)
 import Advent.Coord (Coord(C), coordRow, east, west, cardinal, coordCol, south)
-import Advent.Search (bfsN)
+import Advent.Search (fillN)
 
 -- | The set of five blocks
 --
@@ -123,8 +123,8 @@ clean stuff = Set.filter alive stuff
   where
     ymin = coordRow (minimum stuff)
     step c = [n | n <- cardinal c, 0 <= coordCol n, coordCol n <= 6, coordRow c >= ymin, Set.notMember n stuff]
-    air = bfsN step [C ymin x | x <- [0..6], Set.notMember (C ymin x) stuff]
-    alive x = any (`elem` air) (cardinal x) || coordRow x == ymin
+    air = fillN step [C ymin x | x <- [0..6], Set.notMember (C ymin x) stuff]
+    alive x = any (`Set.member` air) (cardinal x) || coordRow x == ymin
 
 -- | Piece the next piece on the top of the tower returning the updated
 -- piece index, jet index, and tower contents. The tower is pruned to
diff --git a/solutions/src/2022/18.hs b/solutions/src/2022/18.hs
index 2bf8712..9489cf4 100644
--- a/solutions/src/2022/18.hs
+++ b/solutions/src/2022/18.hs
@@ -36,7 +36,7 @@ import Data.Set (Set)
 import Data.Set qualified as Set
 
 import Advent (format)
-import Advent.Search (bfs)
+import Advent.Search (fill)
 import Advent.Coord3 (Coord3(..), boundingBox)
 
 -- |
@@ -53,7 +53,7 @@ main =
 
 -- | Given the the location of the lava cubes, find a bounding box of air surrounding them.
 findAir :: Set Coord3 -> Set Coord3
-findAir cubes = Set.fromList (bfs step (hi + 1))
+findAir cubes = fill step (hi + 1)
   where
     (lo, hi) = fromJust (boundingBox cubes)
     box      = (lo - 1, hi + 1)
diff --git a/solutions/src/2024/12.hs b/solutions/src/2024/12.hs
index 6cb7ec9..3325030 100644
--- a/solutions/src/2024/12.hs
+++ b/solutions/src/2024/12.hs
@@ -50,7 +50,7 @@ module Main (main) where
 
 import Advent (getInputMap, countBy)
 import Advent.Coord (Coord, cardinal, above, right, below, left)
-import Advent.Search (dfs)
+import Advent.Search (fill)
 import Data.List (unfoldr)
 import Data.Map (Map)
 import Data.Map qualified as Map
@@ -72,7 +72,7 @@ regions :: Map Coord Char -> [Set Coord]
 regions = unfoldr \input ->
   [ (region, Map.withoutKeys input region)
   | (start, label) <- Map.lookupMin input
-  , let region = Set.fromList (dfs step start)
+  , let region = fill step start
         step i = [j | j <- cardinal i, Map.lookup j input == Just label]
   ]