o Empty files.
Improvements:
-o Use radix-sort for internal sorting.
-o Parallelization of internal sorting.
o Clean up data types and make sure they cannot overflow. (size_t vs. u64 vs. sh_off_t vs. uns)
-o Buffer sizing in internal sorters.
-o Switching between direct and normal I/O.
-o When merging, choose the output file with less runs instead of always switching?
-o Deal with too rough range estimates in radix splitting.
+o Switching between direct and normal I/O. Should use normal I/O if the input is small enough.
+o How does the speed of radix splitting decrease with increasing number of hash bits?
+ Does it help to use more bits than we need, so that we sort less data in memory?
+o Log messages should show both original and new size of the data. The speed
+ should be probably calculated from the former.