From 50008c0db1c7ba09a3f2ec5d69ec2b9420f5291c Mon Sep 17 00:00:00 2001
From: Sergio Pedri <sergio0694@live.com>
Date: Fri, 23 Jul 2021 00:36:44 +0200
Subject: [PATCH] Optimized CreateUniqueParamNameFromIndex method

---
 .../ExpressionNodes/ExpressionNode.cs         | 38 +++++++++++++------
 ...Microsoft.Toolkit.Uwp.UI.Animations.csproj |  1 +
 .../Properties/SkipLocalsInitAttribute.cs     | 24 ++++++++++++
 3 files changed, 52 insertions(+), 11 deletions(-)
 create mode 100644 Microsoft.Toolkit.Uwp.UI.Animations/Properties/SkipLocalsInitAttribute.cs

diff --git a/Microsoft.Toolkit.Uwp.UI.Animations/Expressions/ExpressionNodes/ExpressionNode.cs b/Microsoft.Toolkit.Uwp.UI.Animations/Expressions/ExpressionNodes/ExpressionNode.cs
index 0b2bcb016cc..e02d4500201 100644
--- a/Microsoft.Toolkit.Uwp.UI.Animations/Expressions/ExpressionNodes/ExpressionNode.cs
+++ b/Microsoft.Toolkit.Uwp.UI.Animations/Expressions/ExpressionNodes/ExpressionNode.cs
@@ -5,6 +5,7 @@
 using System;
 using System.Collections.Generic;
 using System.Numerics;
+using System.Runtime.CompilerServices;
 using Windows.UI;
 using Windows.UI.Composition;
 
@@ -283,10 +284,10 @@ internal void EnsureReferenceInfo()
 
                 // Create a map to store the generated paramNames for each CompObj
                 _compObjToParamNameMap = new Dictionary<CompositionObject, string>();
-                var paramCount = 0;
+                var paramCount = 0u;
                 foreach (var compObj in compObjects)
                 {
-                    string paramName = UniqueParamNameFromIndex(paramCount++); // Guid.NewGuid().ToUppercaseAsciiLetters();
+                    string paramName = CreateUniqueParamNameFromIndex(paramCount++);
 
                     _compObjToParamNameMap.Add(compObj, paramName);
                 }
@@ -314,20 +315,35 @@ internal void EnsureReferenceInfo()
                 }
             }
 
-            // Generates Excel-column-like identifiers, e.g. A, B, ..., Z, AA, AB...
-            string UniqueParamNameFromIndex(int i)
+            // Generates Excel-column-like identifiers, e.g. A, B, ..., Z, AA, BA...
+            // This implementation aggregates characters in reverse order to avoid having to
+            // precompute the exact number of characters in the resulting string. This is not
+            // important in this context as the only critical property to maintain is to have
+            // a unique mapping to each input value to the resulting sequence of letters.
+            [SkipLocalsInit]
+            static unsafe string CreateUniqueParamNameFromIndex(uint i)
             {
-                var alphabetLength = 'Z' - 'A' + 1;
-                var paramName = ((char)('A' + (i % alphabetLength))).ToString();
+                const int alphabetLength = 'Z' - 'A' + 1;
 
-                while (i / alphabetLength > 0)
+                // The total length of the resulting sequence is guaranteed to always
+                // be less than 8, given that log26(4294967295) ≈ 6.8. In this case we
+                // are just allocating the immediate next power of two following that.
+                // Note: this is using a char* buffer instead of Span<char> as the latter
+                // is not referenced here, and we don't want to pull in an extra package.
+                char* characters = stackalloc char[8];
+
+                characters[0] = (char)('A' + (i % alphabetLength));
+
+                int totalCharacters = 1;
+
+                while ((i /= alphabetLength) > 0)
                 {
-                    i = (i / alphabetLength) - 1;
-                    var nextCharacter = (char)('A' + (i % alphabetLength));
-                    paramName = nextCharacter + paramName;
+                    i--;
+
+                    characters[totalCharacters++] = (char)('A' + (i % alphabetLength));
                 }
 
-                return paramName;
+                return new string(characters, 0, totalCharacters);
             }
         }
 
diff --git a/Microsoft.Toolkit.Uwp.UI.Animations/Microsoft.Toolkit.Uwp.UI.Animations.csproj b/Microsoft.Toolkit.Uwp.UI.Animations/Microsoft.Toolkit.Uwp.UI.Animations.csproj
index c514502375d..130fdd806ef 100644
--- a/Microsoft.Toolkit.Uwp.UI.Animations/Microsoft.Toolkit.Uwp.UI.Animations.csproj
+++ b/Microsoft.Toolkit.Uwp.UI.Animations/Microsoft.Toolkit.Uwp.UI.Animations.csproj
@@ -2,6 +2,7 @@
 
   <PropertyGroup>
     <TargetFrameworks>uap10.0.17763</TargetFrameworks>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>
 
   <PropertyGroup>
diff --git a/Microsoft.Toolkit.Uwp.UI.Animations/Properties/SkipLocalsInitAttribute.cs b/Microsoft.Toolkit.Uwp.UI.Animations/Properties/SkipLocalsInitAttribute.cs
new file mode 100644
index 00000000000..4f0d98a5f82
--- /dev/null
+++ b/Microsoft.Toolkit.Uwp.UI.Animations/Properties/SkipLocalsInitAttribute.cs
@@ -0,0 +1,24 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace System.Runtime.CompilerServices
+{
+    /// <summary>
+    /// Used to indicate to the compiler that the <c>.locals init</c> flag should not be set in method headers.
+    /// </summary>
+    /// <remarks>Internal copy from the BCL attribute.</remarks>
+    [AttributeUsage(
+        AttributeTargets.Module |
+        AttributeTargets.Class |
+        AttributeTargets.Struct |
+        AttributeTargets.Interface |
+        AttributeTargets.Constructor |
+        AttributeTargets.Method |
+        AttributeTargets.Property |
+        AttributeTargets.Event,
+        Inherited = false)]
+    internal sealed class SkipLocalsInitAttribute : Attribute
+    {
+    }
+}
\ No newline at end of file