diff --git a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/AdventOfCode2023.FSharp.Lib.fsproj b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/AdventOfCode2023.FSharp.Lib.fsproj
index 29333a6..d989f2b 100644
--- a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/AdventOfCode2023.FSharp.Lib.fsproj
+++ b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/AdventOfCode2023.FSharp.Lib.fsproj
@@ -12,6 +12,7 @@
         <Compile Include="Day2.fs" />
         <Compile Include="Day3.fs" />
         <Compile Include="Day4.fs" />
+        <Compile Include="Day5.fs" />
     </ItemGroup>
 
 </Project>
diff --git a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day4.fs b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day4.fs
index 50c2b3d..1f10a48 100644
--- a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day4.fs
+++ b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day4.fs
@@ -1,15 +1,21 @@
 namespace AdventOfCode2023
 
 open System
-open System.Collections.Generic
 
 [<RequireQualifiedAccess>]
 module Day4 =
 
+    let inline parseByte (chars : ReadOnlySpan<char>) : byte =
+        // Byte.Parse (chars, NumberStyles.None, NumberFormatInfo.InvariantInfo)
+        let mutable answer = 0uy
+        for c in chars do
+            answer <- answer * 10uy + (byte c - 48uy)
+        answer
+
     let part1 (s : string) =
         use lines = StringSplitEnumerator.make '\n' s
         let mutable total = 0
-        let winningNumbers = HashSet ()
+        let winningNumbers = ResizeArray<byte> ()
 
         for line in lines do
             if not (line.IsWhiteSpace ()) then
@@ -30,14 +36,14 @@ module Day4 =
                     if split.Current.[0] = '|' then
                         accumulatingWinning <- false
                     else
-                        winningNumbers.Add (Int32.Parse split.Current) |> ignore
+                        winningNumbers.Add (parseByte split.Current)
                         split.MoveNext () |> ignore
 
                 let mutable answer = 0
 
                 while split.MoveNext () do
                     if not split.Current.IsEmpty then
-                        let n = Int32.Parse split.Current
+                        let n = parseByte split.Current
 
                         if winningNumbers.Contains n then
                             answer <- answer + 1
@@ -51,9 +57,8 @@ module Day4 =
 
     let part2 (s : string) =
         use lines = StringSplitEnumerator.make '\n' s
-        let mutable total = 0
-        let winningNumbers = HashSet ()
-        let winners = ResizeArray ()
+        let winningNumbers = ResizeArray<byte> ()
+        let winners = ResizeArray<int> ()
 
         for line in lines do
             if not (line.IsWhiteSpace ()) then
@@ -74,14 +79,14 @@ module Day4 =
                     if split.Current.[0] = '|' then
                         accumulatingWinning <- false
                     else
-                        winningNumbers.Add (Int32.Parse split.Current) |> ignore
+                        winningNumbers.Add (parseByte split.Current)
                         split.MoveNext () |> ignore
 
                 let mutable answer = 0
 
                 while split.MoveNext () do
                     if not split.Current.IsEmpty then
-                        let n = Int32.Parse split.Current
+                        let n = parseByte split.Current
 
                         if winningNumbers.Contains n then
                             answer <- answer + 1
diff --git a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day5.fs b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day5.fs
new file mode 100644
index 0000000..7afa4cd
--- /dev/null
+++ b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day5.fs
@@ -0,0 +1,16 @@
+namespace AdventOfCode2023
+
+open System
+open System.Collections.Generic
+
+[<RequireQualifiedAccess>]
+module Day5 =
+
+    let part1 (s : string) =
+        use lines = StringSplitEnumerator.make '\n' s
+        0
+
+    let part2 (s : string) =
+        use lines = StringSplitEnumerator.make '\n' s
+
+        0
diff --git a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp/Program.fs b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp/Program.fs
index 0f1bfbb..000ca44 100644
--- a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp/Program.fs
+++ b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp/Program.fs
@@ -98,6 +98,19 @@ module Program =
             Console.WriteLine (part2.ToString ())
             Console.Error.WriteLine ((1_000.0 * float sw.ElapsedTicks / float Stopwatch.Frequency).ToString () + "ms")
 
