1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<head> 4<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 5<title>Optimized CRC Computer</title> 6<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> 7<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 8<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> 9<link rel="up" href="../crc.html" title="Chapter 12. Boost.CRC 1.5"> 10<link rel="prev" href="crc_basic.html" title="Theoretical CRC Computer"> 11<link rel="next" href="crc_function.html" title="CRC Function"> 12</head> 13<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 14<table cellpadding="2" width="100%"><tr> 15<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> 16<td align="center"><a href="../../../index.html">Home</a></td> 17<td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> 18<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 19<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 20<td align="center"><a href="../../../more/index.htm">More</a></td> 21</tr></table> 22<hr> 23<div class="spirit-nav"> 24<a accesskey="p" href="crc_basic.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../crc.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="crc_function.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 25</div> 26<div class="section"> 27<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 28<a name="crc.crc_optimal"></a><a class="link" href="crc_optimal.html" title="Optimized CRC Computer">Optimized CRC Computer</a> 29</h2></div></div></div> 30<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> 31<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> 32 33<span class="keyword">namespace</span> <span class="identifier">boost</span> 34<span class="special">{</span> 35 <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> 36 <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> 37 <span class="keyword">class</span> <span class="identifier">crc_optimal</span><span class="special">;</span> 38<span class="special">}</span> 39</pre> 40<p> 41 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> 42 class template acts as an unaugmented-CRC processor that can accept input at 43 the byte-level. It takes all the Rocksoft™ Model CRC Algorithm parameters 44 as template parameters. Like in <code class="computeroutput"><span class="identifier">crc_basic</span></code>, 45 the <span class="emphasis"><em>WIDTH</em></span> stays as the first parameter and determines 46 the built-in unsigned integer type used for division registers. The other Rocksoft™ 47 Model CRC Algorithm parameters move up to the template parameter field in the 48 same relative order they were in the <code class="computeroutput"><span class="identifier">crc_basic</span></code> 49 constructor. (Some parameters have defaults.) Objects based from <code class="computeroutput"><span class="identifier">crc_optimal</span></code> can either be default-constructed, 50 giving it the same behavior as a <code class="computeroutput"><span class="identifier">crc_basic</span></code> 51 object with the equivalent settings, or use one parameter, which overrides 52 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. 53 </p> 54<p> 55 Besides template parameters and construction, <code class="computeroutput"><span class="identifier">crc_optimal</span></code> 56 differs from <code class="computeroutput"><span class="identifier">crc_basic</span></code> interface-wise 57 by: 58 </p> 59<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 60<li class="listitem"> 61<p class="simpara"> 62 Adding five class-static immutable data members corresponding to the new 63 template parameters. 64 </p> 65<div class="table"> 66<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> 67<div class="table-contents"><table class="table" summary="Additional RMCA Expressions in boost::crc_optimal"> 68<colgroup> 69<col> 70<col> 71</colgroup> 72<thead><tr> 73<th> 74 <p> 75 New Member 76 </p> 77 </th> 78<th> 79 <p> 80 Equivalent 81 </p> 82 </th> 83</tr></thead> 84<tbody> 85<tr> 86<td> 87 <p> 88 <code class="computeroutput"><span class="identifier">truncated_polynominal</span></code> 89 </p> 90 </td> 91<td> 92 <p> 93 <code class="computeroutput"><span class="identifier">get_truncated_polynominal</span></code> 94 </p> 95 </td> 96</tr> 97<tr> 98<td> 99 <p> 100 <code class="computeroutput"><span class="identifier">initial_remainder</span></code> 101 </p> 102 </td> 103<td> 104 <p> 105 <code class="computeroutput"><span class="identifier">get_initial_remainder</span></code> 