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Joel de Guzman 994d4e48cc moving stuff to classic spirit 
[SVN r44163]
2008-04-10 23:51:31 +00:00

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<title> Loops</title>
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<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b> Loops</b></font>
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<p>So far we have introduced a couple of EBNF operators that deal with looping.
We have the <tt>+</tt> positive operator, which matches the preceding symbol
one (1) or more times, as well as the Kleene star <tt>*</tt> which matches the
preceding symbol zero (0) or more times.</p>
<p>Taking this further, we may want to have a generalized loop operator. To some
this may seem to be a case of overkill. Yet there are grammars that are impractical
and cumbersome, if not impossible, for the basic EBNF iteration syntax to specify.
Examples:</p>
<blockquote>
<p><img src="theme/bullet.gif" width="12" height="12"> A file name may have
a maximum of 255 characters only.<br>
<img src="theme/bullet.gif" width="12" height="12"> A specific bitmap file
format has exactly 4096 RGB color information. <br>
<img src="theme/bullet.gif" width="12" height="12"> A 32 bit binary string
(1..32 1s or 0s).</p>
</blockquote>
<p>Other than the Kleene star <tt>*</tt>, the Positive closure <tt>+</tt>, and
the optional <tt>!</tt>, a more flexible mechanism for looping is provided for
by the framework. <br>
</p>
<table width="80%" border="0" align="center">
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<td colspan="2" class="table_title">Loop Constructs</td>
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<td class="table_cells" width="26%"><b>repeat_p (n) [p]</b></td>
<td class="table_cells" width="74%">Repeat <b>p</b> exactly <b>n</b> times</td>
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<td class="table_cells" width="26%"><b>repeat_p (n1, n2) [p]</b></td>
<td class="table_cells" width="74%">Repeat <b>p</b> at least <b>n1</b> times
and at most <b>n2</b> times</td>
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<td class="table_cells" width="26%"><b>repeat_p (n, more) [p] </b></td>
<td class="table_cells" width="74%">Repeat <b>p</b> at least <b>n</b> times,
continuing until <b>p</b> fails or the input is consumed</td>
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<p>Using the <tt>repeat_p</tt> parser, we can now write our examples above:</p>
<p>A file name with a maximum of 255 characters:<br>
</p>
<pre> <span class=identifier>valid_fname_chars </span><span class=special>= </span><span class=comment>/*..*/</span><span class=special>;
</span><span class=identifier>filename </span><span class=special>= </span><span class=identifier>repeat_p</span><span class=special>(</span><span class=number>1</span><span class=special>, </span><span class=number>255</span><span class=special>)[</span><span class=identifier>valid_fname_chars</span><span class=special>];</span></pre>
<p>A specific bitmap file format which has exactly 4096 RGB color information:<span class=special><br>
</span></p>
<pre> <span class=identifier>uint_parser</span><span class=special>&lt;</span><span class=keyword>unsigned</span><span class=special>, </span><span class=number>16</span><span class=special>, </span><span class=number>6</span><span class=special>, </span><span class=number>6</span><span class=special>&gt; </span><span class=identifier>rgb_p</span><span class=special>;
</span><span class=identifier>bitmap </span><span class=special>= </span><span class=identifier>repeat_p</span><span class=special>(</span><span class=number>4096</span><span class=special>)[</span><span class=identifier>rgb_p</span><span class=special>];</span></pre>
<p>As for the 32 bit binary string (1..32 1s or 0s), of course we could have easily
used the <tt>bin_p</tt> numeric parser instead. For the sake of demonstration
however:<span class=special><br>
</span></p>
<pre> <span class=identifier>bin</span><span class=number>32</span> <span class=special>= </span><span class=identifier>lexeme_d</span><span class=special>[</span><span class=identifier>repeat_p</span><span class=special>(</span>1, <span class=number>32</span><span class=special>)[</span><span class=identifier>ch_p</span><span class=special>(</span><span class=literal>'1'</span><span class=special>) </span><span class=special>| </span><span class=literal>'0'</span><span class=special>]];</span></pre>
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<td class="note_box"><img src="theme/note.gif" width="16" height="16"> Loop
parsers are run-time <a href="parametric_parsers.html">parametric</a>.</td>
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</table>
<p>The Loop parsers can be dynamic. Consider the parsing of a binary file of Pascal-style
length prefixed string, where the first byte determines the length of the incoming
string. Here's a sample input:
<blockquote>
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<div align="center">11</div>
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<div align="center">h</div>
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<div align="center">e</div>
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<div align="center">l</div>
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<div align="center">l</div>
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<div align="center">o</div>
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<div align="center"> _</div>
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<div align="center">w</div>
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<div align="center">o</div>
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<div align="center">r</div>
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<div align="center">l</div>
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<div align="center">d</div>
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</blockquote>
<p>This trivial example cannot be practically defined in traditional EBNF. Although
some EBNF syntax allow more powerful repetition constructs other than the Kleene
star, we are still limited to parsing fixed strings. The nature of EBNF forces
the repetition factor to be a constant. On the other hand, Spirit allows the
repetition factor to be variable at run time. We could write a grammar that
accepts the input string above:</p>
<pre><span class=identifier> </span><span class=keyword>int </span><span class=identifier>c</span><span class=special>;
</span><span class=identifier>r </span><span class=special>= </span><span class=identifier>anychar_p</span><span class=special>[</span><span class=identifier>assign_a</span><span class=special>(</span><span class=identifier>c</span><span class=special>)] </span><span class=special>&gt;&gt; </span><span class=identifier>repeat_p</span><span class=special>(</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>ref</span><span class=special>(</span><span class=identifier>c</span><span class=special>))[</span><span class=identifier>anychar_p</span><span class=special>];</span></pre>
<p>The expression</p>
<pre> <span class=identifier>anychar_p</span><span class=special>[</span><span class=identifier>assign_a</span><span class=special>(</span><span class=identifier>c</span><span class=special>)]</span></pre>
<p>extracts the first character from the input and puts it in <tt>c</tt>. What
is interesting is that in addition to constants, we can also use variables as
parameters to <tt>repeat_p</tt>, as demonstrated in </p>
<pre> <span class=identifier>repeat_p</span><span class=special>(</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>ref</span><span class=special>(</span><span class=identifier>c</span><span class=special>)</span><span class=special>)</span><span class=special>[</span><span class=identifier>anychar_p</span><span class=special>]</span></pre>
<p>Notice that <tt>boost::ref</tt> is used to reference the integer <tt>c</tt>.
This usage of <tt>repeat_p</tt> makes the parser defer the evaluation of the
repetition factor until it is actually needed. Continuing our example, since
the value 11 is already extracted from the input, <tt>repeat_p</tt> is is now
expected to loop exactly 11 times.</p>
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<hr size="1">
<p class="copyright">Copyright &copy; 1998-2003 Joel de Guzman<br>
<br>
<font size="2">Use, modification and distribution is subject to the Boost Software
License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
http://www.boost.org/LICENSE_1_0.txt) </font> </p>
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