  Continuing results from JCDCGGG 2016 and 2017, we solve several new cases of
the simple foldability problem -- deciding which crease patterns can be folded
flat by a sequence of (some model of) simple folds. We give new efficient
algorithms for mixed crease patterns, where some creases are assigned
mountain/valley while others are unassigned, for all 1D cases and for 2D
rectangular paper with orthogonal one-layer simple folds. By contrast, we show
strong NP-completeness for mixed orthogonal crease patterns on 2D rectangular
paper with some-layers simple folds, complementing a previous result for
all-layers simple folds. We also prove strong NP-completeness for finite simple
folds (no matter the number of layers) of unassigned orthogonal crease patterns
on arbitrary paper, complementing a previous result for assigned crease
patterns, and contrasting with a previous positive result for infinite
all-layers simple folds. In total, we obtain a characterization of polynomial
vs. NP-hard for all cases -- finite/infinite one/some/all-layers simple folds
of assigned/unassigned/mixed orthogonal crease patterns on
1D/rectangular/arbitrary paper -- except the unsolved case of infinite
all-layers simple folds of assigned orthogonal crease patterns on arbitrary
paper.
