Cache the result of rendering backlog items

It's so much faster when getting lots of updates, especially with the
'Added client for' log storm at startup.

src/buffers/log.rs

@@ -25,12 +25,13 @@ use tracing_error::ErrorLayer;
 use tracing_subscriber::prelude::*;
 
 use super::{Buffer, BufferItem};
+use crate::widgets::Prerender;
 
 /// Maximum number of log lines to be stored in memory
 const MAX_MEM_LOG_LINES: usize = 100;
 
 pub struct LogBuffer {
-  lines: VecDeque<String>,
+  lines: VecDeque<(String, Prerender)>,
   receiver: UnboundedReceiver<String>,
 }
 
@@ -58,7 +59,7 @@ impl Buffer for LogBuffer {
     if self.lines.len() >= MAX_MEM_LOG_LINES {
       self.lines.pop_front();
     }
-    self.lines.push_back(line);
+    self.lines.push_back((line, Prerender::new()));
   }
 
   fn content<'a>(&'a self) -> Box<dyn Iterator<Item = BufferItem<'a>> + 'a> {
@@ -69,12 +70,12 @@ impl Buffer for LogBuffer {
         .into_iter()
         .chain(slice2.into_iter())
         .rev()
-        .cloned()
+        .map(|(line, prerender)| BufferItem {
-        .map(|line| BufferItem {
+          text: line.clone().into_text().unwrap_or_else(|e| {
-          text: line.into_text().unwrap_or_else(|e| {
             tracing::error!("Could not convert line from ANSI codes to ratatui: {}", e);
             Text::raw(line)
           }),
+          prerender,
         }),
     )
   }

src/buffers/mod.rs

@@ -14,6 +14,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+use crate::widgets::Prerender;
 use futures::stream::FuturesUnordered;
 use futures::StreamExt;
 use matrix_sdk::async_trait;
@@ -25,8 +26,9 @@ pub use log::LogBuffer;
 mod room;
 pub use room::RoomBuffer;
 
-pub struct BufferItem<'a> {
+pub struct BufferItem<'buf> {
-  pub text: Text<'a>,
+  pub text: Text<'buf>,
+  pub prerender: &'buf Prerender,
 }
 
 #[async_trait]

src/buffers/room.rs

@@ -33,11 +33,12 @@ use ratatui::text::Text;
 use smallvec::SmallVec;
 
 use super::{Buffer, BufferItem};
+use crate::widgets::Prerender;
 
 pub struct SingleClientRoomBuffer {
   room_id: OwnedRoomId,
   client: Client,
-  items: imbl::vector::Vector<String>,
+  items: imbl::vector::Vector<(String, Prerender)>,
   // TODO: get rid of this trait object, we know it's matrix_sdk_ui::timeline::TimelineStream
   stream: Box<dyn Stream<Item = Vec<VectorDiff<Arc<TimelineItem>>>> + Send + Sync + Unpin>,
   timeline: Arc<Timeline>,
@@ -54,7 +55,7 @@ impl SingleClientRoomBuffer {
     if let Some(changes) = self.stream.next().await {
       for change in changes {
         change
-          .map(|item| self.format_timeline_item(item))
+          .map(|item| (self.format_timeline_item(item), Prerender::new()))
           .apply(&mut self.items);
       }
     }
@@ -275,7 +276,7 @@ impl RoomBuffer {
       timeline: Arc::new(timeline),
       items: items // FIXME: it's always empty. why?
         .into_iter()
-        .map(|item| format!("Initial item: {:#?}", item))
+        .map(|item| (format!("Initial item: {:#?}", item), Prerender::new()))
         .collect(),
       stream: Box::new(stream),
       back_pagination_request: AtomicU16::new(0),
@@ -314,8 +315,9 @@ impl Buffer for RoomBuffer {
         .items
         .iter()
         .rev()
-        .map(|line| BufferItem {
+        .map(|(line, prerender)| BufferItem {
           text: Text::raw(line),
+          prerender,
         }),
     )
   }

src/components/backlog.rs

@@ -14,14 +14,18 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
-use super::Component;
+use std::ops::DerefMut;
+
 use color_eyre::eyre::{Result, WrapErr};
 use crossterm::event::{KeyCode, KeyEvent, KeyEventKind, KeyModifiers};
 use ratatui::{prelude::*, widgets::*};
 
 use crate::components::Action;
+use crate::widgets::prerender::{PrerenderInner, PrerenderValue};
 use crate::widgets::{BottomAlignedParagraph, OverlappableWidget};
 
