diff --git a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/AdventOfCode2023.FSharp.Lib.fsproj b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/AdventOfCode2023.FSharp.Lib.fsproj
index df4939d..7acefda 100644
--- a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/AdventOfCode2023.FSharp.Lib.fsproj
+++ b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/AdventOfCode2023.FSharp.Lib.fsproj
@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
     <TargetFramework>net8.0</TargetFramework>
@@ -13,6 +13,7 @@
     <Compile Include="Arithmetic.fs"/>
     <Compile Include="Rational.fs"/>
     <Compile Include="IntervalSet.fs" />
+    <Compile Include="Direction.fs" />
     <Compile Include="List.fs" />
     <Compile Include="Day1.fs"/>
     <Compile Include="Day2.fs"/>
diff --git a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Arr2D.fs b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Arr2D.fs
index 514292f..656f3c9 100644
--- a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Arr2D.fs
+++ b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Arr2D.fs
@@ -5,6 +5,7 @@
 #nowarn "9"
 #endif
 
+open System
 open Microsoft.FSharp.NativeInterop
 
 [<Struct>]
@@ -100,3 +101,76 @@ module Arr2D =
 #else
         NativePtr.initBlock a.Elements 0uy (uint32 sizeof<'a> * uint32 a.Length)
 #endif
+
+    /// Pass in a buffer of memory which we will use entirely for our own purposes (and may resize)
+    /// to maintain state.
+    /// `empty` is the value in empty cells; we will fill them with `fillWith`.
+    let floodFill
+        (stackBuf : ResizeArray<int>)
+        (s : Arr2D<'a>)
+        (empty : 'a)
+        (fillWith : 'a)
+        (currX : int)
+        (currY : int)
+        : unit
+        =
+        stackBuf.Clear ()
+        stackBuf.Add currX
+        stackBuf.Add currY
+
+        while stackBuf.Count > 0 do
+            let currY = stackBuf.[stackBuf.Count - 1]
+            stackBuf.RemoveAt (stackBuf.Count - 1)
+            let currX = stackBuf.[stackBuf.Count - 1]
+            stackBuf.RemoveAt (stackBuf.Count - 1)
+
+            if currX > 0 then
+                if get s (currX - 1) currY = empty then
+                    set s (currX - 1) currY fillWith
+                    stackBuf.Add (currX - 1)
+                    stackBuf.Add currY
+
+            if currX < s.Width - 1 then
+                if get s (currX + 1) currY = empty then
+                    set s (currX + 1) currY fillWith
+                    stackBuf.Add (currX + 1)
+                    stackBuf.Add currY
+
+            if currY > 0 then
+                if get s currX (currY - 1) = empty then
+                    set s currX (currY - 1) fillWith
+                    stackBuf.Add currX
+                    stackBuf.Add (currY - 1)
+
+            if currY < s.Height - 1 then
+                if get s currX (currY + 1) = empty then
+                    set s currX (currY + 1) fillWith
+                    stackBuf.Add currX
+                    stackBuf.Add (currY + 1)
+
+    /// SIMD go brr
+    let inline count< ^a when 'a : equality and 'a : unmanaged and 'a :> IEquatable<'a>>
+        (arr : Arr2D<'a>)
+        (x : 'a)
+        : int
+        =
+        let span =
+#if DEBUG
+            arr.Elements.AsSpan ()
+#else
+            ReadOnlySpan<'a> (NativePtr.toVoidPtr arr.Elements, arr.Length)
+#endif
+        MemoryExtensions.Count (span, x)
+
+    let print (arr : Arr2D<byte>) =
+        for row = 0 to arr.Height - 1 do
+            for col = 0 to arr.Width - 1 do
+                match get arr col row with
+                | 1uy -> printf "#"
+                | 0uy -> printf "."
+                | 2uy -> printf "O"
+                | _ -> failwith "bad"
+
+            printfn ""
+
+        printfn ""
diff --git a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day10.fs b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day10.fs
index 637373e..1c1ab18 100644
--- a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day10.fs
+++ b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day10.fs
@@ -123,41 +123,6 @@ module Day10 =
 
