Perf: improved GraphemeEnumerator by avoiding double codepoint iteration

src/Avalonia.Base/Media/TextFormatting/FormattedTextSource.cs

@@ -128,11 +128,9 @@ namespace Avalonia.Media.TextFormatting
 
             var graphemeEnumerator = new GraphemeEnumerator(text);
 
-            while (graphemeEnumerator.MoveNext())
+            while (graphemeEnumerator.MoveNext(out var grapheme))
             {
-                var grapheme = graphemeEnumerator.Current;
-
-                finalLength += grapheme.Text.Length;
+                finalLength += grapheme.Length;
 
                 if (finalLength >= length)
                 {

src/Avalonia.Base/Media/TextFormatting/TextCharacters.cs

@@ -140,16 +140,14 @@ namespace Avalonia.Media.TextFormatting
 
             var enumerator = new GraphemeEnumerator(textSpan);
 
-            while (enumerator.MoveNext())
+            while (enumerator.MoveNext(out var grapheme))
             {
-                var grapheme = enumerator.Current;
-
                 if (!grapheme.FirstCodepoint.IsWhiteSpace && glyphTypeface.TryGetGlyph(grapheme.FirstCodepoint, out _))
                 {
                     break;
                 }
 
-                count += grapheme.Text.Length;
+                count += grapheme.Length;
             }
 
             return new UnshapedTextRun(text.Slice(0, count), defaultProperties, biDiLevel);
@@ -184,10 +182,8 @@ namespace Avalonia.Media.TextFormatting
 
             var enumerator = new GraphemeEnumerator(text);
 
-            while (enumerator.MoveNext())
+            while (enumerator.MoveNext(out var currentGrapheme))
             {
-                var currentGrapheme = enumerator.Current;
-
                 var currentScript = currentGrapheme.FirstCodepoint.Script;
 
                 if (!currentGrapheme.FirstCodepoint.IsWhiteSpace && defaultFont != null && defaultFont.TryGetGlyph(currentGrapheme.FirstCodepoint, out _))
@@ -217,7 +213,7 @@ namespace Avalonia.Media.TextFormatting
                     }
                 }
 
-                length += currentGrapheme.Text.Length;
+                length += currentGrapheme.Length;
             }
 
             return length > 0;

src/Avalonia.Base/Media/TextFormatting/Unicode/Grapheme.cs

@@ -1,16 +1,15 @@
-using System;
-
-namespace Avalonia.Media.TextFormatting.Unicode
+namespace Avalonia.Media.TextFormatting.Unicode
 {
     /// <summary>
     /// Represents the smallest unit of a writing system of any given language.
     /// </summary>
     public readonly ref struct Grapheme
     {
-        public Grapheme(Codepoint firstCodepoint, ReadOnlySpan<char> text)
+        public Grapheme(Codepoint firstCodepoint, int offset, int length)
         {
             FirstCodepoint = firstCodepoint;
-            Text = text;
+            Offset = offset;
+            Length = length;
         }
 
         /// <summary>
@@ -19,12 +18,13 @@ namespace Avalonia.Media.TextFormatting.Unicode
         public Codepoint FirstCodepoint { get; }
 
         /// <summary>
-        /// The text of the grapheme cluster
+        /// Gets the starting code unit offset of this grapheme inside its containing text.
         /// </summary>
-        public ReadOnlySpan<char> Text { get; }
+        public int Offset { get; }
 
-        /// <inheritdoc />
-        public override string ToString()
-            => Text.ToString();
+        /// <summary>
+        /// Gets the length of this grapheme, in code units.
+        /// </summary>
+        public int Length { get; }
     }
 }

src/Avalonia.Base/Media/TextFormatting/Unicode/GraphemeEnumerator.cs

@@ -198,10 +198,13 @@ namespace Avalonia.Media.TextFormatting.Unicode
                     break; // nothing but trailers after the final RI
             }
 
