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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="crc.crc_optimal"></a><a class="link" href="crc_optimal.html" title="Optimized CRC Computer">Optimized CRC Computer</a>
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</h2></div></div></div>
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<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">cstdint</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> <span class="comment">// for boost::uintmax_t</span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cstddef</span><span class="special">></span> <span class="comment">// for std::size_t</span>
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<span class="keyword">namespace</span> <span class="identifier">boost</span>
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<span class="special">{</span>
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<span class="keyword">template</span> <span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">Bits</span><span class="special">,</span> <span class="identifier">uintmax_t</span> <span class="identifier">TruncPoly</span><span class="special">,</span> <span class="identifier">uintmax_t</span> <span class="identifier">InitRem</span><span class="special">,</span>
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<span class="identifier">uintmax_t</span> <span class="identifier">FinalXor</span><span class="special">,</span> <span class="keyword">bool</span> <span class="identifier">ReflectIn</span><span class="special">,</span> <span class="keyword">bool</span> <span class="identifier">ReflectRem</span> <span class="special">></span>
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<span class="keyword">class</span> <span class="identifier">crc_optimal</span><span class="special">;</span>
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<span class="special">}</span>
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</pre>
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<p>
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The <span class="bold"><strong><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">crc_optimal</span></code></strong></span>
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class template acts as an unaugmented-CRC processor that can accept input at
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the byte-level. It takes all the Rocksoft™ Model CRC Algorithm parameters
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as template parameters. Like in <code class="computeroutput"><span class="identifier">crc_basic</span></code>,
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the <span class="emphasis"><em>WIDTH</em></span> stays as the first parameter and determines
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the built-in unsigned integer type used for division registers. The other Rocksoft™
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Model CRC Algorithm parameters move up to the template parameter field in the
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same relative order they were in the <code class="computeroutput"><span class="identifier">crc_basic</span></code>
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constructor. (Some parameters have defaults.) Objects based from <code class="computeroutput"><span class="identifier">crc_optimal</span></code> can either be default-constructed,
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giving it the same behavior as a <code class="computeroutput"><span class="identifier">crc_basic</span></code>
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object with the equivalent settings, or use one parameter, which overrides
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the initial remainder value<a href="#ftn.crc.crc_optimal.f0" class="footnote" name="crc.crc_optimal.f0"><sup class="footnote">[3]</sup></a> without permanently affecting the initial-remainder attribute.
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</p>
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<p>
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Besides template parameters and construction, <code class="computeroutput"><span class="identifier">crc_optimal</span></code>
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differs from <code class="computeroutput"><span class="identifier">crc_basic</span></code> interface-wise
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by:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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<p class="simpara">
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Adding five class-static immutable data members corresponding to the new
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template parameters.
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</p>
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<div class="table">
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<a name="crc.crc_optimal.crc_optimal_rmca"></a><p class="title"><b>Table 12.3. Additional RMCA Expressions in boost::crc_optimal</b></p>
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<div class="table-contents"><table class="table" summary="Additional RMCA Expressions in boost::crc_optimal">
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<colgroup>
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<col>
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<col>
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</colgroup>
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<thead><tr>
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<th>
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<p>
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New Member
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</p>
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</th>
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<th>
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<p>
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Equivalent
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</p>
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</th>
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</tr></thead>
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<tbody>
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<tr>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">truncated_polynominal</span></code>
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</p>
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</td>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">get_truncated_polynominal</span></code>
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</p>
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</td>
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</tr>
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<tr>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">initial_remainder</span></code>
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</p>
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</td>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">get_initial_remainder</span></code>
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</p>
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</td>
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</tr>
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<tr>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">reflect_input</span></code>
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</p>
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</td>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">get_reflect_input</span></code>
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</p>
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</td>
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</tr>
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<tr>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">reflect_remainder</span></code>
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</p>
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</td>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">get_reflect_remainder</span></code>
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</p>
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</td>
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</tr>
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<tr>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">final_xor_value</span></code>
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</p>
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</td>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">get_final_xor_value</span></code>
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</p>
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</td>
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</tr>
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</tbody>
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</table></div>
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</div>
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<br class="table-break">
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</li>
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<li class="listitem">
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Removing the <code class="computeroutput"><span class="identifier">process_bit</span></code>
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and <code class="computeroutput"><span class="identifier">process_bits</span></code> member
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functions.
