Efficiency of SparseArray.__getitem__(SparseArray[bool])

[TomAugspurger]

This currently densifies:

pandas/pandas/core/sparse/array.py

Lines 651 to 659 in 56d8e78

	# TODO: I think we can avoid densifying when masking a
	# boolean SparseArray with another. Need to look at the
	# key's fill_value for True / False, and then do an intersection
	# on the indicies of the sp_values.
	if isinstance(key, SparseArray):
	if is_bool_dtype(key):
	key = key.to_dense()
	else:
	key = np.asarray(key)

I haven't investigated it, but we should be able to do a boolean mask as an
intersection sp_values on self and key. If key is SparseDtype[bool, False]
(i.e. False is the fill_value) this should be a lot faster.

[bdrum]

take