+        do
+            let input = Path.Combine (dir.FullName, "day5.txt") |> File.ReadAllText
+            sw.Restart ()
+            let part1 = Day5.part1 input
+            sw.Stop ()
+            Console.WriteLine (part1.ToString ())
+            Console.Error.WriteLine ((1_000.0 * float sw.ElapsedTicks / float Stopwatch.Frequency).ToString () + "ms")
+            sw.Restart ()
+            let part2 = Day5.part2 input
+            sw.Stop ()
+            Console.WriteLine (part2.ToString ())
+            Console.Error.WriteLine ((1_000.0 * float sw.ElapsedTicks / float Stopwatch.Frequency).ToString () + "ms")
+
         endToEnd.Stop ()
 
         Console.Error.WriteLine (
diff --git a/AdventOfCode2023.FSharp/Test/Test.fsproj b/AdventOfCode2023.FSharp/Test/Test.fsproj
index bb7f579..aa21125 100644
--- a/AdventOfCode2023.FSharp/Test/Test.fsproj
+++ b/AdventOfCode2023.FSharp/Test/Test.fsproj
@@ -13,11 +13,13 @@
         <Compile Include="TestDay2.fs" />
         <Compile Include="TestDay3.fs" />
         <Compile Include="TestDay4.fs" />
+        <Compile Include="TestDay5.fs" />
         <EmbeddedResource Include="samples\day1.txt" />
         <EmbeddedResource Include="samples\day1part1.txt" />
         <EmbeddedResource Include="samples\day2.txt" />
         <EmbeddedResource Include="samples\day3.txt" />
         <EmbeddedResource Include="samples\day4.txt" />
+        <EmbeddedResource Include="samples\day5.txt" />
     </ItemGroup>
 
     <ItemGroup>
diff --git a/AdventOfCode2023.FSharp/Test/TestDay5.fs b/AdventOfCode2023.FSharp/Test/TestDay5.fs
new file mode 100644
index 0000000..dc38b6b
--- /dev/null
+++ b/AdventOfCode2023.FSharp/Test/TestDay5.fs
@@ -0,0 +1,43 @@
+namespace AdventOfCode2023.Test
+
+open AdventOfCode2023
+open NUnit.Framework
+open FsUnitTyped
+open System.IO
+
+[<TestFixture>]
+module TestDay5 =
+
+    let sample = Assembly.getEmbeddedResource typeof<Dummy>.Assembly "day5.txt"
+
+    [<Test>]
+    let part1Sample () = sample |> Day5.part1 |> shouldEqual 0
+
+    [<Test>]
+    let part2Sample () = sample |> Day5.part2 |> shouldEqual 0
+
+    [<Test>]
+    let part1Actual () =
+        let s =
+            try
+                File.ReadAllText (Path.Combine (__SOURCE_DIRECTORY__, "../../inputs/day5.txt"))
+            with
+            | :? DirectoryNotFoundException
+            | :? FileNotFoundException ->
+                Assert.Inconclusive ()
+                failwith "unreachable"
+
+        Day5.part1 s |> shouldEqual 0
+
+    [<Test>]
+    let part2Actual () =
+        let s =
+            try
+                File.ReadAllText (Path.Combine (__SOURCE_DIRECTORY__, "../../inputs/day5.txt"))
+            with
+            | :? DirectoryNotFoundException
+            | :? FileNotFoundException ->
+                Assert.Inconclusive ()
+                failwith "unreachable"
+
+        Day5.part2 s |> shouldEqual 0
diff --git a/AdventOfCode2023.FSharp/Test/samples/day5.txt b/AdventOfCode2023.FSharp/Test/samples/day5.txt
new file mode 100644
index 0000000..e69de29