regions() made one linesweeper contour pass per distinct coverage mask,
and each pass walks the whole arrangement, so finding the regions cost
masks x segments. Two crossing blends make thousands of both: the
Ribbons example's EPS export never finished, and Flatten Transparency,
the Flattener Preview, Divide, Shape Builder and Live Paint slowed down
the same way on dense art.
The masks now go into batches whose faces share no vertex (a greedy
colouring of the masks meeting at each vertex), one contour pass per
batch. A face's contours are walked as in its own mask's pass, since
no other face of the batch touches it; each contour's mask is the one
of its batch at its first point (a vertex of the arrangement), and
contours group under the nearest enclosing contour of the same mask,
as grouped() does in that mask's pass. The regions come out identical,
in the same order.
Fixes#173.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U9TrG4w49oSRYbP3cqxA3D
9002f86f73