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, 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
    -F lowercase porter2 \          # lowercase and stem every term (using the Porter2 algorithm)
    --html \                        # strip HTML markup 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 (lexicon) 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.

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

It 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

It outputs:

2

NOTE: This requires the initial file to be lexicographically sorted, as rlookup uses binary search for reverse lookups.

Supported stemmers

• Porter2
• Krovetz

Both are English stemmers. Unfortunately, PISA does not have support for any other languages. Contributions are welcome.

Supported formats

The following raw collection formats are supported:

• 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.
• 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.