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<title>JavaCC: CharStream Classes MiniTutorial</title>
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<h1>JavaCC [tm]: CharStream Classes MiniTutorial</h1>
<p>
This document describes in some detail the methods of the CharStream
classes. Note that some of the details may not be relevant for the
CharStream interface (to be used with USER_CHAR_STREAM).
</p>
<p>
There are 4 different kinds of char stream classes that are generated
based on combinations of various options.
</p>
<ul>
<li> <h3>ASCII_CharStream</h3>
Generated when neither of the two options - <tt>UNICODE_INPUT</tt>
or <tt>JAVA_UNICODE_ESCAPE</tt> is set.
<p>
This class treats the input as a stream of 1-byte (ISO-LATIN1)
characters. Note that this class can also be used to parse
binary files. It just reads a byte and returns it as a 16 bit
quantity to the lexical analyzer. So any character returned by
this class will be in the range <tt>'\u0000'-'\u00ff'</tt>.
</p>
</li>
<li> <h3>ASCII_UCodeESC_CharStream</h3>
Generated when the option <tt>JAVA_UNICODE_ESCAPE</tt> is set
and the <tt>UNICODE_INPUT</tt> option is not set.
<p>
This class treats the input as a stream of 1-byte characters.
However, the special escape sequence
</p>
<p>
<tt>("\\\\")* "\\" ("u")+</tt> - (<i>odd number of backslahes followed
by one or more 'u's.</i>)
</p>
<p>
is treated as a tag indicating that the next 4 bytes following
the tag will be hexadecimal digits forming a 4-digit hex number
whose value will be treated as the value of the character at the
position indicated by the first backslash. Note that this value
can be anything in the range <tt>0x0-0xffff</tt>.
</p>
</li>
<li> <h3>UCode_CharStream</h3>
Generated when the option <tt>UNICODE_INPUT</tt> is set and
the option <tt>JAVA_UNICODE_ESCAPE</tt> is not set.
<p>
This class treats the input as a stream of 2-byte characters. So
it reads 2 bytes <tt>b1</tt> and <tt>b2</tt> and returns them as
a single character using the expression <tt> b1 << 8 | b2 </tt>
assuming bigendian order. So in particular all the characters in
the range <tt>0x00-0xff</tt> are assumed to be stored as 2 bytes
with the first (higher-order) byte being 0.
</p>
</li>
<li> <h3>UCode_UCodeESC_CharStream</h3>
Generated when both the options <tt>UNICODE_INPUT</tt> and
<tt>JAVA_UNICODE_ESCAPE</tt> are set.
<p>
This class input is a stream of 2-byte characters (just
like the UCode_CharStream class) and the special escape sequence
</p>
<p>
<tt>("\\\\")* "\\" ("u")+</tt> - (<i>odd number of backslahes followed
by one or more 'u's.</i>)
</p>
<p>
is treated as a tag indicating that the next 4 2-byte characters
following the tag will be hexadecimal digits forming a 4-digit hex
number whose value will be treated as the value of the character at the
position indicated by the first backslash. Note that this value
can be any value in the range <tt>0x0-0xffff</tt>. Also note that
the backslash(es) and u(s) are all assumed to be given as 2-byte
characters (with the higher order byte value being 0).
</p>
</li>
</ul>
<h4>
Note : None of the above classes can be used to read characters in a
mixed mode, i.e., some characters given as 1-byte characters and others
as 2-byte characters. To do this, you need to set USER_CHAR_STREAM
option to true and define your own char stream.
</h4>
<hr />
<p>
(Throughout the following, we use the notation XXXCharStream that stands
for any of the above described 4 classes.)
</p>
<h3>Constructors</h3>
<ul>
<li><tt>
public XXXCharStream(java.io.InputStream dstream,
int startline, int startcolumn)
</tt>
<p>
Takes an input stream, starting line and column numbers and constructs a
CharStream object. It also creates buffers of initial size 4K for buffering the
characters and also for line and column numbers for each of those characters.
