Fix http://community.sharpdevelop.net/forums/t/11817.aspx - NotImplementedException when word-wrapping bidirectional text

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/Rendering/VisualLineElement.cs

@@ -80,6 +80,15 @@ namespace ICSharpCode.AvalonEdit.Rendering
 		/// </param>
 		public abstract TextRun CreateTextRun(int startVisualColumn, ITextRunConstructionContext context);
 		
+		/// <summary>
+		/// Retrieves the text span immediately before the visual column.
+		/// </summary>
+		/// <remarks>This method is used for word-wrapping in bidirectional text.</remarks>
+		public virtual TextSpan<CultureSpecificCharacterBufferRange> GetPrecedingText(int visualColumnLimit, ITextRunConstructionContext context)
+		{
+			return null;
+		}
+		
 		/// <summary>
 		/// Gets if this VisualLineElement can be split.
 		/// </summary>

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/Rendering/VisualLineText.cs

@@ -57,6 +57,18 @@ namespace ICSharpCode.AvalonEdit.Rendering
 			return new TextCharacters(text, 0, text.Length, this.TextRunProperties);
 		}
 		
+		/// <inheritdoc/>
+		public override TextSpan<CultureSpecificCharacterBufferRange> GetPrecedingText(int visualColumnLimit, ITextRunConstructionContext context)
+		{
+			if (context == null)
+				throw new ArgumentNullException("context");
+			
+			int relativeOffset = visualColumnLimit - VisualColumn;
+			string text = context.Document.GetText(context.VisualLine.FirstDocumentLine.Offset + RelativeTextOffset, relativeOffset);
+			CharacterBufferRange range = new CharacterBufferRange(text, 0, text.Length);
+			return new TextSpan<CultureSpecificCharacterBufferRange>(range.Length, new CultureSpecificCharacterBufferRange(this.TextRunProperties.CultureInfo, range));
+		}
+		
 		/// <inheritdoc/>
 		public override bool CanSplit {
 			get { return true; }

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/Rendering/VisualLineTextSource.cs

@@ -56,12 +56,31 @@ namespace ICSharpCode.AvalonEdit.Rendering
 		
 		public override TextSpan<CultureSpecificCharacterBufferRange> GetPrecedingText(int textSourceCharacterIndexLimit)
 		{
-			throw new NotImplementedException();
+			try {
+				foreach (VisualLineElement element in VisualLine.Elements) {
+					if (textSourceCharacterIndexLimit > element.VisualColumn
+					    && textSourceCharacterIndexLimit <= element.VisualColumn + element.VisualLength)
+					{
+						TextSpan<CultureSpecificCharacterBufferRange> span = element.GetPrecedingText(textSourceCharacterIndexLimit, this);
+						if (span == null)
+							break;
+						int relativeOffset = textSourceCharacterIndexLimit - element.VisualColumn;
+						if (span.Length > relativeOffset)
+							throw new ArgumentException("The returned TextSpan is too long.", element.GetType().Name + ".GetPrecedingText");
+						return span;
+					}
+				}
+				CharacterBufferRange empty = CharacterBufferRange.Empty;
+				return new TextSpan<CultureSpecificCharacterBufferRange>(empty.Length, new CultureSpecificCharacterBufferRange(null, empty));
+			} catch (Exception ex) {
+				Debug.WriteLine(ex.ToString());
+				throw;
+			}
 		}
 		
 		public override int GetTextEffectCharacterIndexFromTextSourceCharacterIndex(int textSourceCharacterIndex)
 		{
-			throw new NotImplementedException();
+			throw new NotSupportedException();
 		}
 	}
 }