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>Theoretical 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="introduction.html" title="Introduction"> 11<link rel="next" href="crc_optimal.html" title="Optimized CRC Computer"> 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="introduction.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_optimal.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_basic"></a><a class="link" href="crc_basic.html" title="Theoretical CRC Computer">Theoretical CRC Computer</a> 29</h2></div></div></div> 30<pre class="programlisting"><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> 31 32<span class="keyword">namespace</span> <span class="identifier">boost</span> 33<span class="special">{</span> 34 <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> 35 <span class="keyword">class</span> <span class="identifier">crc_basic</span><span class="special">;</span> 36<span class="special">}</span> 37</pre> 38<p> 39 The <span class="bold"><strong><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">crc_basic</span></code></strong></span> 40 class template acts as an unaugmented-CRC processor that can accept input at 41 the bit-level. It only takes one Rocksoft™ Model CRC Algorithm parameter 42 as a template parameter, the <span class="emphasis"><em>WIDTH</em></span>, which determines the 43 built-in unsigned integer type used for division registers. The other Rocksoft™ 44 Model CRC Algorithm parameters can be passed on through the constructor. (Most 45 of the parameters have defaults.) 46 </p> 47<p> 48 The integer type used for registers is published as <code class="computeroutput"><span class="identifier">value_type</span></code>, 49 while the Rocksoft™ Model CRC Algorithm attributes can be discovered 50 as: 51 </p> 52<div class="table"> 53<a name="crc.crc_basic.crc_basic_rmca"></a><p class="title"><b>Table 12.1. RMCA Parameters in boost::crc_basic</b></p> 54<div class="table-contents"><table class="table" summary="RMCA Parameters in boost::crc_basic"> 55<colgroup> 56<col> 57<col> 58<col> 59<col> 60</colgroup> 61<thead><tr> 62<th> 63 <p> 64 Parameter 65 </p> 66 </th> 67<th> 68 <p> 69 Member Name 70 </p> 71 </th> 72<th> 73 <p> 74 Kind 75 </p> 76 </th> 77<th> 78 <p> 79 Expression Type 80 </p> 81 </th> 82</tr></thead> 83<tbody> 84<tr> 85<td> 86 <p> 87 <span class="emphasis"><em>WIDTH</em></span> 88 </p> 89 </td> 90<td> 91 <p> 92 <code class="computeroutput"><span class="identifier">bit_count</span></code> 93 </p> 94 </td> 95<td> 96 <p> 97 class-static immutable data member 98 </p> 99 </td> 100<td> 101 <p> 102 <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span></code> 103 </p> 104 </td> 105</tr> 106<tr> 107<td> 108 <p> 109 <span class="emphasis"><em>POLY</em></span> 110 </p> 111 </td> 112<td> 113 <p> 114 <code class="computeroutput"><span class="identifier">get_truncated_polynominal</span></code> 115 </p> 116 </td> 117<td> 118 <p> 119 <code class="computeroutput"><span class="keyword">const</span></code> member function 120 </p> 121 </td> 122<td> 123 <p> 124 <code class="computeroutput"><span class="identifier">value_type</span></code> 125 </p> 126 </td> 127</tr> 128<tr> 129<td> 130 <p> 131 <span class="emphasis"><em>INIT</em></span> 132 </p> 133 </td> 134<td> 135 <p> 136 <code class="computeroutput"><span class="identifier">get_initial_remainder</span></code> 137 </p> 138 </td> 139<td> 140 <p> 141 <code class="computeroutput"><span class="keyword">const</span></code> member function 142 </p> 143 </td> 144<td> 145 <p> 146 <code class="computeroutput"><span class="identifier">value_type</span></code> 147 </p> 148 </td> 149</tr> 150<tr> 151<td> 152 <p> 153 <span class="emphasis"><em>REFIN</em></span> 154 </p> 155 </td> 156<td> 157 <p> 158 <code class="computeroutput"><span class="identifier">get_reflect_input</span></code> 159 </p> 160 </td> 161<td> 162 <p> 163 <code class="computeroutput"><span class="keyword">const</span></code> member function 164 </p> 165 </td> 166<td> 167 <p> 168 <code class="computeroutput"><span class="keyword">bool</span></code> 169 </p> 170 </td> 171</tr> 172<tr> 173<td> 174 <p> 175 <span class="emphasis"><em>REFOUT</em></span> 176 </p> 177 </td> 178<td> 179 <p> 180 <code class="computeroutput"><span class="identifier">get_reflect_remainder</span></code> 181 </p> 182 </td> 183<td> 184 <p> 185 <code class="computeroutput"><span class="keyword">const</span></code> member function 186 </p> 187 </td> 188<td> 189 <p> 190 <code class="computeroutput"><span class="keyword">bool</span></code> 191 </p> 192 </td> 193</tr> 194<tr> 195<td> 196 <p> 197 <span class="emphasis"><em>XOROUT</em></span> 198 </p> 199 </td> 200<td> 201 <p> 202 <code class="computeroutput"><span class="identifier">get_final_xor_value</span></code> 203 </p> 204 </td> 205<td> 206 <p> 207 <code class="computeroutput"><span class="keyword">const</span></code> member function 208 </p> 209 </td> 210<td> 211 <p> 212 <code class="computeroutput"><span class="identifier">value_type</span></code> 213 </p> 214 </td> 215</tr> 216</tbody> 217</table></div> 218</div> 219<br class="table-break"><p> 220 Since most of the parameters are specified at run-time, you can reuse a single 221 computer object for separate runs with differing parameters. The type uses 222 the automatically-defined copy/move/assign and destruction routines. 