Rewrite TextFormatter.RemoveHotKeySpecifier

Uses stackalloc char buffer with fallback to rented array.

Benchmarks/Text/TextFormatter/RemoveHotKeySpecifier.cs

@@ -0,0 +1,97 @@
+using System.Text;
+using BenchmarkDotNet.Attributes;
+using Tui = Terminal.Gui;
+
+namespace Terminal.Gui.Benchmarks.Text.TextFormatter;
+
+/// <summary>
+/// Benchmarks for <see cref="Tui.TextFormatter.RemoveHotKeySpecifier"/> performance fine-tuning.
+/// </summary>
+[MemoryDiagnoser]
+[BenchmarkCategory (nameof(Tui.TextFormatter))]
+public class RemoveHotKeySpecifier
+{
+    // Omit from summary table.
+    private static readonly Rune HotkeySpecifier = (Rune)'_';
+
+    /// <summary>
+    /// Benchmark for previous implementation.
+    /// </summary>
+    [Benchmark]
+    [ArgumentsSource (nameof (DataSource))]
+    public string Previous (string text, int hotPos)
+    {
+        return StringConcatLoop (text, hotPos, HotkeySpecifier);
+    }
+
+    /// <summary>
+    /// Benchmark for current implementation with stackalloc char buffer and fallback to rented array.
+    /// </summary>
+    [Benchmark (Baseline = true)]
+    [ArgumentsSource (nameof (DataSource))]
+    public string Current (string text, int hotPos)
+    {
+        return Tui.TextFormatter.RemoveHotKeySpecifier (text, hotPos, HotkeySpecifier);
+    }
+
+    /// <summary>
+    /// Previous implementation with string concatenation in a loop.
+    /// </summary>
+    public static string StringConcatLoop (string text, int hotPos, Rune hotKeySpecifier)
+    {
+        if (string.IsNullOrEmpty (text))
+        {
+            return text;
+        }
+
+        // Scan 
+        var start = string.Empty;
+        var i = 0;
+
+        foreach (Rune c in text.EnumerateRunes ())
+        {
+            if (c == hotKeySpecifier && i == hotPos)
+            {
+                i++;
+
+                continue;
+            }
+
+            start += c;
+            i++;
+        }
+
+        return start;
+    }
+
+    public IEnumerable<object []> DataSource ()
+    {
+        string[] texts = [
+            "",
+			// Typical scenario.
+			"_Save file (Ctrl+S)",
+			// Medium text, hotkey specifier somewhere in the middle.
+			"Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nulla sed euismod metus. _Phasellus lectus metus, ultricies a commodo quis, facilisis vitae nulla.",
+			// Long text, hotkey specifier almost at the beginning.
+			"Ĺόŕéḿ íṕśúḿ d́όĺόŕ śít́ áḿét́, ćόńśéćt́ét́úŕ ád́íṕíśćíńǵ éĺít́. _Ṕŕáéśéńt́ q́úíś ĺúćt́úś éĺít́. Íńt́éǵéŕ út́ áŕćú éǵét́ d́όĺόŕ śćéĺéŕíśq́úé ḿát́t́íś áć ét́ d́íáḿ. " +
+            "Ṕéĺĺéńt́éśq́úé śéd́ d́áṕíb́úś ḿáśśá, v́éĺ t́ŕíśt́íq́úé d́úí. Śéd́ v́ít́áé ńéq́úé éú v́éĺít́ όŕńáŕé áĺíq́úét́. Út́ q́úíś όŕćí t́éḿṕόŕ, t́éḿṕόŕ t́úŕṕíś íd́, t́éḿṕúś ńéq́úé. " +
+            "Ṕŕáéśéńt́ śáṕíéń t́úŕṕíś, όŕńáŕé v́éĺ ḿáúŕíś át́, v́áŕíúś śúśćíṕít́ áńt́é. Út́ ṕúĺv́íńáŕ t́úŕṕíś ḿáśśá, q́úíś ćúŕśúś áŕćú f́áúćíb́úś íń.",
+			// Long text, hotkey specifier almost at the end.
+			"Ĺόŕéḿ íṕśúḿ d́όĺόŕ śít́ áḿét́, ćόńśéćt́ét́úŕ ád́íṕíśćíńǵ éĺít́. Ṕŕáéśéńt́ q́úíś ĺúćt́úś éĺít́. Íńt́éǵéŕ út́ áŕćú éǵét́ d́όĺόŕ śćéĺéŕíśq́úé ḿát́t́íś áć ét́ d́íáḿ. " +
+            "Ṕéĺĺéńt́éśq́úé śéd́ d́áṕíb́úś ḿáśśá, v́éĺ t́ŕíśt́íq́úé d́úí. Śéd́ v́ít́áé ńéq́úé éú v́éĺít́ όŕńáŕé áĺíq́úét́. Út́ q́úíś όŕćí t́éḿṕόŕ, t́éḿṕόŕ t́úŕṕíś íd́, t́éḿṕúś ńéq́úé. " +
+            "Ṕŕáéśéńt́ śáṕíéń t́úŕṕíś, όŕńáŕé v́éĺ ḿáúŕíś át́, v́áŕíúś śúśćíṕít́ áńt́é. _Út́ ṕúĺv́íńáŕ t́úŕṕíś ḿáśśá, q́úíś ćúŕśúś áŕćú f́áúćíb́úś íń.",
+        ];
+
+        foreach (string text in texts)
+        {
+            int hotPos = text.EnumerateRunes()
+                .Select((r, i) => r == HotkeySpecifier ? i : -1)
+                .FirstOrDefault(i => i > -1, -1);
+
+            yield return [text, hotPos];
+        }
+
+        // Typical scenario but without hotkey and with misleading position.
+        yield return ["Save file (Ctrl+S)", 3];
+    }
+}

Terminal.Gui/Text/TextFormatter.cs

@@ -2444,24 +2444,44 @@ public class TextFormatter
             return text;
         }
 
-        // Scan 
+        const int maxStackallocCharBufferSize = 512; // ~1 kiB
-        var start = string.Empty;
+        char[]? rentedBufferArray = null;
-        var i = 0;
+        try
-
-        foreach (Rune c in text.EnumerateRunes ())
         {
-            if (c == hotKeySpecifier && i == hotPos)
+            Span<char> buffer = text.Length <= maxStackallocCharBufferSize
-            {
+                ? stackalloc char[text.Length]
-                i++;
+                : (rentedBufferArray = ArrayPool<char>.Shared.Rent(text.Length));
 
-                continue;
+            int i = 0;
+            var remainingBuffer = buffer;
+            foreach (Rune c in text.EnumerateRunes ())
+            {
+                if (c == hotKeySpecifier && i == hotPos)
+                {
+                    i++;
+                    continue;
+                }
+                int charsWritten = c.EncodeToUtf16 (remainingBuffer);
+                remainingBuffer = remainingBuffer [charsWritten..];
+                i++;
             }
 
-            start += c;
+            ReadOnlySpan<char> newText = buffer [..^remainingBuffer.Length];
-            i++;
+            // If the resulting string would be the same as original then just return the original.
-        }
+            if (newText.Equals(text, StringComparison.Ordinal))
+            {
+                return text;
+            }
 
-        return start;
+            return new string (newText);
+        }
+        finally
+        {
+            if (rentedBufferArray != null)
+            {
+                ArrayPool<char>.Shared.Return (rentedBufferArray);
+            }
+        }
     }
 
     #endregion // Static Members