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</li>
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<li class="listitem">
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Adding two versions of the <code class="computeroutput"><span class="keyword">operator</span>
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<span class="special">()</span></code> member function. The single-argument
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version forwards to <code class="computeroutput"><span class="identifier">process_byte</span></code>,
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making it suitable to STL algorithms that take (and possibly return) function
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objects<a href="#ftn.crc.crc_optimal.f1" class="footnote" name="crc.crc_optimal.f1"><sup class="footnote">[4]</sup></a>. The argument-less version forwards to <code class="computeroutput"><span class="identifier">checksum</span></code>,
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making it suitable for STL algorithms that expect generator objects<a href="#ftn.crc.crc_optimal.f2" class="footnote" name="crc.crc_optimal.f2"><sup class="footnote">[5]</sup></a>.
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</li>
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<li class="listitem">
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Merging the two <code class="computeroutput"><span class="identifier">reset</span></code> member
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functions into one. (It uses a single parameter that can have a default
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argument).
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</li>
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</ul></div>
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<p>
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The major difference between <code class="computeroutput"><span class="identifier">crc_basic</span></code>
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and <code class="computeroutput"><span class="identifier">crc_optimal</span></code> is the internals.
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Objects from <code class="computeroutput"><span class="identifier">crc_basic</span></code> run
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their CRC algorithm one bit at a time, no matter which unit is used as input.
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Objects from <code class="computeroutput"><span class="identifier">crc_optimal</span></code>, when
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<span class="emphasis"><em>WIDTH</em></span> is at least <code class="computeroutput"><span class="identifier">CHAR_BIT</span></code><a href="#ftn.crc.crc_optimal.f3" class="footnote" name="crc.crc_optimal.f3"><sup class="footnote">[6]</sup></a>, use a byte-indexed table-driven CRC algorithm which is a <span class="bold"><strong>lot</strong></span> faster than processing individual bits.
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</p>
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<p>
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Since all of the parameters are specified at compile-time, you cannot reuse
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a single computer object for separate runs with differing parameters. The type
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uses the automatically-defined copy/move/assign and destruction routines.
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</p>
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<div class="footnotes">
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<br><hr style="width:100; text-align:left;margin-left: 0">
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<div id="ftn.crc.crc_optimal.f0" class="footnote"><p><a href="#crc.crc_optimal.f0" class="para"><sup class="para">[3] </sup></a>
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i.e. The interim-remainder before any input is read.
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</p></div>
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<div id="ftn.crc.crc_optimal.f1" class="footnote"><p><a href="#crc.crc_optimal.f1" class="para"><sup class="para">[4] </sup></a>
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Like <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span></code>.
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</p></div>
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<div id="ftn.crc.crc_optimal.f2" class="footnote"><p><a href="#crc.crc_optimal.f2" class="para"><sup class="para">[5] </sup></a>
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Albeit this object won't change its return value within code that only
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uses it as a generator.
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</p></div>
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<div id="ftn.crc.crc_optimal.f3" class="footnote"><p><a href="#crc.crc_optimal.f3" class="para"><sup class="para">[6] </sup></a>
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i.e. The optimizations are suspended if the <span class="emphasis"><em>WIDTH</em></span> only
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justifies using part of a single byte.
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</p></div>
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</div>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2001, 2003, 2012 Daryle Walker<p>
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Distributed under the Boost Software License, Version 1.0. (See the accompanying
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file LICENSE_1_0.txt or a copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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</p>
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</div></td>
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</tr></table>
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