fix: last line change may disable hunk operation in two-side diff

If the last line is deleted or added, it was impossible to stage,
unstage or discard the last hunk in a two-side diff view, if the hunk
was selected on the side where the last line is missing (that's the left
side for added lines and the right side for deleted lines).

Since the button(s) for hunk operations are on the right side of the
diff view, it was easier to trigger this bug for a deleted last line.

This bug was introduced in commit 6511d15 while rewriting the text
diff views.

This fixes the bug mentioned in
#1950 (comment).

Author: sina-hide

src/ViewModels/TextDiffContext.cs

@@ -191,25 +191,32 @@ public TwoSideTextDiff(Models.DiffOption option, Models.TextDiff diff, TextDiffC
             _option = option;
             _data = diff;
 
-            foreach (var line in diff.Lines)
+            var sourceIndices = new int[diff.Lines.Count];
+
+            for (var sourceIndex = 0; sourceIndex < diff.Lines.Count; sourceIndex++)
             {
+                var line = diff.Lines[sourceIndex];
                 switch (line.Type)
                 {
                     case Models.TextDiffLineType.Added:
+                        sourceIndices[New.Count] = sourceIndex;
                         New.Add(line);
                         break;
                     case Models.TextDiffLineType.Deleted:
+                        sourceIndices[Old.Count] = sourceIndex;
                         Old.Add(line);
                         break;
                     default:
                         FillEmptyLines();
+                        sourceIndices[Old.Count] = sourceIndex;
                         Old.Add(line);
                         New.Add(line);
                         break;
                 }
             }
 
             FillEmptyLines();
+            _sourceIndices = ResizeSourceIndices(sourceIndices);
             TryKeepPrevState(previous, Old);
         }
 
@@ -223,43 +230,9 @@ public override TextDiffContext SwitchMode()
             return new CombinedTextDiff(_option, _data, this);
         }
 
-        public void ConvertsToCombinedRange(ref int startLine, ref int endLine, bool isOldSide)
+        public int ConvertToCombined(int line)
         {
-            endLine = Math.Min(endLine, _data.Lines.Count - 1);
-
-            var oneSide = isOldSide ? Old : New;
-            var firstContentLine = -1;
-            for (int i = startLine; i <= endLine; i++)
-            {
-                var line = oneSide[i];
-                if (line.Type != Models.TextDiffLineType.None)
-                {
-                    firstContentLine = i;
-                    break;
-                }
-            }
-
-            if (firstContentLine < 0)
-                return;
-
-            var endContentLine = -1;
-            for (int i = Math.Min(endLine, oneSide.Count - 1); i >= startLine; i--)
-            {
-                var line = oneSide[i];
-                if (line.Type != Models.TextDiffLineType.None)
-                {
-                    endContentLine = i;
-                    break;
-                }
-            }
-
-            if (endContentLine < 0)
-                return;
-
-            var firstContent = oneSide[firstContentLine];
-            var endContent = oneSide[endContentLine];
-            startLine = _data.Lines.IndexOf(firstContent);
-            endLine = _data.Lines.IndexOf(endContent);
+            return 0 <= line && line < _sourceIndices.Length ? _sourceIndices[line] : line;
         }
 
         private void FillEmptyLines()
@@ -277,5 +250,19 @@ private void FillEmptyLines()
                     New.Add(new Models.TextDiffLine());
             }
         }
+
+        private int[] ResizeSourceIndices(int[] sourceIndices)
+        {
+            var length = Old.Count; // same as `New.Count`
+
+            if (length == sourceIndices.Length)
+                return sourceIndices;
+
+            var resized = new int[length];
+            Array.Copy(sourceIndices, resized, length);
+            return resized;
+        }
+
+        private readonly int[] _sourceIndices;
     }
 }

src/Views/TextDiffView.axaml.cs

@@ -1183,7 +1183,8 @@ protected override void UpdateSelectedChunk(double y)
                     endLine.GetTextLineVisualYPosition(endLine.TextLines[^1], VisualYPosition.TextBottom) - view.VerticalOffset :
                     view.Bounds.Height;
 
-                diff.ConvertsToCombinedRange(ref startIdx, ref endIdx, IsOld);
+                startIdx = diff.ConvertToCombined(startIdx);
+                endIdx = diff.ConvertToCombined(endIdx);
                 TrySetChunk(new(rectStartY, rectEndY - rectStartY, startIdx, endIdx, false, IsOld));
             }
             else
@@ -1229,7 +1230,8 @@ protected override void UpdateSelectedChunk(double y)
                     endLine.GetTextLineVisualYPosition(endLine.TextLines[^1], VisualYPosition.TextBottom) - view.VerticalOffset :
                     view.Bounds.Height;
 
-                diff.ConvertsToCombinedRange(ref startIdx, ref endIdx, IsOld);
+                startIdx = diff.ConvertToCombined(startIdx);
+                endIdx = diff.ConvertToCombined(endIdx);
                 TrySetChunk(new(rectStartY, rectEndY - rectStartY, startIdx, endIdx, true, false));
             }
         }