Fix diff algorithm

UI/ViewModels/PatchDialogViewModel.cs

@@ -372,40 +372,30 @@ namespace pEp.UI.ViewModels
             List<DiffPiece> newLines = sideBySideDiffModel.NewText.Lines;
             List<DiffPiece> oldLines = sideBySideDiffModel.OldText.Lines;
             List<string> block = new List<string>();
-            int unchangedCount = 0;
             bool newBlock = true;
             int oldFileIndex = 0, newFileIndex = 0;
 
             for (int i = 0; i < newLines.Count; i++)
             {
                 // Add lines as necessary
-                if ((newLines[i].Type == ChangeType.Unchanged) &&
-                    (block.Count > 0))
-                {
-                    block.Add(" " + newLines[i].Text);
-                    unchangedCount++;
-                }
-                else if (newLines[i].Type == ChangeType.Imaginary)
+                if (newLines[i].Type == ChangeType.Imaginary)
                 {
                     block.Add("-" + oldLines[i].Text);
-                    unchangedCount = 0;
                 }
                 else if (newLines[i].Type == ChangeType.Inserted)
                 {
                     block.Add("+" + newLines[i].Text);
-                    unchangedCount = 0;
                 }
                 else if (newLines[i].Type == ChangeType.Modified)
                 {
                     block.Add("-" + oldLines[i].Text);
                     block.Add("+" + newLines[i].Text);
-                    unchangedCount = 0;
                 }
 
                 // If a block is being constructed, add additional parts as necessary
                 if (block.Count > 0)
                 {
-                    // If we have a new block, add the three preceding unchanged lines
+                    // First, add the three preceding unchanged lines (if available)
                     if (newBlock)
                     {
                         newBlock = false;
@@ -425,14 +415,32 @@ namespace pEp.UI.ViewModels
                         newFileIndex = newLines[index].Position ?? 1;
                         oldFileIndex = oldLines[index].Position ?? 1;
                     }
-                    else if ((unchangedCount > 2) || (i == newLines.Count - 1))
+
+                    // Then, add the three following lines (if available)
+                    int counter = 0;
+                    while ((++i < newLines.Count) &&
+                           (newLines[i].Type == ChangeType.Unchanged))
+                    {
+                        if (counter++ < 3)
+                        {
+                            block.Add(" " + newLines[i].Text);
+                        }
+                        else
+                        {
+                            break;
+                        }
+                    }
+
+                    // Close block
+                    if ((counter > 3) ||
+                        (i >= newLines.Count))
                     {
                         // Once a block is complete, add descriptor at the beginning and add lines to diff
                         int oldFileLineCount = block.Count(s => !s.StartsWith("+"));
                         int newFileLineCount = block.Count(s => !s.StartsWith("-"));
                         diff += $"@@ -{ oldFileIndex },{ oldFileLineCount } +{ newFileIndex },{ newFileLineCount } @@\n";
 
-                        foreach (var line in block)
+                        foreach (string line in block)
                         {
                             diff += line + '\n';
                         }
@@ -440,6 +448,8 @@ namespace pEp.UI.ViewModels
                         block = new List<string>();
                         newBlock = true;
                     }
+
+                    i--;
                 }
             }