A difference between BSBI and SPIM is that SPIMI adds a posting di-rectly to its postings list (line 10). Instead of first collecting all termID–docID pairs and then sorting them (as we did in BSBI), each postings list is dynamic (i.e., its size is adjusted as it grows) and it is immediately available to collect postings. This has two advantages: It is faster because there is no sorting required, and it saves memory because we keep track of the term a postings