Add support for ECMA-48 escape sequence for crossed-out/strikethrough

ECMA-48 defines rendition mode 9 as:
  crossed-out (characters still legible but marked as to be deleted),
and mode 29 as the inverse:
  not crossed out

Modern terminals (including xterm, tmux, libvte, etc.) support rendering
strikethrough. Further, ncurses since version 6.1 includes the smxx/rmxx
user-defined terminal capabilities in the tmux and xterm-basic
databases.

Note that the new attribute still fits in the eight bits allocated for
Renditions::attributes, so there is no need to increase the size of Cell.

Signed-off-by: Stanislav Spassov <[email protected]>

Author: Stanislav Spassov

src/terminal/terminalframebuffer.cc

@@ -479,7 +479,7 @@ void Renditions::set_rendition( color_type num )
     return;
   }
 
-  bool value = num < 9;
+  bool value = num < 10;
   switch ( num ) {
     case 1:
       set_attribute( bold, value );
@@ -511,6 +511,10 @@ void Renditions::set_rendition( color_type num )
     case 28:
       set_attribute( invisible, value );
       break;
+    case 9:
+    case 29:
+      set_attribute (strikethrough, value);
+      break;
     default:
       break; /* ignore unknown rendition */
   }
@@ -554,6 +558,8 @@ std::string Renditions::sgr( void ) const
     ret.append( ";7" );
   if ( get_attribute( invisible ) )
     ret.append( ";8" );
+  if ( get_attribute( strikethrough ) )
+    ret.append( ";9" );
 
   if ( foreground_color ) {
     // Since foreground_color is a 25-bit field, it is promoted to an int when

src/terminal/terminalframebuffer.h

@@ -59,6 +59,7 @@ class Renditions
     blink,
     inverse,
     invisible,
+    strikethrough,
     SIZE
   } attribute_type;
 

src/tests/emulation-attributes.test

@@ -130,6 +130,8 @@ baseline()
             test_true_color 7
             echo "Invisible:"
             test_true_color 8
+            echo "Strikethrough:"
+            test_true_color 9
             echo "Bold, italic and underline:"
             test_true_color 1 3 4
             ;;