StringExtensions.ToString(IEnumerable<Rune>) stackalloc char buffer with StringBuilder fallback

Author: TheTonttu

Benchmarks/Text/StringExtensions/ToStringEnumerable.cs

@@ -16,27 +16,28 @@ public class ToStringEnumerable
     /// </summary>
     [Benchmark]
     [ArgumentsSource (nameof (DataSource))]
-    public string Previous (IEnumerable<Rune> runes, int size)
+    public string Previous (IEnumerable<Rune> runes, int len)
     {
-        return StringAppendInLoop (runes);
+        return StringConcatInLoop (runes);
     }
 
     /// <summary>
-    /// Benchmark for current implementation with rune chars appending to StringBuilder.
+    /// Benchmark for current implementation with stackalloc char buffer and
+    /// fallback to rune chars appending to StringBuilder.
     /// </summary>
     /// <param name="runes"></param>
     /// <returns></returns>
     [Benchmark (Baseline = true)]
     [ArgumentsSource (nameof (DataSource))]
-    public string Current (IEnumerable<Rune> runes, int size)
+    public string Current (IEnumerable<Rune> runes, int len)
     {
         return Tui.StringExtensions.ToString (runes);
     }
 
     /// <summary>
-    /// Previous implementation with string append in a loop.
+    /// Previous implementation with string concatenation in a loop.
     /// </summary>
-    private static string StringAppendInLoop (IEnumerable<Rune> runes)
+    private static string StringConcatInLoop (IEnumerable<Rune> runes)
     {
         var str = string.Empty;
 
@@ -49,22 +50,35 @@ private static string StringAppendInLoop (IEnumerable<Rune> runes)
     }
 
     public IEnumerable<object []> DataSource ()
+    {
+        // Extra length argument as workaround for the summary grouping
+        // different length collections to same baseline making comparison difficult.
+        foreach (string text in GetTextData ())
+        {
+            Rune [] runes = [..text.EnumerateRunes ()];
+            yield return [runes, runes.Length];
+        }
+    }
+
+    private IEnumerable<string> GetTextData ()
     {
         string textSource =
             """
-			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.
-			Óŕćí v́áŕíúś ńát́όq́úé ṕéńát́íb́úś ét́ ḿáǵńíś d́íś ṕáŕt́úŕíéńt́ ḿόńt́éś, ńáśćét́úŕ ŕíd́íćúĺúś ḿúś. F́úśćé át́ éx́ b́ĺáńd́ít́, ćόńv́áĺĺíś q́úáḿ ét́, v́úĺṕút́át́é ĺáćúś.
-			Śúśṕéńd́íśśé śít́ áḿét́ áŕćú út́ áŕćú f́áúćíb́úś v́áŕíúś. V́ív́áḿúś śít́ áḿét́ ḿáx́íḿúś d́íáḿ. Ńáḿ éx́ ĺéό, ṕh́áŕét́ŕá éú ĺόb́όŕt́íś át́, t́ŕíśt́íq́úé út́ f́éĺíś.
-			""";
+            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.
+            Óŕćí v́áŕíúś ńát́όq́úé ṕéńát́íb́úś ét́ ḿáǵńíś d́íś ṕáŕt́úŕíéńt́ ḿόńt́éś, ńáśćét́úŕ ŕíd́íćúĺúś ḿúś. F́úśćé át́ éx́ b́ĺáńd́ít́, ćόńv́áĺĺíś q́úáḿ ét́, v́úĺṕút́át́é ĺáćúś.
+            Śúśṕéńd́íśśé śít́ áḿét́ áŕćú út́ áŕćú f́áúćíb́úś v́áŕíúś. V́ív́áḿúś śít́ áḿét́ ḿáx́íḿúś d́íáḿ. Ńáḿ éx́ ĺéό, ṕh́áŕét́ŕá éú ĺόb́όŕt́íś át́, t́ŕíśt́íq́úé út́ f́éĺíś.
+            """;
 
-        // Extra argument as workaround for the summary grouping different length collections to same baseline making comparison difficult.
-        int[] sizes = [1, 10, 100, textSource.Length / 2, textSource.Length];
+        int[] lengths = [1, 10, 100, textSource.Length / 2, textSource.Length];
 
-        foreach (int size in sizes)
+        foreach (int length in lengths)
         {
-            yield return [textSource.EnumerateRunes ().Take (size).ToArray (), size];
+            yield return textSource [..length];
         }
+
+        string textLongerThanStackallocThreshold = string.Concat(Enumerable.Repeat(textSource, 10));
+        yield return textLongerThanStackallocThreshold;
     }
 }

Terminal.Gui/Text/StringExtensions.cs

@@ -124,13 +124,43 @@ public static (Rune Rune, int Size) DecodeRune (this string str, int start = 0,
     /// <returns></returns>
     public static string ToString (IEnumerable<Rune> runes)
     {
-        StringBuilder stringBuilder = new();
         const int maxCharsPerRune = 2;
-        Span<char> charBuffer = stackalloc char[maxCharsPerRune];
+        // Max stackalloc ~2 kB
+        const int maxStackallocTextBufferSize = 1048;
+
+        Span<char> runeBuffer = stackalloc char[maxCharsPerRune];
+        // Use stackalloc buffer if rune count is easily available and the count is reasonable.
+        if (runes.TryGetNonEnumeratedCount (out int count))
+        {
+            if (count == 0)
+            {
+                return string.Empty;
+            }
+
+            int maxRequiredTextBufferSize = count * maxCharsPerRune;
+            if (maxRequiredTextBufferSize <= maxStackallocTextBufferSize)
+            {
+                Span<char> textBuffer = stackalloc char[maxRequiredTextBufferSize];
+                Span<char> remainingBuffer = textBuffer;
+                foreach (Rune rune in runes)
+                {
+                    int charsWritten = rune.EncodeToUtf16 (runeBuffer);
+                    ReadOnlySpan<char> runeChars = runeBuffer [..charsWritten];
+                    runeChars.CopyTo (remainingBuffer);
+                    remainingBuffer = remainingBuffer [runeChars.Length..];
+                }
+
+                ReadOnlySpan<char> text = textBuffer[..^remainingBuffer.Length];
+                return text.ToString ();
+            }
+        }
+
+        // Fallback to StringBuilder append.
+        StringBuilder stringBuilder = new();
         foreach (Rune rune in runes)
         {
-            int charsWritten = rune.EncodeToUtf16 (charBuffer);
-            ReadOnlySpan<char> runeChars = charBuffer [..charsWritten];
+            int charsWritten = rune.EncodeToUtf16 (runeBuffer);
+            ReadOnlySpan<char> runeChars = runeBuffer [..charsWritten];
             stringBuilder.Append (runeChars);
         }
         return stringBuilder.ToString ();