diff --git a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/AdventOfCode2023.FSharp.Lib.fsproj b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/AdventOfCode2023.FSharp.Lib.fsproj
index 4ae55e8..10eaced 100644
--- a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/AdventOfCode2023.FSharp.Lib.fsproj
+++ b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/AdventOfCode2023.FSharp.Lib.fsproj
@@ -20,6 +20,7 @@
     <Compile Include="Day7.fs"/>
     <Compile Include="Day8.fs"/>
     <Compile Include="Day9.fs"/>
+    <Compile Include="Day10.fs" />
   </ItemGroup>
 
 </Project>
diff --git a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day10.fs b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day10.fs
new file mode 100644
index 0000000..ed199ec
--- /dev/null
+++ b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day10.fs
@@ -0,0 +1,133 @@
+namespace AdventOfCode2023
+
+#if DEBUG
+#else
+#nowarn "9"
+#endif
+
+open System
+
+[<RequireQualifiedAccess>]
+module Day10 =
+
+    type Pipe =
+        | Empty = '.'
+        | NorthSouth = '|'
+        | EastWest = '-'
+        | NorthEast = 'L'
+        | NorthWest = '7'
+        | SouthEast = 'F'
+        | SouthWest = '7'
+        | Animal = 'S'
+
+    /// Returns first the line number, then the position within that line.
+    /// lineLength includes the newline, as does pos.
+    let inline toRowAndCol (lineLength : int) (lineCount : int) (pos : int) : struct (int * int) =
+        let lineNum = pos / lineLength
+        let withinLine = pos % lineLength
+        struct (lineNum, withinLine)
+
+    let inline ofRowAndCol (lineLength : int) (lineCount : int) (lineNum : int) (col : int) : int =
+        lineNum * lineLength + col
+
+    let inline nextPoint (s : ReadOnlySpan<char>) (lineLength : int) (lineCount : int) (currPos : int) (prevPos : int) =
+        let struct (currLineNum, currCol) = toRowAndCol lineLength lineCount currPos
+        let struct (prevLineNum, prevCol) = toRowAndCol lineLength lineCount prevPos
+        match s.[currPos] with
+        | '|' ->
+            if prevLineNum < currLineNum then
+                ofRowAndCol lineLength lineCount (currLineNum + 1) currCol
+            else
+                ofRowAndCol lineLength lineCount (currLineNum - 1) currCol
+        | '-' ->
+            if prevCol < currCol then
+                ofRowAndCol lineLength lineCount currLineNum (currCol + 1)
+            else
+                ofRowAndCol lineLength lineCount currLineNum (currCol - 1)
+        | 'L' ->
+            if prevLineNum = currLineNum then
+                ofRowAndCol lineLength lineCount (currLineNum - 1) currCol
+            else
+                ofRowAndCol lineLength lineCount currLineNum (currCol + 1)
+        | '7' ->
+            if prevLineNum = currLineNum then
+                ofRowAndCol lineLength lineCount (currLineNum + 1) currCol
+            else
+                ofRowAndCol lineLength lineCount currLineNum (currCol - 1)
+        | 'F' ->
+            if prevLineNum = currLineNum then
+                ofRowAndCol lineLength lineCount (currLineNum + 1) currCol
+            else
+                ofRowAndCol lineLength lineCount currLineNum (currCol + 1)
+        | 'J' ->
+            if prevLineNum = currLineNum then
+                ofRowAndCol lineLength lineCount (currLineNum - 1) currCol
+            else
+                ofRowAndCol lineLength lineCount currLineNum (currCol - 1)
+        | c -> failwithf "unrecognised: %c" c
+
+    let part1 (s : string) =
+        let s = s.AsSpan()
+        let lineCount = s.Count '\n'
+        let lineLength = (s.IndexOf '\n' + 1)
+        let startPos = s.IndexOf 'S'
+
+        let struct (startLine, startCol) = toRowAndCol lineLength lineCount startPos
