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diff --git a/src/boost/libs/crc/crc.html b/src/boost/libs/crc/crc.html new file mode 100644 index 00000000..a11c0200 --- /dev/null +++ b/src/boost/libs/crc/crc.html @@ -0,0 +1,632 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN"> +<html> +<head> +<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> +<title>Boost CRC Library Documentation</title> +</head> + +<body text="black" bgcolor="white" link="blue" vlink="purple" alink="red"> + +<h1><img src="../../boost.png" alt="boost.png (6897 bytes)" +align="middle" width="277" height="86">Header <cite><<a +href="../../boost/crc.hpp">boost/crc.hpp</a>></cite></h1> + +<p>The header <cite><<a +href="../../boost/crc.hpp">boost/crc.hpp</a>></cite> supplies two +class templates in namespace <code>boost</code>. These templates define +objects that can compute the <dfn>CRC</dfn>, or cyclic redundancy code +(or check), of a given stream of data. The header also supplies +function templates to compute a CRC in one step.</p> + +<h2><a name="contents">Contents</a></h2> + +<ol> + <li><a href="#contents">Contents</a></li> + <li><a href="#header">Header Synopsis</a></li> + <li><a href="#rationale">Rationale</a></li> + <li><a href="#background">Background</a> + <ul> + <li><a href="#parameters">CRC Parameters</a></li> + </ul></li> + <li><a href="#crc_basic">Theoretical CRC Computer</a></li> + <li><a href="#crc_optimal">Optimized CRC Computer</a></li> + <li><a href="#usage">Computer Usage</a></li> + <li><a href="#crc_func">CRC Function</a></li> + <li><a href="#a_crc_func">Augmented-CRC Function</a></li> + <li><a href="#crc_ex">Pre-Defined CRC Samples</a></li> + <li><a href="#references">References</a></li> + <li><a href="#credits">Credits</a> + <ul> + <li><a href="#contributors">Contributors</a></li> + <li><a href="#acknowledgements">Acknowledgements</a></li> + <li><a href="#history">History</a></li> + </ul></li> +</ol> + +<h2><a name="header">Header Synopsis</a></h2> + +<blockquote><pre>#include <boost/integer.hpp> <i>// for boost::uint_t</i> +#include <cstddef> <i>// for std::size_t</i> + +namespace boost +{ + +template < std::size_t Bits > + class crc_basic; + +template < std::size_t Bits, <em>impl_def</em> TruncPoly = 0u, + <em>impl_def</em> InitRem = 0u, + <em>impl_def</em> FinalXor = 0u, bool ReflectIn = false, + bool ReflectRem = false > + class crc_optimal; + +template < std::size_t Bits, <em>impl_def</em> TruncPoly, + <em>impl_def</em> InitRem, <em>impl_def</em> FinalXor, + bool ReflectIn, bool ReflectRem > + typename uint_t<Bits>::fast crc( void const *buffer, + std::size_t byte_count ); + +template < std::size_t Bits, <em>impl_def</em> TruncPoly > + typename uint_t<Bits>::fast augmented_crc( void const *buffer, + std::size_t byte_count, + typename uint_t<Bits>::fast initial_remainder = 0u ); + +typedef crc_optimal<16, 0x8005, 0, 0, true, true> crc_16_type; +typedef crc_optimal<16, 0x1021, 0xFFFF, 0, false, false> crc_ccitt_type; +typedef crc_optimal<16, 0x8408, 0, 0, true, true> crc_xmodem_type; + +typedef crc_optimal<32, 0x04C11DB7, 0xFFFFFFFF, 0xFFFFFFFF, true, true> + crc_32_type; + +} +</pre></blockquote> + +<p>The implementation-defined type <var>impl_def</var> stands for the +quickest-to-manipulate built-in unsigned integral type that can +represent at least <var>Bits</var> bits.</p> + +<h2><a name="rationale">Rationale</a></h2> + +<p>A common error detection technique, especially with electronic +communications, is an appended checksum. The transmitter sends its data +bits, followed by the bits of the checksum. The checksum is based on +operations done on the data bit stream. The receiver applies the same +operations on the bits it gets, and then gets the checksum. If the +computed checksum doesn't match the received checksum, then an error +ocurred in the transmission. There is the slight chance that the error +is only in the checksum, and an actually-correct data stream is +rejected. There is also the chance of an error occurring that does not +change the checksum, making that error invisible. CRC is a common +checksum type, used for error detection for hardware interfaces and +encoding formats.</p> + +<h2><a name="background">Background</a></h2> + +<p>CRCs work by computing the remainder of a modulo-2 polynominal +division. The message is treated as the (binary) coefficents of a long +polynominal for the dividend, with the earlier bits of the message fed +first as the polynominal's highest coefficents. A particular CRC +algorithm has another polynominal associated with it to be used as the +divisor. The quotient is ignored. The remainder of the division +considered the checksum. However, the division uses modulo-2 rules (no +carries) for the coefficents.