Once the parsing phase is complete, use the invert command to turn a forward index into an inverted index:

invert - turn forward index into inverted index
Usage: ./invert [OPTIONS]

  -h,--help                   Print this help message and exit
  -i,--input TEXT REQUIRED    Forward index filename
  -o,--output TEXT REQUIRED   Output inverted index basename
  -j,--threads UINT           Thread count
  --term-count UINT REQUIRED  Term count
  -b,--batch-size INT=100000  Number of documents to process at a time

For example, assuming the existence of a forward index in the path path/to/forward/cw09b:

$ mkdir -p path/to/inverted
$ ./invert -i path/to/forward/cw09b -o path/to/inverted/cw09b --term-count `wc -w < path/to/forward/cw09b.terms`

Note that the script requires as parameter the number of terms to be indexed, which is obtained by embedding the wc -w < path/to/forward/cw09b.terms instruction.

Inverted index format

A binary sequence is a sequence of integers prefixed by its length, where both the sequence integers and the length are written as 32-bit little-endian unsigned integers. An inverted index consists of 3 files, <basename>.docs, <basename>.freqs, <basename>.sizes:

  • <basename>.docs starts with a singleton binary sequence where its only integer is the number of documents in the collection. It is then followed by one binary sequence for each posting list, in order of term-ids. Each posting list contains the sequence of document-ids containing the term.
  • <basename>.freqs is composed of a one binary sequence per posting list, where each sequence contains the occurrence counts of the postings, aligned with the previous file (note however that this file does not have an additional singleton list at its beginning).
  • <basename>.sizes is composed of a single binary sequence whose length is the same as the number of documents in the collection, and the i-th element of the sequence is the size (number of terms) of the i-th document.

Reading the inverted index using Python

import os
import numpy as np

class InvertedIndex:
    def __init__(self, index_name):
        index_dir = os.path.join(index_name) = np.memmap(index_name + ".docs", dtype=np.uint32,
        self.freqs = np.memmap(index_name + ".freqs", dtype=np.uint32,

    def __iter__(self):
        i = 2
        while i < len(
            size =[i]
            yield ([i+1:size+i+1], self.freqs[i-1:size+i-1])
            i += size+1

    def __next__(self):
        return self

for i, (docs, freqs) in enumerate(InvertedIndex("cw09b")):
    print(i, docs, freqs)