3. The Input module

This module provides a set of packages with a common interface to access the characters contained in a stream. Various implementations are provided to access files and manipulate standard Ada strings.

A top-level tagged type is provided that must be extended for the various streams. It is assumed that the pointer to the current character in the stream can only go forward, and never backward. As a result, it is possible to implement this package for sockets or other streams where it isn’t even possible to go backward. This also means that one doesn’t have to provide buffers in such cases, and thus that it is possible to provide memory-efficient readers.

Four predefined readers are available, namely String_Input to read characters from a standard Ada string, File_Input to read characters from a standard text file, Http_Input to read from http location and Socket_Input to read from a streaming socket.

All readers share same limitation of total length of input: files bigger than 2GB are not supported.

They all provide the following primitive operations:

Open

Although this operation isn’t exactly overridden, since its parameters depend on the type of stream you want to read from, it is nice to use a standard name for this constructor.

Close

This terminates the stream reader and frees any associated memory. It is no longer possible to read from the stream afterwards.

Next_Char

Return the next Unicode character in the stream. Note this character doesn’t have to be associated specifically with a single byte, but that it depends on the encoding chosen for the stream (see the unicode module documentation for more information).

The next time this function is called, it returns the following character from the stream.

Eof

This function should return True when the reader has already returned the last character from the stream. Note that it is not guarantee that a second call to Eof will also return True.

It is the responsibility of this stream reader to correctly call the decoding functions in the unicode module so as to return one single valid unicode character. No further processing is done on the result of Next_Char. Note that the standard File_Input and String_Input streams can automatically detect the encoding to use for a file, based on a header read directly from the file.

Based on the first four bytes of the stream (assuming this is valid XML), they will automatically detect whether the file was encoded as Utf8, Utf16,… If you are writing your own input streams, consider adding this automatic detection as well.

However, it is always possible to override the default through a call to Set_Encoding. This allows you to specify both the character set (Latin1, …) and the character encoding scheme (Utf8,…).

The user is also encouraged to set the identifiers for the stream they are parsing, through calls to Set_System_Id and Set_Public_Id. These are used when reporting error messages.