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<a name="boost.crc"></a><div class="titlepage"></div>
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<h2><span class="refentrytitle">Function template crc</span></h2>
<p>boost::crc — Compute the (unaugmented) CRC of a memory block. </p>
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<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: &lt;<a class="link" href="../crc/reference.html#header.boost.crc_hpp" title="Header &lt;boost/crc.hpp&gt;">boost/crc.hpp</a>&gt;

</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> Bits<span class="special">,</span> <span class="keyword">typename</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">uint_t</span><span class="special">&lt;</span> <span class="identifier">Bits</span> <span class="special">&gt;</span><span class="special">::</span><span class="identifier">fast</span> TruncPoly<span class="special">,</span>
         <span class="keyword">typename</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">uint_t</span><span class="special">&lt;</span> <span class="identifier">Bits</span> <span class="special">&gt;</span><span class="special">::</span><span class="identifier">fast</span> InitRem<span class="special">,</span>
         <span class="keyword">typename</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">uint_t</span><span class="special">&lt;</span> <span class="identifier">Bits</span> <span class="special">&gt;</span><span class="special">::</span><span class="identifier">fast</span> FinalXor<span class="special">,</span> <span class="keyword">bool</span> ReflectIn<span class="special">,</span>
         <span class="keyword">bool</span> ReflectRem<span class="special">&gt;</span>
  <span class="identifier">uint_t</span><span class="special">&lt;</span> <span class="identifier">Bits</span> <span class="special">&gt;</span><span class="special">::</span><span class="identifier">fast</span> <span class="identifier">crc</span><span class="special">(</span><span class="keyword">void</span> <span class="keyword">const</span> <span class="special">*</span> buffer<span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> byte_count<span class="special">)</span><span class="special">;</span></pre></div>
<div class="refsect1">
<a name="id-1.3.11.11.2.11.4"></a><h2>Description</h2>
<p>Computes the polynomial remainder of a CRC run, assuming that <span class="emphasis"><em>buffer</em></span> and <span class="emphasis"><em>byte_count</em></span> describe a memory block representing the polynomial dividend. The division steps are altered so the result directly gives a checksum, without need to augment the memory block with scratch-space bytes. The first byte is considered the highest order, going down for subsequent bytes.</p>
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<div class="note"><table border="0" summary="Note">
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
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<tr><td align="left" valign="top"><p>Unaugmented-style CRC runs perform modulo-2 polynomial division in an altered order. The trailing <span class="emphasis"><em>Bits</em></span> number of zero-valued bits needed to extracted an (unprocessed) checksum is virtually moved to near the beginning of the message. This is OK since the XOR operation is commutative and associative. It also means that you can get a checksum anytime. Since data is being read byte-wise, a table of pre-computed remainder changes (as XOR masks) can be used to speed computation. </p></td></tr>
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<td><p><span class="term">Parameters:</span></p></td>
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<td><p><span class="term"><code class="computeroutput">buffer</code></span></p></td>
<td><p>The address where the memory block begins. </p></td>
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<td><p><span class="term"><code class="computeroutput">byte_count</code></span></p></td>
<td><p>The number of bytes in the memory block.</p></td>
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<td><p><span class="term">Template Parameters:</span></p></td>
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<td><p><span class="term"><code class="computeroutput">Bits</code></span></p></td>
<td><p>The order of the modulo-2 polynomial divisor. (<span class="emphasis"><em>Width</em></span> from the RMCA) </p></td>
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<td><p><span class="term"><code class="computeroutput">FinalXor</code></span></p></td>
<td><p>The (XOR) bit-mask to be applied to the output remainder, after possible reflection but before returning. (<span class="emphasis"><em>XorOut</em></span> from the RMCA) </p></td>
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<td><p><span class="term"><code class="computeroutput">InitRem</code></span></p></td>
<td><p>The (unaugmented) initial state of the polynomial remainder. (<span class="emphasis"><em>Init</em></span> from the RMCA) </p></td>
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<td><p><span class="term"><code class="computeroutput">ReflectIn</code></span></p></td>
<td><p>If <code class="computeroutput">True</code>, input bytes are read lowest-order bit first, otherwise highest-order bit first. (<span class="emphasis"><em>RefIn</em></span> from the RMCA) </p></td>
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<td><p><span class="term"><code class="computeroutput">ReflectRem</code></span></p></td>
<td><p>If <code class="computeroutput">True</code>, the output remainder is reflected before the XOR-mask. (<span class="emphasis"><em>RefOut</em></span> from the RMCA)</p></td>
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<td><p><span class="term"><code class="computeroutput">TruncPoly</code></span></p></td>
<td><p>The lowest coefficients of the divisor polynomial. The highest-order coefficient is omitted and always assumed to be 1. (<span class="emphasis"><em>Poly</em></span> from the RMCA) </p></td>
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<td><p><span class="term">Requires:</span></p></td>
<td><p>0 &lt; <span class="emphasis"><em>Bits</em></span> &lt;= <code class="computeroutput">std::numeric_limits&lt;uintmax_t&gt;::digits</code> </p></td>
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<td><p><span class="term">Returns:</span></p></td>
<td><p>The checksum, which is the last (interim) remainder plus any output processing.</p></td>
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<td align="right"><div class="copyright-footer">Copyright © 2001, 2003, 2012 Daryle Walker<p>
        Distributed under the Boost Software License, Version 1.0. (See the accompanying
        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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