Blocked sort-based indexing has excellent scaling properties, but it needs
a data structure for mapping terms to termIDs. For very large collections,
this data structure does not fit into memory. A more scalable alternative is
single-pass in-memory indexing or SPIMI. SPIMI uses terms instead of termIDs,
writes each block’s dictionary to disk, and then starts a newdictionary for the
next block. SPIMI can index collections of any size as long as there is enough
disk space available.