Parsing

A forward index is a data structure that stores the term identifiers associated to every document. Conversely, an inverted index stores for each unique term the document identifiers where it appears (usually, associated to a numeric value used for ranking purposes such as the raw frequency of the term within the document).

The objective of the parsing process is to represent a given collection as a forward index. To parse a collection, use the parse_collection command:

parse_collection - parse collection and store as forward index.
Usage: ./bin/parse_collection [OPTIONS]

Options:
  -h,--help                   Print this help message and exit
  -o,--output TEXT REQUIRED   Forward index filename
  -j,--threads UINT           Thread count
  -b,--batch-size INT=100000  Number of documents to process in one thread
  -f,--format TEXT=plaintext  Input format
  --stemmer TEXT              Stemmer type
  --content-parser TEXT       Content parser type
  --debug                     Print debug messages

  Subcommands:
    merge                       Merge previously produced batch files.
                                When parsing process was killed during merging, use this
                                command to finish merging without having to restart building batches.

For example:

$ mkdir -p path/to/forward
$ zcat ClueWeb09B/*/*.warc.gz |   # pass unzipped stream in WARC format
    parse_collection \
    -j 8                          # use up to 8 threads at a time
    -b 10000                      # one thread builds up to 10k documents in memory
    -f warc                       # use WARC
    --stemmer porter2             # stem every term using the Porter2 algorithm
    --content-parser html         # parse HTML content before extracting tokens
    -o path/to/forward/cw09b

In case you get the error -bash: /bin/zcat: Argument list too long, you can pass the unzipped stream using:

$ find ClueWeb09B -name '*.warc.gz' -exec zcat -q {} \;

The parsing process will write the following files:

  • cw09b: forward index in binary format.
  • cw09b.terms: a new-line-delimited list of sorted terms, where term having ID N is on line N, with N starting from 0.
  • cw09b.termlex: a binary representation of the .terms file that is used to look up term identifiers at query time.
  • cw09b.documents: a new-line-delimited list of document titles (e.g., TREC-IDs), where document having ID N is on line N, with N starting from 0.
  • cw09b.doclex: a binary representation of the .documents file that is used to look up document identifiers at query time.
  • cw09b.urls: a new-line-delimited list of URLs, where URL having ID N is on line N, with N starting from 0. Also, keep in mind that each ID corresponds with an ID of the cw09b.documents file.

Generating mapping files

Once the forward index has been generated, a binary document map and lexicon file will be automatically built. However, they can also be built using the lexicon utility by providing the new-line delimited file as input. The lexicon utility also allows efficient look-ups and dumping of these binary mapping files.

Examples of the lexicon command are shown below:

Build, print, or query lexicon
Usage: ./bin/lexicon [OPTIONS] SUBCOMMAND

Options:
    -h,--help                   Print this help message and exit

Subcommands:
    build                       Build a lexicon
    lookup                      Retrieve the payload at index
    rlookup                     Retrieve the index of payload
    print                       Print elements line by line

For example, assume we have the following plaintext, new-line delimited file, example.terms:

aaa
bbb
def
zzz

We can generate a lexicon as follows: ./bin/lexicon build example.terms example.lex

You can dump the binary lexicon back to a plaintext representation: ./bin/lexicon print example.lex which should output:

aaa
bbb
def
zzz

You can retrieve the term with a given identifier: ./bin/lexicon lookup example.lex 2 which outputs def

Finally, you can retrieve the id of a given term: ./bin/lexicon rlookup example.lex def which outputs 2. NOTE: This requires the initial file to be lexicographically sorted, as rlookup depends on binary search.

Supported stemmers

  • Porter2
  • Krovetz

Supported formats

  • plaintext: every line contains the document’s title first, then any number of whitespaces, followed by the content delimited by a new line character.
  • trectext: TREC newswire collections.
  • trecweb: TREC web collections.
  • warc: Web ARChive format as defined in [the format specification](https://iipc.github.io/warc-specifications/specifications/warc-format/warc-1.0/).
  • wapo: TREC Washington Post Corpus.

In case you want to parse a set of files where each one is a document (for example, the collection wiki-large), use the files2trec.py script to format it to TREC (take into account that each relative file path is used as the document ID). Once the file is generated, parse it with the parse_collection command specifying the trectext value for the --format option.