micro-opt scroll_up some more

README.md

@@ -141,7 +141,7 @@ set to 80x24 with 3500 lines of scrollback.  `alacritty` has no scrollback.
 | xterm | 9.863 |
 | Gnome Terminal | 2.391 |
 | Terminator 1.91 | 2.319 |
-| wezterm | 1.033 |
+| wezterm | 0.996 |
 | kitty | 0.899 |
 | urxvt | 0.615 |
 | alacritty | 0.421 |

term/src/cell.rs

@@ -134,7 +134,12 @@ pub struct Cell {
 
 impl Default for Cell {
     fn default() -> Cell {
-        Cell::from_char(' ', &CellAttributes::default())
+        let bytes = [b' ', 0, 0, 0, 0, 0, 0];
+        Cell {
+            len: 1,
+            bytes,
+            attrs: CellAttributes::default()
+        }
     }
 }
 
@@ -185,6 +190,13 @@ impl Cell {
             self.str().width()
         }
     }
+
+    #[inline]
+    pub fn reset(&mut self) {
+        self.len = 1;
+        self.bytes[0] = b' ';
+        self.attrs = CellAttributes::default();
+    }
 }
 
 impl From<char> for Cell {

term/src/line.rs

@@ -58,6 +58,14 @@ impl Line {
         }
     }
 
+    pub fn reset(&mut self, width: usize) {
+        let blank = Cell::default();
+        self.cells.resize(width, blank);
+        for cell in self.cells.iter_mut() {
+            cell.reset();
+        }
+    }
+
     /// Recompose line into the corresponding utf8 string.
     /// In the future, we'll want to decompose into clusters of Cells that share
     /// the same render attributes

term/src/screen.rs

@@ -112,11 +112,16 @@ impl Screen {
         }
 
         let width = line.cells.len();
+        let cell = Cell::from_char(c, attr);
+        if x == width {
+            line.cells.push(cell);
+        } else if x > width {
             // if the line isn't wide enough, pad it out with the default attributes
-        if x >= width {
+            line.cells.resize(x, Cell::default());
-            line.cells.resize(x + 1, Cell::default());
+            line.cells.push(cell);
+        } else {
+            line.cells[x] = cell;
         }
-        line.cells[x] = Cell::from_char(c, attr);
         &line.cells[x]
     }
 
@@ -203,18 +208,37 @@ impl Screen {
             }
         };
 
+        // To avoid thrashing the heap, prefer to move lines that were
+        // scrolled off the top and re-use them at the bottom.
+        let to_move = lines_removed.min(num_rows);
+        let (to_remove, to_add) = {
+            for _ in 0..to_move {
+                let mut line = self.lines.remove(phys_scroll.start);
+                // Make the line like a new one of the appropriate width
+                line.reset(self.physical_cols);
+                if scroll_region.end as usize == self.physical_rows {
+                    self.lines.push(line);
+                } else {
+                    self.lines.insert(phys_scroll.end - lines_removed, line);
+                }
+            }
+            // We may still have some lines to add at the bottom, so
+            // return revised counts for remove/add
+            (lines_removed - to_move, num_rows - to_move)
+        };
+
         // Perform the removal
-        for _ in 0..lines_removed {
+        for _ in 0..to_remove {
             self.lines.remove(phys_scroll.start);
         }
 
         if scroll_region.end as usize == self.physical_rows {
             // It's cheaper to push() than it is insert() at the end
-            for _ in 0..num_rows {
+            for _ in 0..to_add {
                 self.lines.push(Line::new(self.physical_cols));
             }
         } else {
-            for _ in 0..num_rows {
+            for _ in 0..to_add {
                 self.lines.insert(
                     phys_scroll.end - lines_removed,
                     Line::new(self.physical_cols),