106 </p> 107 </td> 108</tr> 109<tr> 110<td> 111 <p> 112 <code class="computeroutput"><span class="identifier">reflect_input</span></code> 113 </p> 114 </td> 115<td> 116 <p> 117 <code class="computeroutput"><span class="identifier">get_reflect_input</span></code> 118 </p> 119 </td> 120</tr> 121<tr> 122<td> 123 <p> 124 <code class="computeroutput"><span class="identifier">reflect_remainder</span></code> 125 </p> 126 </td> 127<td> 128 <p> 129 <code class="computeroutput"><span class="identifier">get_reflect_remainder</span></code> 130 </p> 131 </td> 132</tr> 133<tr> 134<td> 135 <p> 136 <code class="computeroutput"><span class="identifier">final_xor_value</span></code> 137 </p> 138 </td> 139<td> 140 <p> 141 <code class="computeroutput"><span class="identifier">get_final_xor_value</span></code> 142 </p> 143 </td> 144</tr> 145</tbody> 146</table></div> 147</div> 148<br class="table-break"> 149</li> 150<li class="listitem"> 151 Removing the <code class="computeroutput"><span class="identifier">process_bit</span></code> 152 and <code class="computeroutput"><span class="identifier">process_bits</span></code> member 153 functions. 154 </li> 155<li class="listitem"> 156 Adding two versions of the <code class="computeroutput"><span class="keyword">operator</span> 157 <span class="special">()</span></code> member function. The single-argument 158 version forwards to <code class="computeroutput"><span class="identifier">process_byte</span></code>, 159 making it suitable to STL algorithms that take (and possibly return) function 160 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>, 161 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>. 162 </li> 163<li class="listitem"> 164 Merging the two <code class="computeroutput"><span class="identifier">reset</span></code> member 165 functions into one. (It uses a single parameter that can have a default 166 argument). 167 </li> 168</ul></div> 169<p> 170 The major difference between <code class="computeroutput"><span class="identifier">crc_basic</span></code> 171 and <code class="computeroutput"><span class="identifier">crc_optimal</span></code> is the internals. 172 Objects from <code class="computeroutput"><span class="identifier">crc_basic</span></code> run 173 their CRC algorithm one bit at a time, no matter which unit is used as input. 174 Objects from <code class="computeroutput"><span class="identifier">crc_optimal</span></code>, when 175 <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. 176 </p> 177<p> 178 Since all of the parameters are specified at compile-time, you cannot reuse 179 a single computer object for separate runs with differing parameters. The type 180 uses the automatically-defined copy/move/assign and destruction routines. 181 </p> 182<div class="footnotes"> 183<br><hr style="width:100; text-align:left;margin-left: 0"> 184<div id="ftn.crc.crc_optimal.f0" class="footnote"><p><a href="#crc.crc_optimal.f0" class="para"><sup class="para">[3] </sup></a> 185 i.e. The interim-remainder before any input is read. 186 </p></div> 187<div id="ftn.crc.crc_optimal.f1" class="footnote"><p><a href="#crc.crc_optimal.f1" class="para"><sup class="para">[4] </sup></a> 188 Like <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span></code>. 189 </p></div> 190<div id="ftn.crc.crc_optimal.f2" class="footnote"><p><a href="#crc.crc_optimal.f2" class="para"><sup class="para">[5] </sup></a> 191 Albeit this object won't change its return value within code that only 192 uses it as a generator. 193 </p></div> 194<div id="ftn.crc.crc_optimal.f3" class="footnote"><p><a href="#crc.crc_optimal.f3" class="para"><sup class="para">[6] </sup></a> 195 i.e. The optimizations are suspended if the <span class="emphasis"><em>WIDTH</em></span> only 196 justifies using part of a single byte. 197 </p></div> 198</div> 199</div> 200<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 201<td align="left"></td> 202<td align="right"><div class="copyright-footer">Copyright © 2001, 2003, 2012 Daryle Walker<p> 203 Distributed under the Boost Software License, Version 1.0. (See the accompanying 204 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>) 205 </p> 206</div></td> 207</tr></table> 208<hr> 209<div class="spirit-nav"> 210<a accesskey="p" href="crc_basic.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../crc.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="crc_function.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 211</div> 212</body> 213</html> 214
