Before starting our description of the indexing process, a brief introduction is necessary to present some basic concepts. MG4J has to do with documents (e.g., HTML files, mail messages etc.), and every document is composed by fields (e.g., the fields of a mail message will probably be its subject, sender, recipient, body etc.). Although, as we shall see, MG4J will provides support for non-textual fields, its "bread and butter" is with textual fields, and for the time being we shall assume that we are dealing with documents composed of just one textual field.

A textual field (in our simplified view: a document) is a sequence of words: it is up to the factory producing the document to decide how to choose words (e.g., one may want to discard digits or not), and how to segment the document. For instance, the typical letter-to-nonletter transition used to split Western languages does not work very well with, say, Chinese. However, once segmentation has produced suitable words, they must be turned into indexable terms: for instance, you may want to downcase your words, but at the same time you may want to keep "ph" (as in "this soap's ph") separated from "Ph" (as in "Ph.D. degree"). You may also want to make more dramatic transformations, such as stemming, or avoid indexing a term altogether. All these operation are performed by a term processor, which can be specified on the command line. The option --downcase, for instance, selects for you the class it.unimi.dsi.mg4j.index.DowncaseTermProcessor. The chosen processor is recorded into the index structure: this is essential for interpreting queries correctly.

Note that in the design of other search engines segmentation and processing are somehow mixed into a generic tokenisation phase. We prefer to split clearly between linguistic and algorithmic term processing. Linguistic processing depends only on the writing customs of a language, whereas algorithmic processing might be language neutral (we do not exclude, however, that it might be language dependent, too).

If you scan the whole document collection, you can collect all terms that appear in it; the set of all such terms is called the term dictionary. Note that every term in the dictionary appears in some (at least one) document, and probably it will appear in many documents, possibly even many times in some documents. (By the way: terms that appear in just one document are called hapax legomena, and they are far more frequent than one might expect in many collections, especially due to typos).

MG4J, like any other indexing tool, does not treat internally terms as character sequences, but it uses numbers. This means that terms in the dictionary are assigned an index (a number between 0 and the dictionary size minus 1), and that this index is used whenever the application needs to refer to a term. Usually, indices are assigned in lexicographical order: this means that index 0 is assigned to the first term in lexicographic order, index 1 to the next one and so on). The assignment between terms and indices is stored in a suitable data structure, that compactly represents both the dictionary and the map.

There are many possible different representations of this assignment, each with certain memory requirements and each allowing different kind of access to the data.