+        let mutable distance = 1
+        let mutable prevPointA = startPos
+        let mutable prevPointB = startPos
+
+        let mutable pointA =
+            let pos = ofRowAndCol lineLength lineCount startLine (startCol - 1)
+            match s.[pos] with
+            | '-'
+            | 'L'
+            | 'F' -> pos
+            | _ ->
+
+            let pos = ofRowAndCol lineLength lineCount startLine (startCol + 1)
+            match s.[pos] with
+            | '-'
+            | 'J'
+            | '7' -> pos
+            | _ ->
+
+            ofRowAndCol lineLength lineCount (startLine + 1) startCol
+
+        let mutable pointB =
+            let pos = ofRowAndCol lineLength lineCount (startLine - 1) startCol
+            match s.[pos] with
+            | '|'
+            | '7'
+            | 'F' -> pos
+            | _ ->
+
+            let pos = ofRowAndCol lineLength lineCount (startLine + 1) startCol
+            match s.[pos] with
+            | '|'
+            | 'L'
+            | 'J' -> pos
+            | _ ->
+
+            let pos = ofRowAndCol lineLength lineCount startLine (startCol + 1)
+            match s.[pos] with
+            | '-'
+            | 'J'
+            | '7' -> pos
+            | _ -> ofRowAndCol lineLength lineCount startLine (startCol - 1)
+
+        while pointA <> pointB do
+            let currentA = pointA
+            pointA <- nextPoint s lineLength lineCount pointA prevPointA
+            prevPointA <- currentA
+
+            let currentB = pointB
+            pointB <- nextPoint s lineLength lineCount pointB prevPointB
+            prevPointB <- currentB
+
+            distance <- distance + 1
+
+        distance
+
+    let part2 (s : string) =
+        0
diff --git a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp/Program.fs b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp/Program.fs
index f69ac90..3343dae 100644
--- a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp/Program.fs
+++ b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp/Program.fs
@@ -195,6 +195,22 @@ module Program =
             Console.WriteLine (part2.ToString ())
             Console.Error.WriteLine ((1_000.0 * float sw.ElapsedTicks / float Stopwatch.Frequency).ToString () + "ms")
 
+        Console.WriteLine "=====Day 10====="
+
+        do
+            let input = Path.Combine (dir.FullName, "day10.txt") |> File.ReadAllText
+
+            sw.Restart ()
+            let part1 = Day10.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 = Day10.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 7c324d0..4cef56b 100644
--- a/AdventOfCode2023.FSharp/Test/Test.fsproj
+++ b/AdventOfCode2023.FSharp/Test/Test.fsproj
@@ -18,6 +18,7 @@
     <Compile Include="TestDay7.fs"/>
     <Compile Include="TestDay8.fs"/>
     <Compile Include="TestDay9.fs"/>
+    <Compile Include="TestDay10.fs" />
     <EmbeddedResource Include="samples\day1.txt"/>
     <EmbeddedResource Include="samples\day1part1.txt"/>
     <EmbeddedResource Include="samples\day2.txt"/>
@@ -29,6 +30,8 @@
     <EmbeddedResource Include="samples\day8part1.txt"/>
     <EmbeddedResource Include="samples\day8.txt"/>
     <EmbeddedResource Include="samples\day9.txt"/>
+    <EmbeddedResource Include="samples\day10part1.txt" />
+    <Content Include="samples\day10.txt" />
   </ItemGroup>
 
   <ItemGroup>
diff --git a/AdventOfCode2023.FSharp/Test/TestDay10.fs b/AdventOfCode2023.FSharp/Test/TestDay10.fs
new file mode 100644
index 0000000..99ee5b7
--- /dev/null
+++ b/AdventOfCode2023.FSharp/Test/TestDay10.fs
@@ -0,0 +1,102 @@
+namespace AdventOfCode2023.Test
+
+open AdventOfCode2023
+open NUnit.Framework
+open FsUnitTyped
+open System.IO
+
+[<TestFixture>]
+module TestDay10 =
+
+    let part1Sample1 () =
+        """.....