</p> + +<p>See <cite><a href="http://www.ross.net/crc/crcpaper.html">A +Painless Guide to CRC Error Detection Algorithms</a></cite> for complete +information. A clearer guide is at the <a +href="http://www.netrino.com/Embedded-Systems/How-To/CRC-Calculation-C-Code">CRC +Implementation Code in C</a> web page.</p> + +<h3><a name="parameters">CRC Parameters</a></h3> + +<dl> + <dt>Truncated polynominal + <dd>The divisor polynominal has a degree one bit larger than the + checksum (remainder) size. That highest bit is always one, so + it is ignored when describing a particular CRC type. Excluding + this bit makes the divisor fit in the same data type as the + checksum. + + <dt>Initial remainder + <dd>The interim CRC remainder changes as each bit is processed. + Usually, the interim remainder starts at zero, but some CRCs use + a different initial value to avoid "blind spots." A + blind spot is when a common sequence of message bits does not + change certain interim remainder values. + + <dt>Final XOR value + <dd>A CRC remainder can be combined with a defined value, <i>via</i> + a bitwise exclusive-or operation, before being returned to the + user. The value is usually zero, meaning the interim remainder + is returned unchanged. The other common value is an all-ones + value, meaning that the bitwise complement of the interim + remainder is returned. + + <dt>Reflected input + <dd>A message's bits are usually fed a byte at a time, with the + highest bits of the byte treated as the higher bits of the + dividend polynominal. Some CRCs reflect the bits (about the + byte's center, so the first and last bits are switched, + <i>etc.</i>) before feeding. + + <dt>Reflected (remainder) output + <dd>Some CRCs return the reflection of the interim remainder (taking + place <em>before</em> the final XOR value stage). +</dl> + +<h2><a name="crc_basic">Theoretical CRC Computer</a></h2> + +<blockquote><pre>template < std::size_t Bits > +class boost::crc_basic +{ +public: + // Type + typedef <em>implementation_defined</em> value_type; + + // Constant reflecting template parameter + static std::size_t const bit_count = Bits; + + // Constructor + explicit crc_basic( value_type truncated_polynominal, + value_type initial_remainder = 0, value_type final_xor_value = 0, + bool reflect_input = false, bool reflect_remainder = false ); + + // Internal Operations + value_type get_truncated_polynominal() const; + value_type get_initial_remainder() const; + value_type get_final_xor_value() const; + bool get_reflect_input() const; + bool get_reflect_remainder() const; + + value_type get_interim_remainder() const; + void reset( value_type new_rem ); + void reset(); + + // External Operations + void process_bit( bool bit ); + void process_bits( unsigned char bits, std::size_t bit_count ); + void process_byte( unsigned char byte ); + void process_block( void const *bytes_begin, void const *bytes_end ); + void process_bytes( void const *buffer, std::size_t byte_count ); + + value_type checksum() const; + +}; +</pre></blockquote> + +<p>The <code>value_type</code> is the smallest built-in type that can +hold the specified (by <code>Bits</code>) number of bits. This should +be <code>boost::uint_t<Bits>::least</code>, see the <a +href="../integer/doc/html/boost_integer/integer.html">documentation for integer type +selection</a> for details.</p> + +<p>This implementation is slow since it computes its CRC the same way as +in theory, bit by bit. No optimizations are performed. It wastes space +since most of the CRC parameters are specified at run-time as +constructor parameters.</p> + +<h2><a name="crc_optimal">Optimized CRC Computer</a></h2> + +<blockquote><pre>template < std::size_t Bits, <em>impl_def</em> TruncPoly, + <em>impl_def</em> InitRem, <em>impl_def</em> FinalXor, + bool ReflectIn, bool ReflectRem > +class boost::crc_optimal +{ +public: + // Type + typedef <em>implementation_defined</em> value_type; + + // Constants reflecting template parameters + static std::size_t const bit_count = Bits; + static value_type const truncated_polynominal = TruncPoly; + static value_type const initial_remainder = InitRem; + static value_type const final_xor_value = FinalXor; + static bool const reflect_input = ReflectIn; + static bool const reflect_remainder = ReflectRem; + + // Constructor + explicit crc_optimal( value_type init_rem = InitRem ); + + // Internal Operations + value_type get_truncated_polynominal() const; + value_type get_initial_remainder() const; + value_type get_final_xor_value() const; + bool get_reflect_input() const; + bool get_reflect_remainder() const; + + value_type get_interim_remainder() const; + void reset( value_type new_rem = InitRem ); + + // External Operations + void process_byte( unsigned char byte ); + void process_block( void const *bytes_begin, void const *bytes_end ); + void process_bytes( void const *buffer, std::size_t byte_count ); + + value_type checksum() const; + + // Operators + void operator ()( unsigned char byte ); + value_type operator ()() const; + +}; +</pre></blockquote> + +<p>The <code>value_type</code> is the quickest-to-manipulate built-in +type that can hold at least the specified (by <code>Bits</code>) number +of bits. This should be <code>boost::uint_t<Bits>::fast</code>. +See the <a href="../integer/doc/html/boost_integer/integer.html">integer type selection +documentation</a> for details. The <code>TruncPoly</code>, +<code>InitRem</code>, and <code>FinalXor</code> template parameters also +are of this type.