223 </p> 224<p> 225 Here's an example of reuse: 226 </p> 227<p> 228</p> 229<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">unsigned</span><span class="special">,</span> <span class="keyword">unsigned</span><span class="special">></span> <span class="identifier">crc_16_and_xmodem</span><span class="special">(</span> <span class="keyword">void</span> <span class="keyword">const</span> <span class="special">*</span><span class="identifier">b</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">l</span> <span class="special">)</span> 230<span class="special">{</span> 231 <span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">unsigned</span><span class="special">,</span> <span class="keyword">unsigned</span><span class="special">></span> <span class="identifier">result</span><span class="special">;</span> 232 <a class="co" name="crc.crc_basic.c0" href="crc_basic.html#crc.crc_basic.c1"><img src="../../../doc/src/images/callouts/1.png" alt="1" border="0"></a><span class="identifier">boost</span><span class="special">::</span><span class="identifier">crc_basic</span><span class="special"><</span><span class="number">16</span><span class="special">></span> <span class="identifier">crc1</span><span class="special">(</span> <span class="number">0x8005u</span><span class="special">,</span> <span class="number">0u</span><span class="special">,</span> <span class="number">0u</span><span class="special">,</span> <span class="keyword">true</span><span class="special">,</span> <span class="keyword">true</span> <span class="special">);</span> 233 234 <span class="identifier">crc1</span><span class="special">.</span><span class="identifier">process_bytes</span><span class="special">(</span> <span class="identifier">b</span><span class="special">,</span> <span class="identifier">l</span> <span class="special">);</span> 235 <span class="identifier">result</span><span class="special">.</span><span class="identifier">first</span> <span class="special">=</span> <span class="identifier">crc1</span><span class="special">.</span><span class="identifier">checksum</span><span class="special">();</span> 236 <a class="co" name="crc.crc_basic.c2" href="crc_basic.html#crc.crc_basic.c3"><img src="../../../doc/src/images/callouts/2.png" alt="2" border="0"></a><span class="identifier">crc1</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">crc_basic</span><span class="special"><</span><span class="number">16</span><span class="special">>(</span> <span class="number">0x8408u</span><span class="special">,</span> <span class="identifier">crc1</span><span class="special">.</span><span class="identifier">get_initial_remainder</span><span class="special">(),</span> 237 <span class="identifier">crc1</span><span class="special">.</span><span class="identifier">get_final_xor_value</span><span class="special">(),</span> <span class="identifier">crc1</span><span class="special">.</span><span class="identifier">get_reflect_input</span><span class="special">(),</span> 238 <span class="identifier">crc1</span><span class="special">.</span><span class="identifier">get_reflect_remainder</span><span class="special">()</span> <span class="special">);</span> 239 <span class="identifier">crc1</span><span class="special">.</span><span class="identifier">process_bytes</span><span class="special">(</span> <span class="identifier">b</span><span class="special">,</span> <span class="identifier">l</span> <span class="special">);</span> 240 <span class="identifier">result</span><span class="special">.</span><span class="identifier">second</span> <span class="special">=</span> <span class="identifier">crc1</span><span class="special">.</span><span class="identifier">checksum</span><span class="special">();</span> 241 242 <span class="keyword">return</span> <span class="identifier">result</span><span class="special">;</span> 243<span class="special">}</span> 244</pre> 245<p> 246 </p> 247<p> 248 For now, most Rocksoft™ Model CRC Algorithm parameters can only be changed 249 through assignment to the entire object. 250 </p> 251<div class="calloutlist"><table border="0" summary="Callout list"> 252<tr> 253<td width="5%" valign="top" align="left"><p><a name="crc.crc_basic.c1"></a><a href="#crc.crc_basic.c0"><img src="../../../doc/src/images/callouts/1.png" alt="1" border="0"></a> </p></td> 254<td valign="top" align="left"><p> 255 The parameters are based on <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">crc_16_type</span></code>. 