diff --git a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Bench/Inputs.fs b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Bench/Inputs.fs
index 2d29639..58cd579 100644
--- a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Bench/Inputs.fs
+++ b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Bench/Inputs.fs
@@ -14,6 +14,6 @@ module Inputs =
             if isNull dir then
                 failwith "reached root of filesystem without finding inputs dir"
 
-        Array.init 14 (fun day -> Path.Combine (dir.FullName, "inputs", $"day%i{day + 1}.txt") |> File.ReadAllText)
+        Array.init 15 (fun day -> Path.Combine (dir.FullName, "inputs", $"day%i{day + 1}.txt") |> File.ReadAllText)
 
     let inline day (i : int) = days.[i - 1]
diff --git a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Bench/Program.fs b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Bench/Program.fs
index 85029c0..8de9c54 100644
--- a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Bench/Program.fs
+++ b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Bench/Program.fs
@@ -44,7 +44,7 @@ module Benchmarks =
         [<GlobalSetup>]
         member _.Setup () = Run.shouldWrite <- false
 
-        [<Params(11, 12, 13, 14)>]
+        [<Params(11, 12, 13, 14, 15)>]
         member val Day = 0 with get, set
 
         [<Params(false, true)>]
diff --git a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Bench/Run.fs b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Bench/Run.fs
index 90c329c..8f7a676 100644
--- a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Bench/Run.fs
+++ b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Bench/Run.fs
@@ -197,6 +197,18 @@ module Run =
             if shouldWrite then
                 Console.WriteLine output
 
+    let day15 (partTwo : bool) (input : string) =
+        if not partTwo then
+            let output = Day15.part1 input
+
+            if shouldWrite then
+                Console.WriteLine output
+        else
+            let output = Day15.part2 input
+
+            if shouldWrite then
+                Console.WriteLine output
+
     let allRuns =
         [|
             day1
@@ -213,4 +225,5 @@ module Run =
             day12
             day13
             day14
+            day15
         |]
diff --git a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/AdventOfCode2023.FSharp.Lib.fsproj b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/AdventOfCode2023.FSharp.Lib.fsproj
index 1290625..8bdcbba 100644
--- a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/AdventOfCode2023.FSharp.Lib.fsproj
+++ b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/AdventOfCode2023.FSharp.Lib.fsproj
@@ -25,6 +25,7 @@
     <Compile Include="Day12.fs" />
     <Compile Include="Day13.fs" />
     <Compile Include="Day14.fs" />
+    <Compile Include="Day15.fs" />
   </ItemGroup>
 
 </Project>
diff --git a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day15.fs b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day15.fs
new file mode 100644
index 0000000..fa6171a
--- /dev/null
+++ b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day15.fs
@@ -0,0 +1,114 @@
+namespace AdventOfCode2023
+
+open System
+open System.Globalization
+
+[<RequireQualifiedAccess>]
+module Day15 =
+
+    let hash (s : ReadOnlySpan<char>) : int =
+        let mutable v = 0
+
+        for c in s do
+            v <- v + int (byte c)
+            v <- (17 * v) % 256
+
+        v
+
+    let part1 (s : string) =
+        let s = s.AsSpan().TrimEnd ()
+        use chunks = StringSplitEnumerator.make' ',' s
+        let mutable answer = 0
+
+        for chunk in chunks do
+            answer <- answer + hash chunk
+
+        answer
+
+    let inline removeFirst<'a> ([<InlineIfLambda>] toRemove : 'a -> bool) (arr : ResizeArray<'a>) : unit =
+        let mutable i = 0
+
+        while i < arr.Count do
+            if toRemove arr.[i] then
+                for j = i to arr.Count - 2 do
+                    arr.[j] <- arr.[j + 1]
+
+                arr.RemoveAt (arr.Count - 1)
+                i <- arr.Count
+
+            i <- i + 1
+
+    let inline replace
+        ([<InlineIfLambda>] withKey : 'a -> 'key)
+        (key : 'key)
+        (value : 'a)
+        (arr : ResizeArray<'a>)
+        : unit
+        =
+        let mutable i = 0
+
+        while i < arr.Count do
+            if withKey arr.[i] = key then
+                arr.[i] <- value
+                i <- arr.Count
+
+            i <- i + 1
+
+        if i < arr.Count + 1 then
+            // no replacement was made
+            arr.Add value
+
+    let inline getLength (labelAndLength : uint64) : uint32 =
+        (labelAndLength % uint64 UInt32.MaxValue) |> uint32
+
+    let inline getLabel (labelAndLength : uint64) : uint32 =
+        (labelAndLength / uint64 UInt32.MaxValue) |> uint32
+
+    let inline focusingPower (boxNumber : uint32) (arr : ResizeArray<_>) =
+        let mutable answer = 0ul
+
+        for i = 0 to arr.Count - 1 do
+            answer <- answer + (boxNumber + 1ul) * (uint32 i + 1ul) * getLength arr.[i]
+
+        answer
+
+    let inline toUint32 (s : ReadOnlySpan<char>) : uint32 =
+        let mutable answer = 0ul
+
+        for c in s do
+            answer <- answer * 26ul + uint32 (byte c - byte 'a')
+
+        answer
+
+    let inline pack (label : uint32) (focalLength : uint32) : uint64 =
+        uint64 label * uint64 UInt32.MaxValue + uint64 focalLength
+
+    let part2 (s : string) =
+        let s = s.AsSpan().TrimEnd ()
+        use chunks = StringSplitEnumerator.make' ',' s
+        // The max length of a label turns out to be 6, which means we need 26^6 < 2^32 entries.
+        // So we'll use a uint32 instead of our string, to save hopping around memory.
+        // We'll also pack the focal length into the elements, to save tupling.
+        let lenses = Array.init 256 (fun _ -> ResizeArray<uint64> ())
+
+        for chunk in chunks do
+            if chunk.[chunk.Length - 1] = '-' then
+                let label = chunk.Slice (0, chunk.Length - 1)
+                let labelShrunk = toUint32 label
+                removeFirst (fun labelAndLength -> getLabel labelAndLength = labelShrunk) lenses.[hash label]
+            else
+                let equalsPos = chunk.IndexOf '='
+
+                let focalLength =
+                    UInt32.Parse (chunk.Slice (equalsPos + 1), NumberStyles.None, CultureInfo.InvariantCulture)
+
+                let label = chunk.Slice (0, equalsPos)
+                let labelShrunk = toUint32 label
+                replace getLabel labelShrunk (pack labelShrunk focalLength) lenses.[hash label]
+
+        let mutable answer = 0ul
+
+        for i = 0 to 255 do
+            answer <- answer + focusingPower (uint32 i) lenses.[i]
+
+        answer
diff --git a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp/Program.fs b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp/Program.fs
index 8689c4b..9e7b12d 100644
--- a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp/Program.fs
+++ b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp/Program.fs
@@ -284,6 +284,22 @@ module Program =
             Console.WriteLine (part2.ToString ())
             Console.Error.WriteLine ((1_000.0 * float sw.ElapsedTicks / float Stopwatch.Frequency).ToString () + "ms")
 