+.S-7.
+.|.|.
+.L-J.
+.....
+"""
+        |> Day10.part1 |> shouldEqual 4
+
+
+    [<Test>]
+    let part1Sample () =
+        Assembly.getEmbeddedResource typeof<Dummy>.Assembly "day10part1.txt"
+        |> Day10.part1 |> shouldEqual 8
+
+    [<Test>]
+    let part2Sample1 () =
+        """...........
+.S-------7.
+.|F-----7|.
+.||.....||.
+.||.....||.
+.|L-7.F-J|.
+.|..|.|..|.
+.L--J.L--J.
+...........
+"""
+        |> Day10.part2
+        |> shouldEqual 4
+
+    [<Test>]
+    let part2Sample2 () =
+        """..........
+.S------7.
+.|F----7|.
+.||....||.
+.||....||.
+.|L-7F-J|.
+.|..||..|.
+.L--JL--J.
+..........
+"""
+        |> Day10.part2
+        |> shouldEqual 4
+
+    [<Test>]
+    let part2Sample3 () =
+        """.F----7F7F7F7F-7....
+.|F--7||||||||FJ....
+.||.FJ||||||||L7....
+FJL7L7LJLJ||LJ.L-7..
+L--J.L7...LJS7F-7L7.
+....F-J..F7FJ|L7L7L7
+....L7.F7||L7|.L7L7|
+.....|FJLJ|FJ|F7|.LJ
+....FJL-7.||.||||...
+....L---J.LJ.LJLJ...
+"""
+        |> Day10.part2
+        |> shouldEqual 8
+
+    [<Test>]
+    let part2Sample () =
+        Assembly.getEmbeddedResource typeof<Dummy>.Assembly "day10.txt"
+        |> Day10.part2
+        |> shouldEqual 10
+
+    [<Test>]
+    let part1Actual () =
+        let s =
+            try
+                File.ReadAllText (Path.Combine (__SOURCE_DIRECTORY__, "../../inputs/day10.txt"))
+            with
+            | :? DirectoryNotFoundException
+            | :? FileNotFoundException ->
+                Assert.Inconclusive ()
+                failwith "unreachable"
+
+        Day10.part1 s |> shouldEqual 6842
+
+    [<Test>]
+    let part2Actual () =
+        let s =
+            try
+                File.ReadAllText (Path.Combine (__SOURCE_DIRECTORY__, "../../inputs/day10.txt"))
+            with
+            | :? DirectoryNotFoundException
+            | :? FileNotFoundException ->
+                Assert.Inconclusive ()
+                failwith "unreachable"
+
+        Day10.part2 s |> shouldEqual -1
diff --git a/AdventOfCode2023.FSharp/Test/samples/day10.txt b/AdventOfCode2023.FSharp/Test/samples/day10.txt
new file mode 100644
index 0000000..8f950ae
--- /dev/null
+++ b/AdventOfCode2023.FSharp/Test/samples/day10.txt
@@ -0,0 +1,10 @@
+FF7FSF7F7F7F7F7F---7
+L|LJ||||||||||||F--J
+FL-7LJLJ||||||LJL-77
+F--JF--7||LJLJ7F7FJ-
+L---JF-JLJ.||-FJLJJ7
+|F|F-JF---7F7-L7L|7|
+|FFJF7L7F-JF7|JL---7
+7-L-JL7||F7|L7F-7F7|
+L.L7LFJ|||||FJL7||LJ
+L7JLJL-JLJLJL--JLJ.L
diff --git a/AdventOfCode2023.FSharp/Test/samples/day10part1.txt b/AdventOfCode2023.FSharp/Test/samples/day10part1.txt
new file mode 100644
index 0000000..682499d
--- /dev/null
+++ b/AdventOfCode2023.FSharp/Test/samples/day10part1.txt
@@ -0,0 +1,5 @@
+..F7.
+.FJ|.
+SJ.L7
+|F--J
+LJ...