An 'ID database' is a binary file containing a list of file names, a list of tokens, and a sparse matrix indicating which tokens appear in which files.
With this database and some tools to query it, many text-searching tasks become simpler and faster.
For example, you can list all files that reference a particular `#include' file throughout a huge source hierarchy, search for all the memos containing references to a project, or automatically invoke an editor on all files containing references to some function or variable.
Anyone with a large software project to maintain, or a large set of text files to organize, can benefit from the ID utilities.
Although the name `ID' is short for `identifier', the ID utilities handle more than just identifiers; they also treat other kinds of tokens, most notably numeric constants, and the contents of certain character strings.
