perf: fast display (row runs, bulk fills, pixel scroll), terminal draws changed cells only + scrolls pixels, print! batched (was IPC per fmt fragment), snake sleeps on timer

crates/servers/terminal-server/src/grid.rs

@@ -15,12 +15,15 @@ pub struct Grid {
     pub row: usize,
     /// Rows to redraw.
     dirty: Vec<bool>,
+    /// Rows everything moved up by since the screen last caught up: moving
+    /// the pixels is cheaper than drawing every row again.
+    scrolled: usize,
 }
 
 impl Grid {
     pub fn new(cols: usize, rows: usize) -> Self {
         let (cols, rows) = (cols.clamp(1, MAX_COLS), rows.clamp(1, MAX_ROWS));
-        Self { cells: vec![b' '; cols * rows], cols, rows, col: 0, row: 0, dirty: vec![true; rows] }
+        Self { cells: vec![b' '; cols * rows], cols, rows, col: 0, row: 0, dirty: vec![true; rows], scrolled: 0 }
     }
 
     pub fn row(&self, r: usize) -> &[u8] {
@@ -37,6 +40,12 @@ impl Grid {
         self.dirty[..rows].iter_mut().enumerate().filter_map(|(r, d)| core::mem::take(d).then_some(r))
     }
 
+    /// How far to move the screen up before drawing the dirty rows, which
+    /// already account for it.
+    pub fn take_scrolled(&mut self) -> usize {
+        core::mem::take(&mut self.scrolled)
+    }
+
     fn mark(&mut self, r: usize) {
         self.dirty[r] = true;
     }
@@ -53,7 +62,16 @@ impl Grid {
         self.cells.copy_within(self.cols.., 0);
         let last = (self.rows - 1) * self.cols;
         self.cells[last..].fill(b' ');
-        self.dirty.fill(true);
+        // Rows still to redraw move up with their text.
+        self.dirty.copy_within(1.., 0);
+        self.mark(self.rows - 1);
+        if self.scrolled + 1 >= self.rows {
+            // Scrolled a whole screen: just draw it all.
+            self.touch_all();
+            self.scrolled = 0;
+        } else {
+            self.scrolled += 1;
+        }
     }
 
     /// Writes one byte at the cursor: `\n` moves to the next line, other
@@ -90,5 +108,6 @@ impl Grid {
         self.col = 0;
         self.row = 0;
         self.dirty.fill(true);
+        self.scrolled = 0;
     }
 }