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AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day15.fs

@@ -58,11 +58,17 @@ module Day15 =
             // no replacement was made
             arr.Add value
 
-    let inline focusingPower (boxNumber : int) (arr : ResizeArray<_ * _>) : int =
+    let inline getLength (labelAndLength : uint64) : uint32 =
-        let mutable answer = 0
+        (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 + 1) * (i + 1) * snd arr.[i]
+            answer <- answer + (boxNumber + 1ul) * (uint32 i + 1ul) * getLength arr.[i]
 
         answer
 
@@ -74,31 +80,35 @@ module Day15 =
 
         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.
-        let lenses = Array.init 256 (fun _ -> ResizeArray<uint32 * _> ())
+        // 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 (label2 : uint32, _focalLength) -> label2 = labelShrunk) lenses.[hash label]
+                removeFirst (fun labelAndLength -> getLabel labelAndLength = labelShrunk) lenses.[hash label]
             else
                 let equalsPos = chunk.IndexOf '='
 
                 let focalLength =
-                    Int32.Parse (chunk.Slice (equalsPos + 1), NumberStyles.None, CultureInfo.InvariantCulture)
+                    UInt32.Parse (chunk.Slice (equalsPos + 1), NumberStyles.None, CultureInfo.InvariantCulture)
 
                 let label = chunk.Slice (0, equalsPos)
                 let labelShrunk = toUint32 label
-                replace fst labelShrunk (labelShrunk, focalLength) lenses.[hash label]
+                replace getLabel labelShrunk (pack labelShrunk focalLength) lenses.[hash label]
 
-        let mutable answer = 0
+        let mutable answer = 0ul
 
         for i = 0 to 255 do
-            answer <- answer + focusingPower i lenses.[i]
+            answer <- answer + focusingPower (uint32 i) lenses.[i]
 
         answer

AdventOfCode2023.FSharp/Test/TestDay15.fs

@@ -1,7 +1,5 @@
 namespace AdventOfCode2023.Test
 
-open System
-
 open AdventOfCode2023
 open NUnit.Framework
 open FsUnitTyped
@@ -19,7 +17,7 @@ module TestDay15 =
 
     [<Test>]
     let part2Sample () =
-        sample |> Day15.part2 |> shouldEqual 145
+        sample |> Day15.part2 |> shouldEqual 145ul
 
     [<Test>]
     let part1Actual () =
@@ -45,4 +43,4 @@ module TestDay15 =
                 Assert.Inconclusive ()
                 failwith "unreachable"
 
-        Day15.part2 s |> shouldEqual 248279
+        Day15.part2 s |> shouldEqual 248279ul