Shave a few cycles off Grid::scroll_up

This implementation avoids a few extra transformations by operating in
buffer-space. However, there is still a transformation from the logical
buffer-space to the underlying vector.

src/grid/mod.rs

@@ -334,7 +334,6 @@ impl<T: Copy + Clone> Grid<T> {
     /// scroll_up moves lines at the bottom towards the top
     ///
     /// This is the performance-sensitive part of scrolling.
-    #[inline]
     pub fn scroll_up(
         &mut self,
         region: &Range<index::Line>,
@@ -356,17 +355,21 @@ impl<T: Copy + Clone> Grid<T> {
                 selection.rotate(*positions as isize);
             }
 
-            // Now, restore any lines outside the scroll region
-            for idx in (*region.end .. *self.num_lines()).rev() {
-                // First do the swap
-                self.raw.swap_lines(Line(idx), Line(idx) - positions);
+            // // This next loop swaps "fixed" lines outside of a scroll region
+            // // back into place after the rotation. The work is done in buffer-
+            // // space rather than terminal-space to avoid redundant
+            // // transformations.
+            let fixed_lines = *self.num_lines() - *region.end;
+
+            for i in 0..fixed_lines {
+                self.raw.swap(i, i + *positions);
             }
 
             // Finally, reset recycled lines
             //
             // Recycled lines are just above the end of the scrolling region.
             for i in 0..*positions {
-                self.raw[region.end - i - 1].reset(&template);
+                self.raw[i + fixed_lines].reset(&template);
             }
         } else {
             // Subregion rotation

src/grid/storage.rs

@@ -171,12 +171,25 @@ impl<T> Storage<T> {
         self.len
     }
 
-    /// Compute actual index in underlying storage given the requested index.
     #[inline]
+    pub fn raw_len(&self) -> usize {
+        self.inner.len()
+    }
+
+    /// Compute actual index in underlying storage given the requested index.
     fn compute_index(&self, requested: usize) -> usize {
         (requested + self.zero) % self.inner.len()
     }
 
+    #[inline]
+    pub fn line_offset(&self) -> usize {
+        self.inner.len() + self.zero + *self.visible_lines
+    }
+
+    pub fn compute_line_index(&self, a: Line) -> usize {
+        (self.line_offset() - *a) % self.inner.len()
+    }
+
     pub fn swap_lines(&mut self, a: Line, b: Line) {
         let offset = self.inner.len() + self.zero + *self.visible_lines;
         let a = (offset - *a) % self.inner.len();
@@ -184,6 +197,19 @@ impl<T> Storage<T> {
         self.inner.swap(a, b);
     }
 
+    /// Swap two lines in raw buffer
+    ///
+    /// # Panics
+    ///
+    /// `swap` will panic if either `a` or `b` are out-of-bounds of the
+    /// underlying storage.
+    pub fn swap(&mut self, a: usize, b: usize) {
+        let a = self.compute_index(a);
+        let b = self.compute_index(b);
+
+        self.inner.swap(a, b);
+    }
+
     /// Iterator over *logical* entries in the storage
     ///
     /// This *does not* iterate over hidden entries.