Add zj and zk command for scrolling the window

README.asciidoc

@@ -148,6 +148,8 @@ window.
  * _zz_ or _zc_: center the main selection in the window
  * _zt_: scroll to put the main selection on the top line of the window
  * _zb_: scroll to put the main selection on the bottom line of the window
+ * _zj_: scroll the window count line downward
+ * _zk_: scroll the window count line upward
 
 Multi Selection
 ---------------

src/main.cc

@@ -167,6 +167,12 @@ void do_disp_cmd(Context& context)
         case 'b':
             context.window().display_selection_at(window.dimensions().line-1);
             break;
+        case 'j':
+            context.window().scroll( std::max<LineCount>(1, context.numeric_param()));
+            break;
+        case 'k':
+            context.window().scroll(-std::max<LineCount>(1, context.numeric_param()));
+            break;
         }
     });
 }

src/window.cc

@@ -52,6 +52,11 @@ void Window::center_selection()
     display_selection_at(m_dimensions.line/2_line);
 }
 
+void Window::scroll(LineCount offset)
+{
+    m_position.line = std::max(0_line, m_position.line + offset);
+}
+
 void Window::update_display_buffer()
 {
     scroll_to_keep_cursor_visible_ifn();

src/window.hh

@@ -34,6 +34,7 @@ public:
 
     void center_selection();
     void display_selection_at(LineCount line);
+    void scroll(LineCount offset);
     void update_display_buffer();
 
     DisplayCoord display_position(const BufferIterator& it);