</p> + +<p>This implementation is fast since it uses as many optimizations as +practical. All of the CRC parameters are specified at compile-time as +template parameters. No individual bits are considered; only whole +bytes are passed. A table of interim CRC values versus byte values is +pre-computed when the first object using a particular bit size, +truncated polynominal, and input reflection state is processed.</p> + +<h2><a name="usage">Computer Usage</a></h2> + +<p>The two class templates have different policies on where the CRC's +parameters go. Both class templates use the number of bits in the CRC +as the first template parameter. The theoretical computer class +template has the bit count as its only template parameter, all the other +CRC parameters are entered through the constructor. The optimized +computer class template obtains all its CRC parameters as template +parameters, and instantiated objects are usually +default-constructed.</p> + +<p>The CRC parameters can be inspected at run-time with the following +member functions: <code>get_truncated_polynominal</code>, +<code>get_initial_remainder</code>, <code>get_final_xor_value</code>, +<code>get_reflect_input</code>, and <code>get_reflect_remainder</code>. +The fast computer also provides compile-time constants for its CRC +parameters.</p> + +<p>The <code>get_interim_remainder</code> member function returns the +internal state of the CRC remainder. It represents the unreflected +remainder of the last division. Saving an interim remainder allows the +freezing of CRC processing, as long as the other CRC parameters and the +current position of the bit stream are saved. Restarting a frozen +stream involves constructing a new computer with the most of the old +computer's parameters. The only change is to use the frozen remainder +as the new computer's initial remainder. Then the interrupted bit +stream can be fed as if nothing happened. The fast CRC computer has a +special constructor that takes one argument, an interim remainder, for +this purpose (overriding the initial remainder CRC parameter).</p> + +<p>The <code>reset</code> member functions reset the internal state of +the CRC remainder to the given value. If no value is given, then the +internal remainder is set to the initial remainder value when the object +was created. The remainder must be unreflected. When a CRC calculation +is finished, calling <code>reset</code> lets the object be reused for a +new session.</p> + +<p>After any construction, both CRC computers work the same way. +Feeding new data to a computer is in a seperate operation(s) from +extracting the current CRC value from the computer. The following table +lists the feeding and extracting operations.</p> + +<table cellpadding="5" border="1"> + <caption>Regular CRC Operations</caption> + <tr> + <th>Operation</th> + <th>Description</th> + </tr> + <tr> + <td><code>void process_bit( bool bit );</code></td> + <td>Feeds the single <var>bit</var> to the computer, updating + the interim CRC. It is only defined for the slow CRC + computer.</td> + </tr> + <tr> + <td><code>void process_bits( unsigned char bits, std::size_t + bit_count );</code></td> + <td>Acts as applying <code>process_bit</code> to the lowest + <var>bit_count</var> bits given in <var>bits</var>, most + significant relevant bit first. The results are undefined + if <var>bit_count</var> exceeds the number of bits per byte. + It is only defined for the slow CRC computer.</td> + </tr> + <tr> + <td><code>void process_byte( unsigned char byte );</code></td> + <td>Acts as applying <code>process_bit</code> to the all the + bits in <var>byte</var>. If reflection is not desired, the + bits are fed from the most to least significant. The bits + are fed in the opposite order if reflection is desired.</td> + </tr> + <tr> + <td><code>void process_block( void const *bytes_begin, void + const *bytes_end );</code></td> + <td>Acts as applying <code>process_byte</code> to each byte in + the given memory block. This memory block starts at + <var>bytes_begin</var> and finishes before + <var>bytes_end</var>. The bytes are processed in that + order.