+        Console.WriteLine "=====Day 15====="
+
+        do
+            let input = Path.Combine (dir.FullName, "day15.txt") |> File.ReadAllText
+
+            sw.Restart ()
+            let part1 = Day15.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 = Day15.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 75f79a1..2a6094c 100644
--- a/AdventOfCode2023.FSharp/Test/Test.fsproj
+++ b/AdventOfCode2023.FSharp/Test/Test.fsproj
@@ -23,6 +23,7 @@
     <Compile Include="TestDay12.fs" />
     <Compile Include="TestDay13.fs" />
     <Compile Include="TestDay14.fs" />
+    <Compile Include="TestDay15.fs" />
     <EmbeddedResource Include="samples\day1.txt"/>
     <EmbeddedResource Include="samples\day1part1.txt"/>
     <EmbeddedResource Include="samples\day2.txt"/>
@@ -40,6 +41,7 @@
     <EmbeddedResource Include="samples\day12.txt" />
     <EmbeddedResource Include="samples\day13.txt" />
     <EmbeddedResource Include="samples\day14.txt" />
+    <EmbeddedResource Include="samples\day15.txt" />
   </ItemGroup>
 
   <ItemGroup>
diff --git a/AdventOfCode2023.FSharp/Test/TestDay15.fs b/AdventOfCode2023.FSharp/Test/TestDay15.fs
new file mode 100644
index 0000000..29b3519
--- /dev/null
+++ b/AdventOfCode2023.FSharp/Test/TestDay15.fs
@@ -0,0 +1,46 @@
+namespace AdventOfCode2023.Test
+
+open AdventOfCode2023
+open NUnit.Framework
+open FsUnitTyped
+open System.IO
+
+[<TestFixture>]
+module TestDay15 =
+
+    [<Test>]
+    let sample = Assembly.getEmbeddedResource typeof<Dummy>.Assembly "day15.txt"
+
+    [<Test>]
+    let part1Sample () =
+        sample |> Day15.part1 |> shouldEqual 1320
+
+    [<Test>]
+    let part2Sample () =
+        sample |> Day15.part2 |> shouldEqual 145ul
+
+    [<Test>]
+    let part1Actual () =
+        let s =
+            try
+                File.ReadAllText (Path.Combine (__SOURCE_DIRECTORY__, "../../inputs/day15.txt"))
+            with
+            | :? DirectoryNotFoundException
+            | :? FileNotFoundException ->
+                Assert.Inconclusive ()
+                failwith "unreachable"
+
+        Day15.part1 s |> shouldEqual 521434
+
+    [<Test>]
+    let part2Actual () =
+        let s =
+            try
+                File.ReadAllText (Path.Combine (__SOURCE_DIRECTORY__, "../../inputs/day15.txt"))
+            with
+            | :? DirectoryNotFoundException
+            | :? FileNotFoundException ->
+                Assert.Inconclusive ()
+                failwith "unreachable"
+
+        Day15.part2 s |> shouldEqual 248279ul
diff --git a/AdventOfCode2023.FSharp/Test/samples/day15.txt b/AdventOfCode2023.FSharp/Test/samples/day15.txt
new file mode 100644
index 0000000..4f58f74
--- /dev/null
+++ b/AdventOfCode2023.FSharp/Test/samples/day15.txt
@@ -0,0 +1 @@
+rn=1,cm-,qp=3,cm=2,qp-,pc=4,ot=9,ab=5,pc-,pc=6,ot=7