         distance
 
-    let floodFill (stackBuf : ResizeArray<_>) (s : Arr2D<byte>) (currX : int) (currY : int) =
-        stackBuf.Clear ()
-        stackBuf.Add currX
-        stackBuf.Add currY
-
-        while stackBuf.Count > 0 do
-            let currY = stackBuf.[stackBuf.Count - 1]
-            stackBuf.RemoveAt (stackBuf.Count - 1)
-            let currX = stackBuf.[stackBuf.Count - 1]
-            stackBuf.RemoveAt (stackBuf.Count - 1)
-
-            if currX > 0 then
-                if Arr2D.get s (currX - 1) currY = 0uy then
-                    Arr2D.set s (currX - 1) currY 2uy
-                    stackBuf.Add (currX - 1)
-                    stackBuf.Add currY
-
-            if currX < s.Width - 1 then
-                if Arr2D.get s (currX + 1) currY = 0uy then
-                    Arr2D.set s (currX + 1) currY 2uy
-                    stackBuf.Add (currX + 1)
-                    stackBuf.Add currY
-
-            if currY > 0 then
-                if Arr2D.get s currX (currY - 1) = 0uy then
-                    Arr2D.set s currX (currY - 1) 2uy
-                    stackBuf.Add currX
-                    stackBuf.Add (currY - 1)
-
-            if currY < s.Height - 1 then
-                if Arr2D.get s currX (currY + 1) = 0uy then
-                    Arr2D.set s currX (currY + 1) 2uy
-                    stackBuf.Add currX
-                    stackBuf.Add (currY + 1)
-
     let print (s : Arr2D<byte>) =
         for y = 0 to s.Height - 1 do
             for x = 0 to s.Width - 1 do
@@ -310,12 +275,12 @@ module Day10 =
         let stackBuf = ResizeArray ()
 
         for line = 0 to system.Height - 1 do
-            floodFill stackBuf system 0 line
-            floodFill stackBuf system (system.Width - 1) line
+            Arr2D.floodFill stackBuf system 0uy 2uy 0 line
+            Arr2D.floodFill stackBuf system 0uy 2uy (system.Width - 1) line
 
         for col = 0 to system.Width - 1 do
-            floodFill stackBuf system col 0
-            floodFill stackBuf system col (system.Height - 1)
+            Arr2D.floodFill stackBuf system 0uy 2uy col 0
+            Arr2D.floodFill stackBuf system 0uy 2uy col (system.Height - 1)
 
         let mutable answer = 0
 
diff --git a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day14.fs b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day14.fs
index 3631a35..cf1426d 100644
--- a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day14.fs
+++ b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day14.fs
@@ -137,19 +137,6 @@ module Day14 =
                 else // current = 0uy
                     pos <- pos + 1
 
-    let print (board : Arr2D<byte>) =
-        for row = 0 to board.Height - 1 do
-            for col = 0 to board.Width - 1 do
-                match Arr2D.get board col row with
-                | 0uy -> printf "."
-                | 1uy -> printf "O"
-                | 2uy -> printf "#"
-                | _ -> failwith "bad value"
-
-            printfn ""
-
-        printfn ""
-
     let score (board : Arr2D<byte>) =
         let mutable answer = 0ul
 
diff --git a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day16.fs b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day16.fs
index a44c8c3..24df0d9 100644
--- a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day16.fs
+++ b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day16.fs
@@ -10,22 +10,8 @@ open System
 [<RequireQualifiedAccess>]
 module Day16 =
 
-    type Direction =
-        | Left = 0
-        | Right = 1
-        | Up = 2
-        | Down = 3
-
-    let inline dirToInt (d : Direction) =
-        match d with
-        | Direction.Left -> 0us
-        | Direction.Right -> 1us
-        | Direction.Up -> 2us
-        | Direction.Down -> 3us
-        | _ -> failwith "Bad"
-
     let inline storeDirectionAndPos (numCols : int) (col : int) (row : int) (direction : Direction) : uint16 =
-        4us * uint16 (col + numCols * row) + dirToInt direction
+        4us * uint16 (col + numCols * row) + Direction.toUInt direction
 
     let inline getDirection (input : uint16) =
         match input % 4us with
diff --git a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Direction.fs b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Direction.fs
new file mode 100644
index 0000000..52ae655
--- /dev/null
+++ b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Direction.fs
@@ -0,0 +1,24 @@
+namespace AdventOfCode2023
+
+type Direction =
+    | Left = 0
+    | Right = 1
+    | Up = 2
+    | Down = 3
+
+module Direction =
+    let inline toUInt (d : Direction) =
+        match d with
+        | Direction.Left -> 0us
+        | Direction.Right -> 1us
+        | Direction.Up -> 2us
+        | Direction.Down -> 3us
+        | _ -> failwith "Bad"
+
+    let inline ofChar (c : char) : Direction =
+        match c with
+        | 'L' -> Direction.Left
+        | 'R' -> Direction.Right
+        | 'U' -> Direction.Up
+        | 'D' -> Direction.Down
+        | c -> failwith $"Bad: %c{c}"