feat: highlight character-level changes in diff panes

src/diff.rs

@@ -154,6 +154,8 @@ pub struct AlignedRow {
     pub before: Option<usize>,
     pub after: Option<usize>,
     pub kind: RowKind,
+    /// Changed grapheme ranges, paired as [before, after]; empty on unpaired rows.
+    pub changes: Vec<[std::ops::Range<usize>; 2]>,
 }
 
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
@@ -958,6 +960,7 @@ impl DiffState {
                 lines,
                 changed_lines,
             );
+            refresh_character_changes(&mut self.rows, file, changed_lines.0..changed_lines.2);
             self.left_after = self.left_after.min(self.max_after);
             self.live_revision = Some(revision);
             return;
@@ -1150,8 +1153,10 @@ impl DiffState {
                 before: None,
                 after: Some(after_len),
                 kind: RowKind::Equal,
+                changes: Vec::new(),
             });
         }
+        refresh_character_changes(&mut self.rows, file, 0..file.after.len());
         self.reset_excerpts();
     }
 
@@ -1310,10 +1315,43 @@ fn append_rows(
             before,
             after,
             kind,
+            changes: Vec::new(),
         });
     }
 }
 
+fn refresh_character_changes(
+    rows: &mut [AlignedRow],
+    file: &ChangedFile,
+    after_rows: std::ops::Range<usize>,
+) {
+    for row in rows {
+        if row.kind == RowKind::Changed
+            && let Some(after) = row.after.filter(|index| after_rows.contains(index))
+        {
+            row.changes = character_changes(
+                &file.before[row.before.expect("paired changed row")],
+                &file.after[after],
+            );
+        }
+    }
+}
+
+fn character_changes(before: &str, after: &str) -> Vec<[std::ops::Range<usize>; 2]> {
+    if before == after {
+        return Vec::new();
+    }
+    let before: Vec<_> = before.graphemes(true).collect();
+    let after: Vec<_> = after.graphemes(true).collect();
+    similar::capture_diff_slices(similar::Algorithm::Myers, &before, &after)
+        .into_iter()
+        .filter_map(|op| {
+            let (tag, before, after) = op.as_tag_tuple();
+            (tag != similar::DiffTag::Equal).then_some([before, after])
+        })
+        .collect()
+}
+
 fn line_width(line: &str) -> usize {
     #[cfg(test)]
     tests::WIDTH_SCANS.with(|scans| scans.set(scans.get() + 1));
@@ -1502,6 +1540,7 @@ mod tests {
                 } else {
                     RowKind::Equal
                 },
+                changes: Vec::new(),
             })
             .collect();
         state.reset_excerpts();
@@ -2631,6 +2670,26 @@ mod tests {
         state
     }
 
+    #[test]
+    fn character_ranges_follow_insertions_deletions_unicode_and_live_edits() {
+        for (before, after, expected) in [
+            ("ab", "a界b", vec![[1..1, 1..2]]),
+            ("a界b", "ab", vec![[1..2, 1..1]]),
+            ("xe\u{301}👩‍🚀z", "xe\u{300}👩‍🔬z", vec![[1..3, 1..3]]),
+        ] {
+            let state = live_state(vec![before.into()], vec![after.into()], false);
+            assert_eq!(state.rows[0].changes, expected);
+        }
+        let mut state = live_state(vec!["abc".into()], vec!["aXbc".into()], false);
+        state.refresh_after(&["aXYbc".into()], 1);
+        assert_eq!(state.rows[0].changes, vec![[1..1, 1..3]]);
+        state.refresh_after(&["abYZc".into()], 2);
+        assert_eq!(state.rows[0].changes, vec![[2..2, 2..4]]);
+        state.refresh_after(&["abc".into()], 3);
+        assert_eq!(state.rows[0].kind, RowKind::Equal);
+        assert!(state.rows[0].changes.is_empty());
+    }
+
     #[test]
     fn reverting_loaded_file_keeps_unloaded_changes_selectable() {
         let mut state = live_state(vec!["base".into()], vec!["changed".into()], false);