[meld] dirdiff: Refactor comparison marker to use row references

[Kai Willadsen <kaiw src gnome org>]

commit a6338386042dda23d7e77d7a3f3e14f0afa400d4
Author: Kai Willadsen <kai willadsen gmail com>
Date:   Sat Mar 12 10:25:14 2022 +1000

    dirdiff: Refactor comparison marker to use row references
    
    The goal here is to have the marker survive tree modifications, and also
    to make the comparison that we do for rendering emblems somewhat more
    correct. Because handling TreeRowReferences is slightly more annoying
    than dealing with pure TreeIters, I've moved the marker to be a
    NamedTuple to make the rest of our logic appear slightly simpler.

 meld/dirdiff.py | 77 ++++++++++++++++++++++++++++++++++++++++++---------------
 1 file changed, 57 insertions(+), 20 deletions(-)
---
diff --git a/meld/dirdiff.py b/meld/dirdiff.py
index 5e8de5a2..36e71606 100644
--- a/meld/dirdiff.py
+++ b/meld/dirdiff.py
@@ -28,7 +28,7 @@ import unicodedata
 from collections import namedtuple
 from decimal import Decimal
 from mmap import ACCESS_COPY, mmap
-from typing import DefaultDict, List, Optional, Tuple
+from typing import DefaultDict, List, NamedTuple, Optional, Tuple
 
 from gi.repository import Gdk, Gio, GLib, GObject, Gtk
 
@@ -316,6 +316,49 @@ class CanonicalListing:
         return sorted(filled(v) for v in self.items.values())
 
 
+class ComparisonMarker(NamedTuple):
+    """A stable row + pane marker
+
+    This marker is used for selecting a specific file or folder when
+    the user wants to compare paths that don't have matching names, and
+    so aren't aligned in our tree view.
+    """
+
+    pane: int
+    row: Gtk.TreeRowReference
+
+    def get_iter(self) -> Gtk.TreeIter:
+        return self.row.get_model().get_iter(self.row.get_path())
+
+    def matches_iter(self, pane: int, it: Gtk.TreeIter) -> bool:
+        return (
+            pane == self.pane and
+            self.row.get_model().get_path(it) == self.row.get_path()
+        )
+
+    @classmethod
+    def from_selection(
+        cls,
+        treeview: Gtk.TreeView,
+        pane: int,
+    ) -> "ComparisonMarker":
+
+        if pane is None or pane == -1:
+            raise ValueError("Invalid pane for marker")
+
+        model = treeview.get_model()
+        _, selected_paths = treeview.get_selection().get_selected_rows()
+
+        # We'll assume that in any multi-select, the first row was the
+        # intended mark.
+        selected_row = Gtk.TreeRowReference.new(model, selected_paths[0])
+
+        return cls(
+            pane=pane,
+            row=selected_row,
+        )
+
+
 @Gtk.Template(resource_path='/org/gnome/meld/ui/dirdiff.ui')
 class DirDiff(Gtk.VBox, tree.TreeviewCommon, MeldDoc):
 
@@ -1278,9 +1321,9 @@ class DirDiff(Gtk.VBox, tree.TreeviewCommon, MeldDoc):
             self.set_action_enabled('folder-expand', is_single_foldable_row)
             self.set_action_enabled('folder-compare', True)
             self.set_action_enabled('folder-mark', True)
-            self.set_action_enabled('folder-compare-marked',
-                                    self.marked is not None and
-                                    self.marked.get('pane', -1) != pane)
+            self.set_action_enabled(
+                'folder-compare-marked',
+                self.marked is not None and self.marked.pane != pane)
             self.set_action_enabled('folder-delete', is_valid)
             self.set_action_enabled('folder-copy-left', is_valid and pane > 0)
             self.set_action_enabled(
@@ -1466,14 +1509,13 @@ class DirDiff(Gtk.VBox, tree.TreeviewCommon, MeldDoc):
         if selected is None:
             return
 
-        old_it = self.marked['mark'] if self.marked else None
-        it = self.model.get_iter(selected[0])
-
-        self.marked = {'mark': it, 'pane': pane}
+        old_mark_it = self.marked.get_iter() if self.marked else None
+        self.marked = ComparisonMarker.from_selection(
+            self.treeview[pane], pane)
 
-        self._update_item_state(it)
-        if old_it:
-            self._update_item_state(old_it)
+        self._update_item_state(self.marked.get_iter())
+        if old_mark_it:
+            self._update_item_state(old_mark_it)
 
     def action_diff_marked(self, *args):
         pane = self._get_focused_pane()
@@ -1484,13 +1526,13 @@ class DirDiff(Gtk.VBox, tree.TreeviewCommon, MeldDoc):
         if selected is None:
             return
 
-        mark, mark_pane = self.marked['mark'], self.marked['pane']
-        marked_path = self.model.value_paths(mark)[mark_pane]
+        mark_it = self.marked.get_iter()
+        marked_path = self.model.value_paths(mark_it)[self.marked.pane]
         selected_path = self.model.value_paths(selected)[pane]
 
         # Maintain the pane ordering in the new comparison, regardless
         # of which pane is the marked one.
-        if pane < mark_pane:
+        if pane < self.marked.pane:
             row_paths = [selected_path, marked_path]
         else:
             row_paths = [marked_path, selected_path]
@@ -1713,12 +1755,7 @@ class DirDiff(Gtk.VBox, tree.TreeviewCommon, MeldDoc):
                 self.model.set_path_state(
                     it, j, state, isdir[j], display_text=name_overrides[j])
 
-                if (
-                    self.marked and
-                    self.marked["pane"] == j and
-                    self.model.get_string_from_iter(self.marked["mark"]) ==
-                    self.model.get_string_from_iter(it)
-                ):
+                if self.marked and self.marked.matches_iter(j, it):
                     emblem = EMBLEM_SELECTED
                 else:
                     emblem = EMBLEM_NEW if j in newest else None