[Text] Clone CodePoint ReadAt for Span

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

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.Runtime.CompilerServices;
 
 namespace Avalonia.Media.TextFormatting.Unicode
@@ -222,6 +223,71 @@ namespace Avalonia.Media.TextFormatting.Unicode
 
             return new Codepoint(code);
         }
+        
+        /// <summary>
+        /// Reads the <see cref="Codepoint"/> at specified position.
+        /// </summary>
+        /// <param name="text">The buffer to read from.</param>
+        /// <param name="index">The index to read at.</param>
+        /// <param name="count">The count of character that were read.</param>
+        /// <returns></returns>
+        public static Codepoint ReadAt(ReadOnlySpan<char> text, int index, out int count)
+        {
+            count = 1;
+
+            if (index >= text.Length)
+            {
+                return ReplacementCodepoint;
+            }
+
+            var code = text[index];
+
+            ushort hi, low;
+
+            //# High surrogate
+            if (0xD800 <= code && code <= 0xDBFF)
+            {
+                hi = code;
+
+                if (index + 1 == text.Length)
+                {
+                    return ReplacementCodepoint;
+                }
+
+                low = text[index + 1];
+
+                if (0xDC00 <= low && low <= 0xDFFF)
+                {
+                    count = 2;
+                    return new Codepoint((uint)((hi - 0xD800) * 0x400 + (low - 0xDC00) + 0x10000));
+                }
+
+                return ReplacementCodepoint;
+            }
+
+            //# Low surrogate
+            if (0xDC00 <= code && code <= 0xDFFF)
+            {
+                if (index == 0)
+                {
+                    return ReplacementCodepoint;
+                }
+
+                hi = text[index - 1];
+
+                low = code;
+
+                if (0xD800 <= hi && hi <= 0xDBFF)
+                {
+                    count = 2;
+                    return new Codepoint((uint)((hi - 0xD800) * 0x400 + (low - 0xDC00) + 0x10000));
+                }
+
+                return ReplacementCodepoint;
+            }
+
+            return new Codepoint(code);
+        }
 
         /// <summary>
         /// Returns <see langword="true"/> if <paramref name="cp"/> is between

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

@@ -30,7 +30,7 @@ namespace Avalonia.Media.TextFormatting.Unicode
                 return false;
             }
 
-            Current = Codepoint.ReadAt(_text, 0, out var count);
+            Current = Codepoint.ReadAt(_text.Span, 0, out var count);
 
             _text = _text.Skip(count);
 

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

@@ -245,7 +245,7 @@ namespace Avalonia.Media.TextFormatting.Unicode
 
                     if (CurrentCodeUnitOffset < _buffer.Length)
                     {
-                        CurrentCodepoint = Codepoint.ReadAt(_buffer, CurrentCodeUnitOffset,
+                        CurrentCodepoint = Codepoint.ReadAt(_buffer.Span, CurrentCodeUnitOffset,
                             out _codeUnitLengthOfCurrentScalar);
                     }
                     else

tests/Avalonia.UnitTests/MockTextShaperImpl.cs

@@ -18,7 +18,7 @@ namespace Avalonia.UnitTests
             {
                 var glyphCluster = i + text.OffsetToFirstChar;
 
-                var codepoint = Codepoint.ReadAt(characterBufferRange, i, out var count);
+                var codepoint = Codepoint.ReadAt(characterBufferRange.Span, i, out var count);
 
                 var glyphIndex = typeface.GetGlyph(codepoint);