Move buffer copy to its own function, it's more readable

src/components/backlog.rs

@@ -124,20 +124,24 @@ impl Component for Backlog {
           value: PrerenderValue::Rendered(buf),
         }) if *key == text_area.width => {
           // We already rendered it, copy the buffer.
-          assert_eq!(buf.area().width, text_area.width);
+          assert_eq!(text_area.width, buf.area.width);
-          for (y, line) in buf
+          let top_padding = text_area.height.saturating_sub(buf.area.height);
-            .content()
+          let skip_top_lines = buf.area.height.saturating_sub(text_area.height);
-            .chunks(u16::min(buf.area().width, text_area.width) as usize)
+          copy_buffer(
-            .enumerate()
+            buf,
-            .skip(buf.area().height.saturating_sub(text_area.height) as usize)
+            Rect {
-          {
+              x: 0,
-            for (x, cell) in line.into_iter().enumerate() {
+              y: 0,
-              *frame.buffer_mut().get_mut(
+              width: text_area.width,
-                text_area.x + (x as u16),
+              height: buf.area.height - skip_top_lines,
-                text_area.y + text_area.height + (y as u16) - buf.area().height,
+            },
-              ) = cell.clone();
+            frame.buffer_mut(),
-            }
+            Rect {
-          }
+              y: text_area.y + top_padding,
+              height: text_area.height - top_padding,
+              ..text_area
+            },
+          );
 
           buf.area().height
         },
@@ -145,28 +149,37 @@ impl Component for Backlog {
           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
+          // If the whole widget fits in the text_area, copy the drawn result to a buffer
-          let mut buf = ratatui::buffer::Buffer::empty(Rect {
+          // for caching
-            x: 0,
+          if height <= text_area.height {
-            y: 0,
+            let mut buf = ratatui::buffer::Buffer::empty(Rect {
-            width: text_area.width,
+              x: 0,
-            height,
+              y: 0,
-          });
+              width: text_area.width,
-          for y in 0..height {
+              height,
-            // TODO: only copy the width actually drawn by the widget
+            });
-            for x in 0..text_area.width {
+            let top_padding = text_area.height.saturating_sub(buf.area.height);
-              *buf.get_mut(x, y) = frame
+            copy_buffer(
-                .buffer_mut()
+              frame.buffer_mut(),
-                .get(text_area.x + x, text_area.y + text_area.height + y - height)
+              Rect {
-                .clone()
+                y: text_area.y + top_padding,
-            }
+                height: text_area.height - top_padding,
+                ..text_area
+              },
+              &mut buf,
+              Rect {
+                x: 0,
+                y: 0,
+                width: text_area.width, // TODO: only copy the width actually drawn by the widget
+                height: height,
+              },
+            );
+            *prerender = Some(PrerenderInner {
+              key: text_area.width,
+              value: PrerenderValue::Rendered(buf),
+            });
           }
 
-          *prerender = Some(PrerenderInner {
-            key: text_area.width,
-            value: PrerenderValue::Rendered(buf),
-          });
-
           height
         },
       };
@@ -185,3 +198,44 @@ impl Component for Backlog {
     Ok(())
   }
 }
+
+fn copy_buffer(src: &Buffer, src_area: Rect, dst: &mut Buffer, dst_area: Rect) {
+  assert_eq!(src_area.width, dst_area.width, "width mismatch");
+  assert_eq!(src_area.height, dst_area.height, "height mismatch");
+
+  assert!(
+    src_area.x + src_area.width <= src.area.width,
+    "src width overflow"
+  );
+  assert!(
+    src_area.y + src_area.height <= src.area.height,
+    "src height overflow"
+  );
+  assert!(
+    dst_area.x + dst_area.width <= dst.area.width,
+    "dst width overflow"
+  );
+  assert!(
+    dst_area.y + dst_area.height <= dst.area.height,
+    "dst height overflow"
+  );
+
+  for (y, line) in src
+    .content()
+    .chunks(src.area.width as usize)
+    .skip(src_area.y as usize)
+    .take(dst_area.height as usize)
+    .enumerate()
+  {
+    for (x, cell) in line
+      .into_iter()
+      .skip(src_area.x as usize)
+      .take(src_area.width as usize)
+      .enumerate()
+    {
+      *dst.get_mut(dst_area.x + (x as u16), dst_area.y + (y as u16)) = src
+        .get(src_area.x + (x as u16), src_area.y + (y as u16))
+        .clone();
+    }
+  }
+}