feat: add diff view component

crates/editor/tests/diff_view_bench.rs

@@ -0,0 +1,109 @@
+use std::cell::RefCell;
+use std::rc::Rc;
+use std::time::{Duration, Instant};
+
+use gpui::{
+    AppContext as _, Context, Entity, IntoElement, Modifiers, ParentElement as _, Render,
+    ScrollDelta, ScrollEvent, Styled as _, TestAppContext, VisualTestContext, Window, div, point,
+    px,
+};
+use gpui_native_editor::diff_view::DiffViewState;
+
+/// Deterministic old/new pairs of roughly `lines` lines each, with changes
+/// sprinkled through.
+fn large_diff_pair(lines: usize) -> (String, String) {
+    let mut old = String::with_capacity(lines * 48);
+    let mut new = String::with_capacity(lines * 48);
+    let mut index = 0usize;
+    while old.lines().count() < lines {
+        let i = index;
+        let comment = format!("// Handler {i} in the generated dispatch table.\n");
+        old.push_str(&comment);
+        new.push_str(&comment);
+        old.push_str(&format!("fn handle_{i}(payload: &str) -> bool {{\n"));
+        new.push_str(&format!("fn handle_{i}(payload: &Payload) -> bool {{\n"));
+        old.push_str(&format!("    payload.len() < {i} + 16\n"));
+        new.push_str(&format!(
+            "    payload.size() < {i} + 16 && !payload.is_empty()\n"
+        ));
+        if i.is_multiple_of(5) {
+            old.push_str(&format!("    log::debug(\"handler {i} ran\");\n"));
+        }
+        if i.is_multiple_of(7) {
+            new.push_str("    metrics::count(\"handle\");\n");
+        }
+        old.push_str("}\n\n");
+        new.push_str("}\n\n");
+        index += 1;
+    }
+    (old, new)
+}
+
+struct Host {
+    diff: Entity<DiffViewState>,
+}
+
+impl Render for Host {
+    fn render(&mut self, _window: &mut Window, _cx: &mut Context<Self>) -> impl IntoElement {
+        div().size_full().child(self.diff.clone())
+    }
+}
+
+#[test]
+fn scrolling_a_large_diff_is_not_pathologically_slow() {
+    let mut cx = TestAppContext::single();
+    cx.update(gpui_native_theme::Theme::init);
+
+    let (old, new) = large_diff_pair(5000);
+    let started = Instant::now();
+    let state = DiffViewState::new(
+        "bench-diff",
+        &old,
+        &new,
+        gpui_native_editor::highlight::Language::Rust,
+    );
+    let diff_elapsed = started.elapsed();
+
+    let state_cell: Rc<RefCell<Option<Entity<DiffViewState>>>> = Rc::new(RefCell::new(None));
+    let cell = state_cell.clone();
+    let window = cx.add_window(move |_, cx| {
+        let diff = cx.new(|_| state);
+        cell.borrow_mut().replace(diff.clone());
+        Host { diff }
+    });
+    let state = state_cell.borrow().clone().expect("state captured");
+    let mut visual = VisualTestContext::from_window(window.into(), &cx);
+    visual.update(|window, cx| {
+        window.draw(cx).clear(cx);
+    });
+
+    let row_count = cx.update(|cx| state.read(cx).rows().len());
+    assert!(row_count > 5000, "changes add rows on top of the context");
+
+    let started = Instant::now();
+    for _ in 0..30 {
+        visual.simulate_event(ScrollEvent {
+            position: point(px(300.), px(300.)),
+            delta: ScrollDelta::Pixels(point(px(0.), px(-240.))),
+            modifiers: Modifiers::default(),
+            ..Default::default()
+        });
+        visual.update(|window, cx| {
+            window.draw(cx).clear(cx);
+        });
+    }
+    let scroll_elapsed = started.elapsed();
+
+    println!(
+        "diffing and highlighting into {row_count} rows took {diff_elapsed:?}; 30 wheel frames took {scroll_elapsed:?} ({:?} per frame)",
+        scroll_elapsed / 30
+    );
+    assert!(
+        diff_elapsed < Duration::from_secs(3),
+        "diffing took {diff_elapsed:?}"
+    );
+    assert!(
+        scroll_elapsed < Duration::from_secs(2),
+        "scrolling 30 frames took {scroll_elapsed:?}"
+    );
+}