implement scrolling down/up by half/full page

src/textarea.rs

@@ -45,13 +45,36 @@ pub enum Scrolling {
     /// textarea.scroll((1, 0));
     /// assert_eq!(textarea.cursor(), (3, 0));
     /// ```
-    Delta { rows: i16, cols: i16 },
+    Delta {
+        rows: i16,
+        cols: i16,
+    },
+    PageDown,
+    PageUp,
+    HalfPageDown,
+    HalfPageUp,
 }
 
 impl Scrolling {
     fn scroll(self, viewport: &mut Viewport) {
         let (rows, cols) = match self {
             Self::Delta { rows, cols } => (rows, cols),
+            Self::PageDown => {
+                let (_, _, _, height) = viewport.rect();
+                (height as i16, 0)
+            }
+            Self::PageUp => {
+                let (_, _, _, height) = viewport.rect();
+                (-(height as i16), 0)
+            }
+            Self::HalfPageDown => {
+                let (_, _, _, height) = viewport.rect();
+                ((height as i16) / 2, 0)
+            }
+            Self::HalfPageUp => {
+                let (_, _, _, height) = viewport.rect();
+                (-(height as i16) / 2, 0)
+            }
         };
         viewport.scroll(rows, cols);
     }

src/widget.rs

@@ -31,13 +31,17 @@ impl Viewport {
         ((u >> 16) as u16, u as u16)
     }
 
-    pub fn position(&self) -> (u16, u16, u16, u16) {
+    pub fn rect(&self) -> (u16, u16, u16, u16) {
         let u = self.0.load(Ordering::Relaxed);
         let width = (u >> 48) as u16;
         let height = (u >> 32) as u16;
-        let row_top = (u >> 16) as u16;
-        let col_top = u as u16;
+        let row = (u >> 16) as u16;
+        let col = u as u16;
+        (row, col, width, height)
+    }
 
+    pub fn position(&self) -> (u16, u16, u16, u16) {
+        let (row_top, col_top, width, height) = self.rect();
         let row_bottom = row_top.saturating_add(height).saturating_sub(1);
         let col_bottom = col_top.saturating_add(width).saturating_sub(1);