The algorithm parses documents into

The algorithm parses documents into termID–docID pairs and accumulates the pairs in memory until a block of a fixed size is full(PARSENEXTBLOCK in Figure 4.2). We choose the block size to fit comfortably into memory to permit a fast in-memory sort. The block is then inverted and written to disk. Inversion involves two steps. First, we sort the termID–docID pairs. Next, we collect all termID–docID pairs with the same termID into a postings list, where a posting is simply a docID. The result, an inverted index for the block we have just read,is then written to disk. Applying this to Reuters-RCV1 and assuming we can fit 10 million termID–docID pairs into memory, we end up with ten blocks, each an inverted index of one part of the collection.