Empty input was asserting originally, but such strict behavior doesn't
seem to be useful in practice as all application code would need to
explicitly make sure that shape() isn't called if the input or the input
range is empty. In particular, I hit this assert as soon as I rebuilt
and ran magnum-player, and the assertion didn't really uncover any bug
or make anything better by firing.
Treating zero glyph output as a failure also isn't good -- rendering
for example a string of whitespace may also result in zero glyphs on the
output, which isn't a failure. Plus it's currently unclear what should
actually be a failure -- neither FreeType nor HarfBuzz really have a
concept of the shaping operation failing, hb_shape() returns void -- so
special-casing glyphCount() being zero as a failure just doesn't make
sense.
This thus means the doScript(), doLanguage() and doDirection()
implementations are now called always, regardless of whether shape() was
called at all, or whether it produced any glyphs. This allows the shaper
for example to give back a detected script etc even if the actual
input range to shape was empty.
Vladimír Vondruš
eaaa34b3fb