</p>
</li>
<li><tt> public XXXCharStream(java.io.InputStream dstream,
int startline, int startcolumn, int buffersize)
</tt>
<p>
Takes an input stream, starting line and column numbers and constructs a
CharStream object. It also creates buffers of initial size <tt>buffsize</tt> for buffering the
characters and also for line and column numbers for each of those characters.
</p>
<p>
So when you have an estimate on the maximum size of any token that can occur,
you can use that size to optimize the buffer sizes. Note, however, that
these sizes are only initial sizes and they will be expanded as and when
needed (in 2K steps).
</p>
</li>
</ul>
<h3>Methods</h3>
<p>
All the following methods will be static or nonstatic depending on
whether the STATIC option is true or false at the generation time. Also only
those methods that users can use in their lexical actions (using the
<tt>input_stream</tt> variable of the lexical analyzer) are documented
here. Rest of the (public) methods are very tightly coupled with the
implementation of the lexical analyzer and thus <b> should not </b> be
used in lexical actions. In the future when we adopt version 1.1 of the Java [tm] programming language, we will
streamline this by making that part of the interface an innerclass to
the lexical analyzer.
</p>
<ul>
<li> <tt>public final char readChar() throws java.io.IOException</tt>
<p>
This method returns the next "character" in the input according
to the rules of the CharStream class as described above. It will
throw <tt>java.io.IOException</tt> if it reaches EOF during the
process of "constructing" the character. It also updates the line
and column number and buffers the character for any possible
backtracking that may be required later. It also stores the line
and column numbers for the same purpose.
</p>
</li>
<li> <tt> public final int getBeginLine() </tt>
<p>
This method returns the line number for the beginning of the
current match.
</p>
</li>
<li> <tt> public final int getBeginColumn() </tt>
<p>
This method returns the column number for the beginning of the
current match.
</p>
</li>
<li> <tt> public final int getEndLine() </tt>
<p>
This method returns the line number for the ending of the
current match.
</p>
</li>
<li> <tt> public final int getEndColumn() </tt>
<p>
This method returns the column number for the ending of the
current match.
</p>
</li>
<li> <tt> public final void backup(int amount) </tt>
<p>
This method puts back <tt>amount</tt> number of characters
into the stream. Note that the amount indicates the number
of characters as constructed by <tt>readChar</tt>. Since the
buffers used are circular buffers, it cannot check for
illegal <tt>amount</tt> values, it just wraps around. So it
is the user's responsibility to make sure that those many
characters are really produced before a call to this method.
</p>
</li>
<li><tt> public final String GetImage()</tt>
<p>
Returns the image of the current match. As far as the XXXCharStream
is concerned, all characters after the last call to the private method
<tt>BeginToken</tt> are considered a part of the current match.
</p>
</li>
<li><tt> public void ReInit(java.io.InputStream dstream,
int startline, int startcolumn)</tt>
<p>
This method reinitializes the XXXCharStream classes with a (possibly
new) input stream and starting line and column numbers.
</p>
</li>
<li><tt> public void ReInit(java.io.InputStream dstream,
int startline, int startcolumn, int buffersize)</tt>
<p>
This method reinitializes the XXXCharStream classes with a (possibly
new) input stream and starting line and column numbers and adjusts
the size of the buffers to <tt>buffersize</tt>, by extending them.
Note that if the value of <tt>buffersize</tt> is less than the current
buffer sizes, they remain unchanged.
</p>
</li>
<li><tt> public void adjustBeginLineColumn(int newLine, int newCol)</tt>
<p>
This method adjusts the line and column number of the beginning of
the current match to <tt>newLine</tt> and <tt>newCol</tt> and
also adjusts the line and column numbers for all the characters
in the lookahead buffer.
</p>
</li>
</ul>
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