+use super::Component;
+
 #[derive(Default)]
 pub struct Backlog {
   scroll: u64,
@@ -71,8 +75,25 @@ impl Component for Backlog {
       let Some(item) = items.next() else {
         break;
       };
-      let widget = BottomAlignedParagraph::new(item.text);
+      let expected_height = match item.prerender.0.lock().unwrap().deref_mut() {
+        Some(PrerenderInner {
+          key,
+          value: PrerenderValue::Rendered(buf),
+        }) if *key == text_area.width => buf.area().height as u64,
+        Some(PrerenderInner {
+          key,
+          value: PrerenderValue::NotRendered(height),
+        }) if *key == text_area.width => *height,
+        prerender => {
+          let widget = BottomAlignedParagraph::new(item.text.clone());
           let expected_height = widget.height(text_area.width);
+          *prerender = Some(PrerenderInner {
+            key: text_area.width,
+            value: PrerenderValue::NotRendered(expected_height),
+          });
+          expected_height
+        },
+      };
 
       if scroll.saturating_sub(expected_height) > text_area.height.into() {
         // Paragraph is too far down, not displayed
@@ -80,7 +101,8 @@ impl Component for Backlog {
         continue;
       }
 
-      let widget = widget.scroll(scroll);
+      // TODO: cache this
+      let widget = BottomAlignedParagraph::new(item.text).scroll(scroll);
       let height = widget.render_overlap(text_area, frame.buffer_mut());
       text_area.height = text_area.height.saturating_sub(height);
 
@@ -96,8 +118,59 @@ impl Component for Backlog {
 
     // Render other widgets
     for item in items {
+      let height = match item.prerender.0.lock().unwrap().deref_mut() {
+        Some(PrerenderInner {
+          key,
+          value: PrerenderValue::Rendered(buf),
+        }) if *key == text_area.width => {
+          // We already rendered it, copy the buffer.
+          assert_eq!(buf.area().width, text_area.width);
+          for (y, line) in buf
+            .content()
+            .chunks(u16::min(buf.area().width, text_area.width) as usize)
+            .enumerate()
+            .skip(buf.area().height.saturating_sub(text_area.height) as usize)
+          {
+            for (x, cell) in line.into_iter().enumerate() {
+              *frame.buffer_mut().get_mut(
+                text_area.x + (x as u16),
+                text_area.y + text_area.height + (y as u16) - buf.area().height,
+              ) = cell.clone();
+            }
+          }
+
+          buf.area().height
+        },
+        prerender => {
           let widget = BottomAlignedParagraph::new(item.text);
           let height = widget.render_overlap(text_area, frame.buffer_mut());
+
+          // Copy the drawn result to a buffer for caching
+          let mut buf = ratatui::buffer::Buffer::empty(Rect {
+            x: 0,
+            y: 0,
+            width: text_area.width,
+            height,
+          });
+          for y in 0..height {
+            // TODO: only copy the width actually drawn by the widget
+            for x in 0..text_area.width {
+              *buf.get_mut(x, y) = frame
+                .buffer_mut()
+                .get(text_area.x + x, text_area.y + text_area.height + y - height)
+                .clone()
+            }
+          }
+
+          *prerender = Some(PrerenderInner {
+            key: text_area.width,
+            value: PrerenderValue::Rendered(buf),
+          });
+
+          height
+        },
+      };
+
       assert!(area.height >= height, "{:?} {}", area, height);
       text_area.height = text_area.height.saturating_sub(height); // Remove lines at the bottom used by this paragraph
       if text_area.height == 0 {

src/widgets/mod.rs

@@ -20,6 +20,9 @@ use ratatui::widgets::Widget;
 mod bottom_aligned_paragraph;
 pub use bottom_aligned_paragraph::BottomAlignedParagraph;
 
+pub(crate) mod prerender;
+pub use prerender::Prerender;
+
 #[rustfmt::skip] // reflow is vendored from ratatui, let's avoid changes
 mod reflow;
 

src/widgets/prerender.rs

@@ -0,0 +1,39 @@
+/*
+ * Copyright (C) 2023  Valentin Lorentz
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License version 3,
+ * as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+use std::sync::{Arc, Mutex};
+
+/// Storage for the result of pre-computations by the UI, with interior mutability
+#[derive(Clone)]
+pub(crate) struct PrerenderInner {
+  pub(crate) key: u16, // width
+  pub(crate) value: PrerenderValue,
+}
+
+#[derive(Clone)]
+pub(crate) enum PrerenderValue {
+  Rendered(ratatui::buffer::Buffer),
+  NotRendered(u64), // Height only. The widget was not rendered because off-screen.
+}
+
+#[derive(Clone)]
+pub struct Prerender(pub(crate) Arc<Mutex<Option<PrerenderInner>>>);
+
+impl Prerender {
+  pub fn new() -> Prerender {
+    Prerender(Arc::new(Mutex::new(None)))
+  }
+}