SparseBlock stores only selected rows and columns.
Initial implementation without checks and asserts
Signed-off-by: Csaba Kiraly <csaba.kiraly@gmail.com>
It is faster to store and process data availability as bitmaps.
It is also enough, as we will not do anything with the data itself.
Signed-off-by: Csaba Kiraly <csaba.kiraly@gmail.com>