Base class for implementing an input stream representing a document returned from a data source
A dtsInputStream is an abstraction of a document with seek() and read() methods for accessing data and a dtsFileInfo with file description information (modification date, size, etc.)
A dtsDataSource object returns dtsInputStream objects in response to the getNextDoc callback function.
Derived classes override seek() and read() to provide access to the data associated with the document.
The derived class DMemoryInputStream can create a dtsInputStream from data in a memory buffer.
The derived class DFileInputStream can create a dtsInputStream from a disk file.