terminal: preserve content during resize

This patch eliminates the flickering of the terminal during resize.

repos/gems/src/server/terminal/main.cc

@@ -190,8 +190,39 @@ void Terminal::Main::_handle_config()
 		bool const reconstruct = !_text_screen_surface.constructed() ||
 		                          _text_screen_surface->size() != new_geometry.size();
 		if (reconstruct) {
+
+			typedef Text_screen_surface<PT>::Snapshot Snapshot;
+			Constructible<Snapshot> snapshot { };
+
+			size_t const snapshot_bytes  = _text_screen_surface.constructed()
+			                             ? Snapshot::bytes_needed(*_text_screen_surface)
+			                             : 0,
+			             preserved_bytes = 32*1024,
+			             needed_bytes    = snapshot_bytes + preserved_bytes,
+			             avail_bytes     = _env.pd().avail_ram().value;
+
+			bool const preserve_content = (needed_bytes < avail_bytes);
+
+			if (!preserve_content)
+				warning("not enough spare RAM to preserve content (",
+				        "need ", Genode::Number_of_bytes(needed_bytes), ", "
+				        "have ", Genode::Number_of_bytes(avail_bytes), ")");
+
+			if (preserve_content && _text_screen_surface.constructed())
+				snapshot.construct(_heap, *_text_screen_surface);
+
+			Position const orig_cursor_pos = _text_screen_surface.constructed()
+			                               ? _text_screen_surface->cursor_pos()
+			                               : Position();
+
 			_text_screen_surface.construct(_heap, _font->font(),
 			                               _color_palette, _framebuffer);
+
+			if (snapshot.constructed())
+				_text_screen_surface->import(*snapshot);
+
+			_text_screen_surface->cursor_pos(orig_cursor_pos);
+
 			_terminal_size = _text_screen_surface->size();
 
 		} else {

repos/gems/src/server/terminal/text_screen_surface.h

@@ -92,6 +92,34 @@ class Terminal::Text_screen_surface
 			bool valid() const { return columns*lines > 0; }
 		};
 
+		/**
+		 * Snapshot of text-screen content
+		 */
+		class Snapshot
+		{
+			private:
+
+				friend class Text_screen_surface;
+
+				Cell_array<Char_cell> _cell_array;
+
+			public:
+
+				static size_t bytes_needed(Text_screen_surface const &surface)
+				{
+					return Cell_array<Char_cell>::bytes_needed(surface.size().w(),
+					                                           surface.size().h());
+				}
+
+				Snapshot(Allocator &alloc, Text_screen_surface const &from)
+				:
+					_cell_array(from._cell_array.num_cols(),
+					            from._cell_array.num_lines(), alloc)
+				{
+					_cell_array.import_from(from._cell_array);
+				}
+		};
+
 	private:
 
 		Font          const &_font;
@@ -133,6 +161,10 @@ class Terminal::Text_screen_surface
 			_cell_array.mark_all_lines_as_dirty(); /* trigger refresh */
 		}
 
+		Position cursor_pos() const { return _character_screen.cursor_pos(); }
+
+		void cursor_pos(Position pos) { _character_screen.cursor_pos(pos); }
+
 		void redraw()
 		{
 			PT *fb_base = _framebuffer.pixel<PT>();
@@ -238,6 +270,11 @@ class Terminal::Text_screen_surface
 			_decoder.insert(c.c);
 		}
 
+		void import(Snapshot const &snapshot)
+		{
+			_cell_array.import_from(snapshot._cell_array);
+		}
+
 		/**
 		 * Return size in colums/rows
 		 */

repos/os/include/terminal/cell_array.h

@@ -94,6 +94,13 @@ class Cell_array
 				_array[i] = new (alloc) CELL[num_cols];
 		}
 
+		static Genode::size_t bytes_needed(unsigned num_cols, unsigned num_lines)
+		{
+			return sizeof(Char_cell_line[num_lines])
+			     + sizeof(bool[num_lines])
+			     + sizeof(CELL[num_cols])*num_lines;
+		}
+
 		~Cell_array()
 		{
 			for (unsigned i = 0; i < _num_lines; i++)
@@ -115,11 +122,23 @@ class Cell_array
 			_line_dirty[line] = true;
 		}
 
-		CELL get_cell(int column, int line)
+		CELL get_cell(int column, int line) const
 		{
 			return _array[line][column];
 		}
 
+		void import_from(Cell_array const &other)
+		{
+			unsigned const num_cols  = Genode::min(_num_cols,  other._num_cols),
+			               num_lines = Genode::min(_num_lines, other._num_lines);
+
+			for (unsigned line = 0; line < num_lines; line++)
+				for (unsigned column = 0; column < num_cols; column++)
+					_array[line][column] = other.get_cell(column, line);
+
+			mark_all_lines_as_dirty();
+		}
+
 		bool line_dirty(int line) { return _line_dirty[line]; }
 
 		void mark_line_as_clean(int line)
@@ -167,8 +186,8 @@ class Cell_array
 				_line_dirty[pos.y] = true;
 		}
 
-		unsigned num_cols()  { return _num_cols; }
-		unsigned num_lines() { return _num_lines; }
+		unsigned num_cols()  const { return _num_cols; }
+		unsigned num_lines() const { return _num_lines; }
 };
 
 #endif /* _TERMINAL__CELL_ARRAY_H_ */

repos/os/include/terminal/char_cell_array_character_screen.h

@@ -160,6 +160,12 @@ class Char_cell_array_character_screen : public Terminal::Character_screen
 
 		Terminal::Position cursor_pos() const { return _cursor_pos; }
 
+		void cursor_pos(Terminal::Position pos)
+		{
+			_cursor_pos.x = Genode::min(_boundary.width  - 1, pos.x);
+			_cursor_pos.y = Genode::min(_boundary.height - 1, pos.y);
+		}
+
 		void output(Terminal::Character c)
 		{
 			if (c.ascii() > 0x10) {