The algorithm parses documents into termID–docID pairs and accumulates
the pairs inmemory until a block of a fixed size is full (PARSENEXTBLOCK
Figure 4.3 Merging in blocked sort-based indexing. Two blocks (“postings lists to
be merged”) are loaded from disk into memory, merged in memory (“merged postings
lists”) and written back to disk. We show terms instead of termIDs for better
readability.