Add "Go to Last Change" feature

Jumps to the first offset associated with the change at the top of
the Undo stack.  We generally jump to the code/data offset, not the
specific line affected.  It's possible to do better (and we do, for
Notes), but probably not worthwhile.

SourceGen/DisasmProject.cs

@@ -1830,6 +1830,20 @@ namespace SourceGen {
             }
         }
 
+        /// <summary>
+        /// Returns the change at the top of the list, i.e. the one that would be popped off
+        /// if we hit "undo".
+        /// </summary>
+        /// <returns>The change set.  The caller must not modify it.</returns>
+        public ChangeSet GetTopChange() {
+            Debug.Assert(mUndoList.Count > 0);
+            Debug.Assert(mUndoTop > 0);
+            return mUndoList[mUndoTop - 1];
+        }
+
+        /// <summary>
+        /// Generate undo/redo history, for the debug window.
+        /// </summary>
         public string DebugGetUndoRedoHistory() {
             StringBuilder sb = new StringBuilder();
             sb.Append("Bracketed change will be overwritten by next action\r\n\r\n");

SourceGen/MainController.cs

@@ -2570,6 +2570,58 @@ namespace SourceGen {
             return new NavStack.Location(offset, lineDelta, isNote);
         }
 
+        public void GotoLastChange() {
+            ChangeSet cs = mProject.GetTopChange();
+            Debug.Assert(cs.Count > 0);
+
+            // Get the offset from the first change in the set.  Ignore the rest.
+            UndoableChange uc = cs[0];
+            int offset;
+            bool isNote = false;
+            if (uc.HasOffset) {
+                offset = uc.Offset;
+
+                // If we altered a Note, and didn't remove it, jump to the note instead of
+                // the nearby code/data.
+                //
+                // TODO(someday): we can do similar things for comment edits, e.g. if it's
+                // SetLongComment we can find the line on which the comment starts and
+                // pass that as a line delta.
+                if (uc.Type == UndoableChange.ChangeType.SetNote &&
+                        uc.NewValue != null) {
+                    isNote = true;
+                }
+            } else if (uc.Type == UndoableChange.ChangeType.SetProjectProperties) {
+                // some chance it modified the EQU statements... jump there
+                offset = 0;
+            } else if (uc.Type == UndoableChange.ChangeType.SetTypeHint) {
+                TypedRangeSet newSet = (TypedRangeSet)uc.NewValue;
+                if (newSet.Count == 0) {
+                    // unexpected
+                    Debug.Assert(false);
+                    return;
+                }
+
+                // Get the offset of the first entry.
+                IEnumerator<TypedRangeSet.Tuple> iter =
+                    (IEnumerator<TypedRangeSet.Tuple>)newSet.GetEnumerator();
+                iter.MoveNext();
+                TypedRangeSet.Tuple firstOffset = iter.Current;
+                offset = firstOffset.Value;
+            } else {
+                Debug.Assert(false);
+                return;
+            }
+
+            if (isNote) {
+                GoToLocation(new NavStack.Location(offset, 0, true),
+                    GoToMode.JumpToNote, true);
+            } else {
+                GoToLocation(new NavStack.Location(offset, 0, false),
+                    GoToMode.JumpToCodeData, true);
+            }
+        }
+
         public bool CanNavigateBackward() {
             return mNavStack.HasBackward;
         }

SourceGen/RuntimeData/Help/end-notes.html

@@ -18,7 +18,9 @@
 school in the late 1980s, I read Don Lancaster's
 <i>Enhancing Your Apple II, Vol. 1</i> (available for download
 <a href="https://www.tinaja.com/ebksamp1.shtml">here</a>).  This
-included a very detailed methodology for disassembling 6502 software.
+included a very detailed methodology for disassembling 6502 software
+(nicely reformatted
+<a href="https://www.tinaja.com/ebooks/tearing_rework.pdf">here</a>).
 I wanted to give it a try, so I generated a monitor listing of an
 operating system (called "RDOS") that SSI used on their games, and
 printed it out on my Epson RX-80 -- tractor feed paper was helpful for

SourceGen/RuntimeData/Help/mainwin.html

@@ -372,11 +372,23 @@ with a '+'.  If you have a label that is also a valid hexadecimal
 address, like "FEED", the label takes precedence.  To jump to the address
 write "$FEED" instead.</p>
 
