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

@@ -25,7 +25,7 @@ module Day15 =
 
         answer
 
-    let removeFirst<'a> (toRemove : 'a -> bool) (arr : ResizeArray<'a>) : unit =
+    let inline removeFirst<'a> ([<InlineIfLambda>] toRemove : 'a -> bool) (arr : ResizeArray<'a>) : unit =
         let mutable i = 0
 
         while i < arr.Count do
@@ -38,7 +38,13 @@ module Day15 =
 
             i <- i + 1
 
-    let replace (withKey : 'a -> 'key) (key : 'key) (value : 'a) (arr : ResizeArray<'a>) : unit =
+    let inline replace
+        ([<InlineIfLambda>] withKey : 'a -> 'key)
+        (key : 'key)
+        (value : 'a)
+        (arr : ResizeArray<'a>)
+        : unit
+        =
         let mutable i = 0
 
         while i < arr.Count do
@@ -60,23 +66,35 @@ module Day15 =
 
         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 part2 (s : string) =
         let s = s.AsSpan().TrimEnd ()
         use chunks = StringSplitEnumerator.make' ',' s
-        let lenses = Array.init 256 (fun _ -> ResizeArray<string * _> ())
+        // 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 * _> ())
 
         for chunk in chunks do
             if chunk.[chunk.Length - 1] = '-' then
-                let label = chunk.Slice(0, chunk.Length - 1).ToString ()
-                removeFirst (fun (label2, _focalLength) -> label2 = label) lenses.[hash (label.AsSpan ())]
+                let label = chunk.Slice (0, chunk.Length - 1)
+                let labelShrunk = toUint32 label
+                removeFirst (fun (label2 : uint32, _focalLength) -> label2 = labelShrunk) lenses.[hash label]
             else
                 let equalsPos = chunk.IndexOf '='
 
                 let focalLength =
                     Int32.Parse (chunk.Slice (equalsPos + 1), NumberStyles.None, CultureInfo.InvariantCulture)
 
-                let label = chunk.Slice(0, equalsPos).ToString ()
-                replace fst label (label, focalLength) lenses.[hash (label.AsSpan ())]
+                let label = chunk.Slice (0, equalsPos)
+                let labelShrunk = toUint32 label
+                replace fst labelShrunk (labelShrunk, focalLength) lenses.[hash label]
 
         let mutable answer = 0