256 </p></td> 257</tr> 258<tr> 259<td width="5%" valign="top" align="left"><p><a name="crc.crc_basic.c3"></a><a href="#crc.crc_basic.c2"><img src="../../../doc/src/images/callouts/2.png" alt="2" border="0"></a> </p></td> 260<td valign="top" align="left"><p> 261 Change the parameters to match <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">crc_xmodem_type</span></code>. 262 </p></td> 263</tr> 264</table></div> 265<p> 266 This example necessarily demonstrates input and output. There is one output 267 member function, <code class="computeroutput"><span class="identifier">checksum</span></code> that 268 returns the remainder from the CRC steps plus any post-processing reflection 269 and/or XOR-masking. There are several options to submit input data for processing: 270 </p> 271<div class="table"> 272<a name="crc.crc_basic.crc_basic_input"></a><p class="title"><b>Table 12.2. Member Functions for Input in boost::crc_basic</b></p> 273<div class="table-contents"><table class="table" summary="Member Functions for Input in boost::crc_basic"> 274<colgroup> 275<col> 276<col> 277</colgroup> 278<thead><tr> 279<th> 280 <p> 281 Member Function 282 </p> 283 </th> 284<th> 285 <p> 286 Input Type/Style 287 </p> 288 </th> 289</tr></thead> 290<tbody> 291<tr> 292<td> 293 <p> 294 <code class="computeroutput"><span class="identifier">process_bit</span></code> 295 </p> 296 </td> 297<td> 298 <p> 299 A single bit. 300 </p> 301 </td> 302</tr> 303<tr> 304<td> 305 <p> 306 <code class="computeroutput"><span class="identifier">process_bits</span></code> 307 </p> 308 </td> 309<td> 310 <p> 311 A specified number of bits within the given byte. Reading starts 312 from the highest-order desired bit. 313 </p> 314 </td> 315</tr> 316<tr> 317<td> 318 <p> 319 <code class="computeroutput"><span class="identifier">process_byte</span></code> 320 </p> 321 </td> 322<td> 323 <p> 324 An entire byte. The bits within the byte are read in an order consistent 325 with <code class="computeroutput"><span class="identifier">get_reflect_input</span></code>. 326 </p> 327 </td> 328</tr> 329<tr> 330<td> 331 <p> 332 <code class="computeroutput"><span class="identifier">process_block</span></code> 333 </p> 334 </td> 335<td> 336 <p> 337 All bytes in the range. The range is defined by a pointer to the 338 first byte and another pointer to one (byte) past-the-end. 339 </p> 340 </td> 341</tr> 342<tr> 343<td> 344 <p> 345 <code class="computeroutput"><span class="identifier">process_bytes</span></code> 346 </p> 347 </td> 348<td> 349 <p> 350 All bytes in the range. The range is defined by a pointer to the 351 first byte and a count of number of bytes to read. 352 </p> 353 </td> 354</tr> 355</tbody> 356</table></div> 357</div> 358<br class="table-break"><p> 359 The input functions currently do not return anything. 360 </p> 361<p> 362 Persistent objects mean that the data doesn't have to be in one block: 363 </p> 364<p> 365</p> 366<pre class="programlisting"><span class="keyword">unsigned</span> <span class="identifier">combined_crc_16</span><span class="special">(</span> <span class="keyword">unsigned</span> <span class="identifier">block_count</span><span class="special">,</span> <span class="special">...</span> <span class="special">)</span> 367<span class="special">{</span> 368 <a class="co" name="crc.crc_basic.c4" href="crc_basic.html#crc.crc_basic.c5"><img src="../../../doc/src/images/callouts/1.png" alt="1" border="0"></a><span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">std</span><span class="special">;</span> 369 370 <a class="co" name="crc.crc_basic.c6" href="crc_basic.html#crc.crc_basic.c7"><img src="../../../doc/src/images/callouts/2.png" alt="2" border="0"></a><span class="identifier">boost</span><span class="special">::</span><span class="identifier">crc_basic</span><span class="special"><</span><span class="number">16</span><span class="special">></span> <span class="identifier">crc1</span><span class="special">(</span> <span class="number">0x8005u</span><span class="special">,</span> <span class="number">0u</span><span class="special">,</span> <span class="number">0u</span><span class="special">,</span> <span class="keyword">true</span><span class="special">,</span> <span class="keyword">true</span> <span class="special">);</span> 371 <span class="identifier">va_list</span> <span class="identifier">ap</span><span class="special">;</span> 372 373 <span class="identifier">va_start</span><span class="special">(</span> <span class="identifier">ap</span><span class="special">,</span> <span class="identifier">block_count</span> <span class="special">);</span> 374 <span class="keyword">while</span> <span class="special">(</span> <span class="identifier">block_count</span><span class="special">--</span> <span class="special">)</span> 375 <span class="special">{</span> 376 <span class="keyword">void</span> <span