+            const uint gb9Mask =
+                (1U << (int)GraphemeBreakClass.Extend) |
+                (1U << (int)GraphemeBreakClass.ZWJ) |
+                (1U << (int)GraphemeBreakClass.SpacingMark);
+
             // rules GB9, GB9a
-            while (_currentType is GraphemeBreakClass.Extend
-                or GraphemeBreakClass.ZWJ
-                or GraphemeBreakClass.SpacingMark)
+            while (((1U << (int)_currentType) & gb9Mask) != 0U)
             {
                 ReadNextCodepoint();
             }

src/Avalonia.Controls/TextBox.cs

@@ -963,10 +963,8 @@ namespace Avalonia.Controls
 
                 var graphemeEnumerator = new GraphemeEnumerator(input.AsSpan());
 
-                while (graphemeEnumerator.MoveNext())
+                while (graphemeEnumerator.MoveNext(out var grapheme))
                 {
-                    var grapheme = graphemeEnumerator.Current;
-
                     if (grapheme.FirstCodepoint.IsBreakChar)
                     {
                         if (lineCount + 1 > MaxLines)
@@ -979,7 +977,7 @@ namespace Avalonia.Controls
                         }
                     }
 
-                    length += grapheme.Text.Length;
+                    length += grapheme.Length;
                 }
 
                 if (length < input.Length)

tests/Avalonia.Base.UnitTests/Media/TextFormatting/GraphemeBreakClassTrieGeneratorTests.cs

@@ -40,9 +40,9 @@ namespace Avalonia.Base.UnitTests.Media.TextFormatting
 
             var enumerator = new GraphemeEnumerator(text);
 
-            enumerator.MoveNext();
+            enumerator.MoveNext(out var g);
 
-            var actual = enumerator.Current.Text;
+            var actual = text.AsSpan(g.Offset, g.Length);
 
             bool pass = actual.Length == grapheme.Length;
 
@@ -86,9 +86,9 @@ namespace Avalonia.Base.UnitTests.Media.TextFormatting
 
             var count = 0;
 
-            while (enumerator.MoveNext())
+            while (enumerator.MoveNext(out var grapheme))
             {
-                Assert.Equal(1, enumerator.Current.Text.Length);
+                Assert.Equal(1, grapheme.Length);
 
                 count++;
             }

tests/Avalonia.Skia.UnitTests/Media/TextFormatting/TextLayoutTests.cs

@@ -151,9 +151,10 @@ namespace Avalonia.Skia.UnitTests.Media.TextFormatting
 
                 while (true)
                 {
-                    while (inner.MoveNext())
+                    Grapheme grapheme;
+                    while (inner.MoveNext(out grapheme))
                     {
-                        j += inner.Current.Text.Length;
+                        j += grapheme.Length;
 
                         if (j + i > text.Length)
                         {
@@ -184,14 +185,14 @@ namespace Avalonia.Skia.UnitTests.Media.TextFormatting
                         }
                     }
 
-                    if (!outer.MoveNext())
+                    if (!outer.MoveNext(out grapheme))
                     {
                         break;
                     }
 
                     inner = new GraphemeEnumerator(text);
 
-                    i += outer.Current.Text.Length;
+                    i += grapheme.Length;
                 }
             }
 
@@ -979,13 +980,11 @@ namespace Avalonia.Skia.UnitTests.Media.TextFormatting
 
                 var graphemeEnumerator = new GraphemeEnumerator(text);
 
-                while (graphemeEnumerator.MoveNext())
+                while (graphemeEnumerator.MoveNext(out var grapheme))
                 {
-                    var grapheme = graphemeEnumerator.Current;
+                    var textStyleOverrides = new[] { new ValueSpan<TextRunProperties>(i, grapheme.Length, new GenericTextRunProperties(Typeface.Default, 12, foregroundBrush: Brushes.Red)) };
 
-                    var textStyleOverrides = new[] { new ValueSpan<TextRunProperties>(i, grapheme.Text.Length, new GenericTextRunProperties(Typeface.Default, 12, foregroundBrush: Brushes.Red)) };
-
-                    i += grapheme.Text.Length;
+                    i += grapheme.Length;
 
                     var layout = new TextLayout(
                         text,