Implement page up and page down

babi.py

@@ -24,6 +24,13 @@ class Margin(NamedTuple):
     def body_lines(self) -> int:
         return curses.LINES - self.header - self.footer
 
+    @property
+    def page_size(self) -> int:
+        if self.body_lines <= 2:
+            return 1
+        else:
+            return self.body_lines - 2
+
     @classmethod
     def from_screen(cls, screen: '_curses._CursesWindow') -> 'Margin':
         if curses.LINES == 1:
@@ -95,6 +102,22 @@ class Position:
     def end(self, margin: Margin, lines: List[str]) -> None:
         self.x = self.x_hint = len(lines[self.cursor_line])
 
+    def page_up(self, margin: Margin, lines: List[str]) -> None:
+        if self.cursor_line < margin.body_lines:
+            self.cursor_line = self.file_line = 0
+        else:
+            pos = self.file_line - margin.page_size
+            self.cursor_line = self.file_line = pos
+        self._set_x_after_vertical_movement(lines)
+
+    def page_down(self, margin: Margin, lines: List[str]) -> None:
+        if self.file_line + margin.body_lines >= len(lines):
+            self.cursor_line = len(lines) - 1
+        else:
+            pos = self.file_line + margin.page_size
+            self.cursor_line = self.file_line = pos
+        self._set_x_after_vertical_movement(lines)
+
     DISPATCH = {
         curses.KEY_DOWN: down,
         curses.KEY_UP: up,
@@ -102,6 +125,8 @@ class Position:
         curses.KEY_RIGHT: right,
         curses.KEY_HOME: home,
         curses.KEY_END: end,
+        curses.KEY_PPAGE: page_up,
+        curses.KEY_NPAGE: page_down,
     }
 
     def dispatch(self, key: int, margin: Margin, lines: List[str]) -> None:
@@ -345,6 +370,10 @@ def c_main(stdscr: '_curses._CursesWindow', args: argparse.Namespace) -> None:
             pos.home(margin, lines)
         elif keyname == b'^E':
             pos.end(margin, lines)
+        elif keyname == b'^Y':
+            pos.page_up(margin, lines)
+        elif keyname == b'^V':
+            pos.page_down(margin, lines)
         elif keyname == b'^X':
             return
         elif keyname == b'^Z':

tests/babi_test.py

@@ -118,6 +118,13 @@ class PrintsErrorRunner(Runner):
                 f'Last cursor position: {pos}',
             )
 
+    def get_screen_line(self, n):
+        return self.screenshot().splitlines()[n]
+
+    def get_cursor_line(self):
+        _, y = self._get_cursor_position()
+        return self.get_screen_line(y)
+
     @contextlib.contextmanager
     def resize(self, width, height):
         current_w, current_h = self.get_pane_size()
@@ -234,7 +241,7 @@ def test_status_clearing_behaviour():
 
 def test_reacts_to_resize():
     with run() as h, and_exit(h):
-        first_line = h.screenshot().splitlines()[0]
+        first_line = h.get_screen_line(0)
         with h.resize(40, 20):
             # the first line should be different after resize
             h.await_text_missing(first_line)
@@ -304,6 +311,56 @@ def test_arrow_key_movement(tmpdir):
         h.await_cursor_position(x=19, y=3)
 
 
+@pytest.mark.parametrize(
+    ('page_up', 'page_down'),
+    (('PageUp', 'PageDown'), ('^Y', '^V')),
+)
+def test_page_up_and_page_down(tmpdir, page_up, page_down):
+    f = tmpdir.join('f')
+    f.write('\n'.join(f'line_{i}' for i in range(10)))
+
+    with run(str(f), height=10) as h, and_exit(h):
+        h.press('Down')
+        h.press('Down')
+        h.press(page_up)
+        h.await_cursor_position(x=0, y=1)
+
+        h.press(page_down)
+        h.await_text('line_8')
+        h.await_cursor_position(x=0, y=1)
+        assert h.get_cursor_line() == 'line_6'
+
+        h.press(page_up)
+        h.await_text_missing('line_8')
+        h.await_cursor_position(x=0, y=1)
+        assert h.get_cursor_line() == 'line_0'
+
+        h.press(page_down)
+        h.press(page_down)
+        h.await_cursor_position(x=0, y=5)
+        assert h.get_cursor_line() == ''
+        h.press('Up')
+        h.await_cursor_position(x=0, y=4)
+        assert h.get_cursor_line() == 'line_9'
+
+
+def test_page_up_page_down_size_small_window(tmpdir):
+    f = tmpdir.join('f')
+    f.write('\n'.join(f'line_{i}' for i in range(10)))
+
+    with run(str(f), height=4) as h, and_exit(h):
+        h.press('PageDown')
+        h.await_text('line_2')
+        h.await_cursor_position(x=0, y=1)
+        assert h.get_cursor_line() == 'line_1'
+
+        h.press('Down')
+        h.press('PageUp')
+        h.await_text_missing('line_2')
+        h.await_cursor_position(x=0, y=1)
+        assert h.get_cursor_line() == 'line_0'
+
+
 def test_scrolling_arrow_key_movement(tmpdir):
     f = tmpdir.join('f')
     f.write('\n'.join(f'line_{i}' for i in range(10)))
@@ -319,7 +376,7 @@ def test_scrolling_arrow_key_movement(tmpdir):
         h.press('Down')
         h.await_text('line_8')
         h.await_cursor_position(x=0, y=4)
-        assert h.screenshot().splitlines()[4] == 'line_8'
+        assert h.get_cursor_line() == 'line_8'
         # we should not have scrolled after 3 up presses
         for _ in range(3):
             h.press('Up')
@@ -382,7 +439,7 @@ def test_resize_scrolls_up(tmpdir):
             h.await_text_missing('line_0')
             h.await_cursor_position(x=0, y=3)
             # make sure we're still on the same line
-            assert h.screenshot().splitlines()[3] == 'line_7'
+            assert h.get_cursor_line() == 'line_7'
 
 
 def test_resize_scroll_does_not_go_negative(tmpdir):
@@ -401,7 +458,7 @@ def test_resize_scroll_does_not_go_negative(tmpdir):
         for _ in range(2):
             h.press('Up')
 
-        assert h.screenshot().splitlines()[1] == 'line_0'
+        assert h.get_screen_line(1) == 'line_0'
 
 
 def test_very_narrow_window_status():