class="keyword">const</span> <span class="special">*</span> <span class="keyword">const</span> <span class="identifier">bs</span> <span class="special">=</span> <span class="identifier">va_arg</span><span class="special">(</span> <span class="identifier">ap</span><span class="special">,</span> <span class="keyword">void</span> <span class="keyword">const</span> <span class="special">*</span> <span class="special">);</span> 377 <span class="identifier">size_t</span> <span class="keyword">const</span> <span class="identifier">bl</span> <span class="special">=</span> <span class="identifier">va_arg</span><span class="special">(</span> <span class="identifier">ap</span><span class="special">,</span> <span class="identifier">size_t</span> <span class="special">);</span> 378 379 <a class="co" name="crc.crc_basic.c8" href="crc_basic.html#crc.crc_basic.c9"><img src="../../../doc/src/images/callouts/3.png" alt="3" border="0"></a><span class="identifier">crc1</span><span class="special">.</span><span class="identifier">process_bytes</span><span class="special">(</span> <span class="identifier">bs</span><span class="special">,</span> <span class="identifier">bl</span> <span class="special">);</span> 380 <span class="special">}</span> 381 <span class="identifier">va_end</span><span class="special">(</span> <span class="identifier">ap</span> <span class="special">);</span> 382 383 <span class="keyword">return</span> <span class="identifier">crc1</span><span class="special">.</span><span class="identifier">checksum</span><span class="special">();</span> 384<span class="special">}</span> 385</pre> 386<p> 387 </p> 388<p> 389 No CRC operation throws, so there is no need for extra protection between the 390 varargs macro calls. 391 </p> 392<div class="calloutlist"><table border="0" summary="Callout list"> 393<tr> 394<td width="5%" valign="top" align="left"><p><a name="crc.crc_basic.c5"></a><a href="#crc.crc_basic.c4"><img src="../../../doc/src/images/callouts/1.png" alt="1" border="0"></a> </p></td> 395<td valign="top" align="left"><p> 396 C-style variable-argument routines are or may be macros. 397 </p></td> 398</tr> 399<tr> 400<td width="5%" valign="top" align="left"><p><a name="crc.crc_basic.c7"></a><a href="#crc.crc_basic.c6"><img src="../../../doc/src/images/callouts/2.png" alt="2" border="0"></a> </p></td> 401<td valign="top" align="left"><p> 402 The parameters are based on <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">crc_16_type</span></code>. 403 </p></td> 404</tr> 405<tr> 406<td width="5%" valign="top" align="left"><p><a name="crc.crc_basic.c9"></a><a href="#crc.crc_basic.c8"><img src="../../../doc/src/images/callouts/3.png" alt="3" border="0"></a> </p></td> 407<td valign="top" align="left"><p> 408 The <code class="computeroutput"><span class="identifier">va_arg</span></code> calls were within 409 the <code class="computeroutput"><span class="identifier">process_bytes</span></code> call, 410 but I remembered that calling order is not guaranteed among function arguments, 411 so I need explicit object declarations to enforce the extraction order. 412 </p></td> 413</tr> 414</table></div> 415<p> 416 The input-influenced state of a computer object may be read with the <code class="computeroutput"><span class="identifier">get_interim_remainder</span></code> member function. An 417 object may be loaded with the same kind of state with the one-argument version 418 of the <code class="computeroutput"><span class="identifier">reset</span></code> member function. 419 The state is the remainder from the CRC operations performed on all the already-entered 420 input, without any output post-processing. 421 </p> 422<p> 423 The two functions can be used together to save the state to a persistent medium 424 and restore it later. Note that the Rocksoft™ Model CRC Algorithm parameters 425 have to saved/restored via other means, if they're not fixed within a program. 426 </p> 427<p> 428 Calling <code class="computeroutput"><span class="identifier">reset</span></code> with no arguments 429 sets the interim remainder to what <span class="emphasis"><em>INIT</em></span> was set at object 430 construction. This lets one computer object be used for independent runs with 431 separate data messages using the same CRC standard. 432 </p> 433</div> 434<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 435<td align="left"></td> 436<td align="right"><div class="copyright-footer">Copyright © 2001, 2003, 2012 Daryle Walker<p> 437 Distributed under the Boost Software License, Version 1.0. (See the accompanying 438 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>) 439 </p> 440</div></td> 441</tr></table> 442<hr> 443<div class="spirit-nav"> 444<a accesskey="p" href="introduction.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_optimal.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 445</div> 446</body> 447</html> 448
