Improve perf of InputText/Multiline.

Author: mellinoe

src/ImGui.NET/ImGui.Manual.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Numerics;
+using System.Runtime.CompilerServices;
 using System.Text;
 
 namespace ImGuiNET
@@ -88,22 +89,25 @@ public static bool InputText(
                 Encoding.UTF8.GetBytes(labelPtr, label.Length, labelBytes, labelByteCount);
             }
 
-            int bytesNeeded = Encoding.UTF8.GetByteCount(input);
-            int stackBufSize = Math.Max((int)maxLength, bytesNeeded);
+            int originalByteCount = Encoding.UTF8.GetByteCount(input);
+            int stackBufSize = Math.Max((int)maxLength, originalByteCount);
             byte* bufBytes = stackalloc byte[stackBufSize];
             fixed (char* u16Ptr = input)
             {
                 Encoding.UTF8.GetBytes(u16Ptr, input.Length, bufBytes, stackBufSize);
             }
 
+            byte* originalBufBytes = stackalloc byte[originalByteCount];
+            Unsafe.CopyBlock(originalBufBytes, bufBytes, (uint)originalByteCount);
+
             byte result = ImGuiNative.igInputText(
                 labelBytes,
                 bufBytes,
                 (uint)stackBufSize,
                 flags,
                 callback,
                 user_data.ToPointer());
-            if (!Util.AreStringsEqual(input, bufBytes))
+            if (!Util.AreStringsEqual(originalBufBytes, originalByteCount, bufBytes))
             {
                 input = Util.StringFromPtr(bufBytes);
             }
@@ -148,14 +152,17 @@ public static bool InputTextMultiline(
                 Encoding.UTF8.GetBytes(labelPtr, label.Length, labelBytes, labelByteCount);
             }
 
-            int bytesNeeded = Encoding.UTF8.GetByteCount(input);
-            int stackBufSize = Math.Max((int)maxLength, bytesNeeded);
+            int originalByteCount = Encoding.UTF8.GetByteCount(input);
+            int stackBufSize = Math.Max((int)maxLength, originalByteCount);
             byte* bufBytes = stackalloc byte[stackBufSize];
             fixed (char* u16Ptr = input)
             {
                 Encoding.UTF8.GetBytes(u16Ptr, input.Length, bufBytes, stackBufSize);
             }
 
+            byte* originalBufBytes = stackalloc byte[originalByteCount];
+            Unsafe.CopyBlock(originalBufBytes, bufBytes, (uint)originalByteCount);
+
             byte result = ImGuiNative.igInputTextMultiline(
                 labelBytes,
                 bufBytes,
@@ -164,7 +171,7 @@ public static bool InputTextMultiline(
                 flags,
                 callback,
                 user_data.ToPointer());
-            if (!Util.AreStringsEqual(input, bufBytes))
+            if (!Util.AreStringsEqual(originalBufBytes, originalByteCount, bufBytes))
             {
                 input = Util.StringFromPtr(bufBytes);
             }

src/ImGui.NET/Util.cs

@@ -15,23 +15,14 @@ public static unsafe string StringFromPtr(byte* ptr)
             return Encoding.UTF8.GetString(ptr, characters);
         }
 
-        internal static unsafe bool AreStringsEqual(string a, byte* b)
+        internal static unsafe bool AreStringsEqual(byte* a, int aLength, byte* b)
         {
-            if (a.Length == 0) { return b[0] == 0; }
-
-            int aCount = Encoding.UTF8.GetByteCount(a);
-            byte* aBytes = stackalloc byte[aCount];
-            fixed (char* labelPtr = a)
-            {
-                Encoding.UTF8.GetBytes(labelPtr, a.Length, aBytes, aCount);
-            }
-
-            for (int i = 0; i < aCount; i++)
+            for (int i = 0; i < aLength; i++)
             {
-                if (aBytes[i] != b[i]) { return false; }
+                if (a[i] != b[i]) { return false; }
             }
 
-            if (b[aCount] != 0) { return false; }
+            if (b[aLength] != 0) { return false; }
 
             return true;
         }