feat: add expandable diff excerpts with configurable context

src/tui.rs

@@ -32,7 +32,7 @@ use unicode_segmentation::UnicodeSegmentation;
 use unicode_width::UnicodeWidthStr;
 
 use crate::{
-    diff::{ChangedFile, DiffState, FileStatus, Focus, Phase, RowKind},
+    diff::{ChangedFile, DiffState, DisplayRow, FileStatus, Focus, Phase, RowKind},
     editor::{
         Completion, Cursor, Editor, Hover, Input, Mode, Palette, Selection, StartupDiagnostic,
     },
@@ -1206,7 +1206,7 @@ fn draw_diff(
     }
     frame.render_widget(
         Paragraph::new(
-            "^E explorer · Tab focus · i filter · Enter open/toggle · ↑/↓ scroll/tree · ←/→ horizontal · Esc close",
+            "^E explorer · Tab focus · i filter · Enter open/toggle · ↑/↓ scroll/tree · [/] expand above/below · ←/→ horizontal · Esc close",
         )
         .style(Style::default().fg(Color::DarkGray)),
         help,
@@ -1253,14 +1253,16 @@ fn draw_diff_pane(
     );
     let focus = if before { Focus::Before } else { Focus::After };
     let content = diff_panel(frame, area, &title, diff.focus == focus);
-    if diff.rows.is_empty() {
+    if diff.display_rows.is_empty() {
         frame.render_widget(
-            Paragraph::new(
-                file.notice
-                    .as_deref()
-                    .map(display_text)
-                    .unwrap_or_else(|| "Empty file".into()),
-            ),
+            Paragraph::new(file.notice.as_deref().map(display_text).unwrap_or_else(|| {
+                if diff.rows.is_empty() {
+                    "Empty file"
+                } else {
+                    "No text changes"
+                }
+                .into()
+            })),
             content,
         );
         return;
@@ -1278,8 +1280,8 @@ fn draw_diff_pane(
         0
     };
     let width = usize::from(content.width).saturating_sub(gutter);
-    for (aligned_row, row) in diff
-        .rows
+    for (display_row, row) in diff
+        .display_rows
         .iter()
         .enumerate()
         .skip(diff.top)
@@ -1287,10 +1289,25 @@ fn draw_diff_pane(
     {
         let row_area = Rect::new(
             content.x,
-            content.y + (aligned_row - diff.top) as u16,
+            content.y + (display_row - diff.top) as u16,
             content.width,
             1,
         );
+        let aligned_row = match row {
+            DisplayRow::Line(index) => *index,
+            DisplayRow::Fold { start, end } => {
+                frame.render_widget(
+                    Paragraph::new(format!(
+                        "… {} unchanged lines hidden · [/] expand context",
+                        end - start
+                    ))
+                    .style(Style::default().fg(Color::DarkGray)),
+                    row_area,
+                );
+                continue;
+            }
+        };
+        let row = &diff.rows[aligned_row];
         let index = if before { row.before } else { row.after };
         let Some(index) = index else {
             let style = Style::default().fg(Color::Rgb(72, 72, 72)).bg(Color::Reset);
@@ -2546,6 +2563,72 @@ mod tests {
         );
     }
 
+    #[test]
+    fn diff_excerpts_render_original_lines_and_expand_without_losing_changes() {
+        let before: Vec<_> = (0..80)
+            .map(|index| format!("let row_{index:03} = {index};"))
+            .collect();
+        let mut after = before.clone();
+        after[30] = "let first_change = 900;".into();
+        after[60] = "let second_change = 901;".into();
+        let mut diff = diff_comparison(vec![ChangedFile {
+            path: "excerpts.rs".into(),
+            old_path: None,
+            status: FileStatus::Modified,
+            before,
+            after,
+            hunks: [30, 60]
+                .map(|index| crate::diff::Hunk {
+                    before_start: index,
+                    before_len: 1,
+                    after_start: index,
+                    after_len: 1,
+                })
+                .into(),
+            notice: None,
+        }]);
+        diff.set_context_options(1, 3);
+        diff.handle(Input::Ctrl('e'));
+        let mut terminal = Terminal::new(ratatui::backend::TestBackend::new(160, 24)).unwrap();
+        let mut panels = WheelPanels::default();
+        render_diff(&mut terminal, &diff, &mut panels);
+        let text = screen_text(&terminal);
+        for visible in [
+            "30  let row_029",
+            "31- let row_030",
+            "31+ let first_change",
+            "61+ let second_change",
+        ] {
+            assert!(text.contains(visible), "missing {visible}: {text}");
+        }
+        for hidden in ["row_028", "row_032", "row_058", "row_062"] {
+            assert!(!text.contains(hidden), "unexpected {hidden}: {text}");
+        }
+        let before_area = panels.diff.as_ref().unwrap().regions[1].0;
+        let after_area = panels.diff.as_ref().unwrap().regions[2].0;
+        let buffer = terminal.backend().buffer();
+        assert_eq!(
+            buffer[(before_area.x + 5, before_area.y + 3)].fg,
+            Color::Magenta
+        );
+        assert_eq!(
+            buffer[(before_area.x + 5, before_area.y + 3)].bg,
+            Color::Rgb(64, 24, 24)
+        );
+        assert_eq!(
+            buffer[(after_area.x + 5, after_area.y + 3)].bg,
+            Color::Rgb(24, 48, 32)
+        );
+        diff.handle(Input::Char('['));
+        diff.handle(Input::Home);
+        render_diff(&mut terminal, &diff, &mut panels);
+        let text = screen_text(&terminal);
+        assert!(text.contains("row_026"));
+        assert!(!text.contains("row_025"));
+        assert!(text.contains("first_change"));
+        assert!(text.contains("second_change"));
+    }
+
     #[test]
     fn diff_view_aligns_deletion_holes_numbers_colors_and_shared_vertical_scroll() {
         let mut diff = diff_comparison(vec![deletion_file()]);