-<p>When you jump around, by double-clicking on an opcode or an entry
-in one of the side windows, the currently-selected line is added to
-a navigation stack.  You can use the arrows on the toolbar to navigate
-forward or backward, or Navigate &gt; Nav Forward and
-Navigate &gt; Nav Backward.  (You can use Alt+Left/Right Arrow, or
+<p>When you edit something, lines throughout the listing can change.  This
+is different from a source code editor, where editing a line just changes
+that line.  To allow you to watch the effects changes have, the undo/redo
+commands try to keep the listing in the same position.
+If you want to go to the place where the last change (i.e. the change
+that will be undone by the next Undo operation) was made,
+Navigate &gt; Go to Last Change will jump to the first offset
+associated with the most recent change.
+If the last change was to the project properties, it will jump to the
+first offset in the file.</p>
+
+<p>When you jump around, e.g. by double-clicking on an opcode or an entry
+in one of the side windows, the previously-selected line is added to
+a navigation stack.  You can use Navigate &gt; Nav Forward and
+Navigate &gt; Nav Backward to move forward and backward through the
+stack.  (The curly arrows on the left side of the toolbar may be more
+convenient.  You can use Alt+Left/Right Arrow, or
 Ctrl+- / Ctrl+Shift+-, as keyboard shortcuts.)</p>
 
 

SourceGen/UndoableChange.cs

@@ -206,6 +206,7 @@ namespace SourceGen {
         /// <returns>Change record.</returns>
         public static UndoableChange CreateTypeHintChange(TypedRangeSet undoSet,
                 TypedRangeSet newSet) {
+            Debug.Assert(undoSet != null && newSet != null);
             if (newSet.Count == 0) {
                 Debug.WriteLine("Empty hint change?");
             }

SourceGen/WpfGui/MainWindow.xaml

@@ -124,6 +124,7 @@ limitations under the License.
                     <KeyGesture>Ctrl+G</KeyGesture>
                 </RoutedUICommand.InputGestures>
             </RoutedUICommand>
+            <RoutedUICommand x:Key="GotoLastChangeCmd" Text="Go to Last Change"/>
             <RoutedUICommand x:Key="HintAsCodeEntryPointCmd" Text="Hint As Code Entry Point"/>
             <RoutedUICommand x:Key="HintAsDataStartCmd" Text="Hint As Data Start"/>
             <RoutedUICommand x:Key="HintAsInlineDataCmd" Text="Hint As Inline Data"/>
@@ -239,6 +240,8 @@ limitations under the License.
                         CanExecute="CanFormatAddressTable" Executed="FormatAddressTableCmd_Executed"/>
         <CommandBinding Command="{StaticResource GotoCmd}"
                         CanExecute="IsProjectOpen" Executed="GotoCmd_Executed"/>
+        <CommandBinding Command="{StaticResource GotoLastChangeCmd}"
+                        CanExecute="CanUndo" Executed="GotoLastChangeCmd_Executed"/>
         <CommandBinding Command="Help"
                         Executed="HelpCmd_Executed"/>
         <CommandBinding Command="{StaticResource HintAsCodeEntryPointCmd}"
@@ -345,6 +348,7 @@ limitations under the License.
                 <MenuItem Command="{StaticResource NavigateForwardCmd}"/>
                 <MenuItem Command="{StaticResource NavigateBackwardCmd}"/>
                 <MenuItem Command="{StaticResource GotoCmd}"/>
+                <MenuItem Command="{StaticResource GotoLastChangeCmd}"/>
                 <Separator/>
                 <MenuItem Command="Find"/>
                 <MenuItem Command="{StaticResource FindNextCmd}"/>

SourceGen/WpfGui/MainWindow.xaml.cs

@@ -1211,6 +1211,10 @@ namespace SourceGen.WpfGui {
             mMainCtrl.Goto();
         }
 
+        private void GotoLastChangeCmd_Executed(object sender, ExecutedRoutedEventArgs e) {
+            mMainCtrl.GotoLastChange();
+        }
+
         private void HelpCmd_Executed(object sender, ExecutedRoutedEventArgs e) {
             mMainCtrl.ShowHelp();
         }