Bidirectional Text Editing

Cursor position

The cursor position contains both character index and on which side the cursor is in logical text-ordering.

Although more complex then keeping just track of just a character index, it allows for more correct cursor movement in bidirectional text.

Glossary and Symbols

• Display ordering:
• Logical ordering:
• LTR: text written in a left-to-right language.
• RTL: text written in a right-to-left language.
• Leading: the cursor position before a character in logical ordering
• Trailing: the cursor position after a character in logical ordering.
• East: The cursor position to the right of a character in display order.
• West: The cursor position to the left of a character in display order.
• Neighbor:

Symbols used in examples:

• lower-case: text written in LTR
• upper-case: text written in RTL
• ’^’: primary cursor position
• ’[’: LTR cursor
• ’]’: RTL cursor

Rendering the insert cursors

There are two cursors when editing text, the primary cursor which is the actual cursor position, the secondary cursor is the neighboring cursor.

The LTR cursor is where left-to-right text will be inserted, it is displayed with a little flag at the top pointing to the right. The RTL cursor is the reverse.

The primary cursor is the actual cursor position; the secondary is the neighboring cursor position in logical text-ordering, the secondary cursor is drawn in a different color.

When the primary and secondary cursor share the same position in display order, then the secondary cursor suppressed as is the LTR/RTL-flag on the primary cursor. This means in most cases when writing in one direction it will seem like there is just a single cursor.

In the example below we have the text “theREDfox”. “RED” is written in a RTL language, which makes the display order of the text “theDERfox”. The example show the cursor to be before the letter ‘R’ in logical order.

• Because the ‘R’ is a RTL character, the primary cursor will appear on its right and will be a RTL cursor.
• The neighboring cursor in display order is behind the ‘e’ character. Since this is a LTR character, the secondary cursor will be displayed on the right side of the ‘e’ and will be a LTR cursor.
• The primary and secondary cursors are not neighbors in display order, so both cursors will be displayed.

logical order:
  t   h   e   R   E   D   f   o   x
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7
             ^

display order:
  t   h   e[  D   E   R]  f   o   x
 0 1 2 3 4 5 1 0 9 8 7 6 2 3 4 5 6 7
                       ^