Again, the RTL property controls whether textboxes will be
right-aligned, and that kind of stuff. It can't be font-bound, since
Space Station supports Hebrew characters and we want to be able to
support, say, a Hebrew translation or Hebrew levels in the future
without having to make a dedicated (or duplicated) font for it.
Therefore it's a property of both the language pack as well as custom
levels - like custom levels already had a <font> tag, they now also
have an <rtl> tag that sets this property.
Right now, we'll have to hardcode it so the menu option for the Arabic
font sets the <rtl> property to 1, and all the other options set it to
0. But it's future-proof in that we can later decide to split the
option for Space Station into an LTR option and an RTL option (so both
"english/..." and "עברית" would select Space Station, but one sets the
RTL property to 0 and the other sets it to 1).
I'm now using SheenBidi to reorder RTL and bidirectional text properly
at text rendering time! For Arabic this is still missing reshaping, but
everything's looking really promising now!
The code changes are really non-invasive. The changes to Font.cpp are
absolutely minimal:
1305+ if (bidi_should_transform(text))
1306+ {
1307+ text = bidi_transform(text);
1308+ }
There's now a FontBidi.cpp, which implements these two functions,
notably bidi_transform(), which takes a UTF-8 encoded string and
returns another UTF-8 encoded string that has bidi reorderings and
reshapings applied.
In that function, SheenBidi gives us information about where in the
input string runs start and end, and on a basic level, all we need to
do there is to concatenate the parts together in the order that we're
given them, and to reverse the RTL runs (recognizable by odd levels).
As this is a proof-of-concept, bidi_should_transform() still always
returns true, applying the bidi algorithm to all languages and all
strings. I'm thinking of enabling bidi only when the language/font
metadata enables RTL (which could be for the interface or for a custom
level), or outside of that, at least when RTL characters are detected
(such as Arabic or Hebrew Unicode blocks).
