ANS inverted index compression

ANS inverted index compression based on the inverted index framework from https://github.com/ot/partitioned_elias_fano

Collection parsing

Parsing WARC colllections was done with this script: https://github.com/mpetri/TikaLuceneWarc

The next sections are almost unmodified from the https://github.com/ot/partitioned_elias_fano repo:

Building the code

The code is tested on Linux with GCC 4.9 and OSX Mavericks with Clang.

The following dependencies are needed for the build.

• CMake >= 2.8, for the build system
• Boost >= 1.42

The code depends on several git submodules. If you have cloned the repository without --recursive, you will need to perform the following commands before building:

$ git submodule init
$ git submodule update

To build, it should be sufficient to do:

$ cmake . -DCMAKE_BUILD_TYPE=Release
$ make

It is also preferable to perform a make test, which runs the unit tests.

Running the experiments

The directory test/test_data contains a small document collection used in the unit tests. The binary format of the collection is described in the next section.

To create an index use the command create_freq_index. The available index types are listed in index_types.hpp. For example, to create an index using the optimal partitioning algorithm using the test collection, execute the command:

$ ./create_freq_index opt test/test_data/test_collection test_collection.index.opt --check

where test/test_data/test_collection is the basename of the collection, that is the name without the .{docs,freqs,sizes} extensions, and test_collection.index.opt is the filename of the output index. --check perform a verification step to check the correctness of the index.

To perform BM25 queries it is necessary to build an additional file containing the parameters needed to compute the score, such as the document lengths. The file can be built with the following command:

$ ./create_wand_data test/test_data/test_collection test_collection.wand

Now it is possible to query the index. The command queries parses each line of the standard input as a tab-separated collection of term-ids, where the i-th term is the i-th list in the input collection. An example set of queries is again in test/test_data.

$ ./queries opt test_collection.index.opt test_collection.wand < test/test_data/queries

Create ANS based indexes

./create_freq_index block_ansmsbmedmaxmerged test/test_data/test_collection test_collection.index.anspacked2 --check

Collection input 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.

A collection 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.

Authors

• Matthias Petri Matthias.Petri@gmail.com
• Alistair Moffat ammoffat@unimelb.edu.au

Partitioned Elias-Fano framework developed by

• Giuseppe Ottaviano giuott@gmail.com
• Rossano Venturini rossano@di.unipi.it