</td> + </tr> + <tr> + <td><code>void process_bytes( void const *buffer, std::size_t + byte_count );</code></td> + <td>Acts as applying <code>process_byte</code> to each byte in + the given memory block. This memory block starts at + <var>buffer</var> and lasts for <var>byte_count</var> bytes. + The bytes are processed in ascending order.</td> + </tr> + <tr> + <td><code>value_type checksum() const;</code></td> + <td>Returns the CRC checksum of the data passed in so far, + possibly after applying the remainder-reflection and + exclusive-or operations.</td> + </tr> + <tr> + <td><code>void operator ()( unsigned char byte );</code></td> + <td>Calls <code>process_byte</code>. This member function lets + its object act as a (stateful) function object. It is only + defined for the fast CRC computer.</td> + </tr> + <tr> + <td><code>value_type operator ()() const;</code></td> + <td>Calls <code>checksum</code>. This member function lets + its object act as a generator function object. It is only + defined for the fast CRC computer.</td> + </tr> +</table> + +<p>You can use them like this:</p> + +<blockquote><pre>#include <boost/crc.hpp> <i>// for boost::crc_basic, boost::crc_optimal</i> +#include <boost/cstdint.hpp> <i>// for boost::uint16_t</i> + +#include <algorithm> <i>// for std::for_each</i> +#include <cassert> <i>// for assert</i> +#include <cstddef> <i>// for std::size_t</i> +#include <iostream> <i>// for std::cout</i> +#include <ostream> <i>// for std::endl</i> + + +// Main function +int +main () +{ + // This is "123456789" in ASCII + unsigned char const data[] = { 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, + 0x38, 0x39 }; + std::size_t const data_len = sizeof( data ) / sizeof( data[0] ); + + // The expected CRC for the given data + boost::uint16_t const expected = 0x29B1; + + // Simulate CRC-CCITT + boost::crc_basic<16> crc_ccitt1( 0x1021, 0xFFFF, 0, false, false ); + crc_ccitt1.process_bytes( data, data_len ); + assert( crc_ccitt1.checksum() == expected ); + + // Repeat with the optimal version (assuming a 16-bit type exists) + boost::crc_optimal<16, 0x1021, 0xFFFF, 0, false, false> crc_ccitt2; + crc_ccitt2 = std::for_each( data, data + data_len, crc_ccitt2 ); + assert( crc_ccitt2() == expected ); + + std::cout << "All tests passed." << std::endl; + return 0; +} +</pre></blockquote> + +<h2><a name="crc_func">CRC Function</a></h2> + +<blockquote><pre>template < std::size_t Bits, <em>impl_def</em> TruncPoly, + <em>impl_def</em> InitRem, <em>impl_def</em> FinalXor, + bool ReflectIn, bool ReflectRem > +typename boost::uint_t<Bits>::fast +boost::crc( void const *buffer, std::size_t byte_count ); +</pre></blockquote> + +<p>The <code>boost::crc</code> function template computes the CRC of a +given data block. The data block starts at the address given by +<var>buffer</var> and lasts for <var>byte_count</var> bytes. The CRC +parameters are passed through template arguments, identical to the +optimized CRC computer (<a href="#crc_optimal">see above</a>). In fact, +such a computer is used to implement this function.</p> + +<h2><a name="a_crc_func">Augmented-CRC Function</a></h2> + +<blockquote><pre>template < std::size_t Bits, <em>impl_def</em> TruncPoly > +typename boost::uint_t<Bits>::fast +boost::augmented_crc( void const *buffer, std::size_t byte_count, + typename boost::uint_t<Bits>::fast initial_remainder = 0u ); +</pre></blockquote> + +<p>All the other CRC-computing function or class templates work assuming +that the division steps start immediately on the first message bits. +The <code>boost::augmented_crc</code> function template has a +different division order. Instead of combining (<i>via</i> bitwise +exclusive-or) the current message bit with the highest bit of a separate +remainder, these templates shift a new message bit into the low bit of a +remainder register as the highest bit is shifted out. The new method +means that the bits in the inital remainder value are processed before +any of the actual message bits are processed. To compensate, the real +CRC can only be extracted after feeding enough zero bits (the same count +as the register size) after the message bits.</p> + +<p>The template parameters of the function template are +the CRC's bit size (<code>Bits</code>) and the truncated polynominal +(<code>TruncPoly</code>). The function parameters are the starting address of +the data block to be worked on (<var>buffer</var>), the number of bytes in that +data block (<var>byte_count</var>), and the incoming value of the remainder +(<var>initial_remainder</var>). That last parameter defaults to zero if it is +ommitted.</p> + +<p>This function template is useful if the bytes of the CRC directly +follow the message's bytes. First, set the bytes of where the CRC will +go to zero. Then use <code>augmented_crc</code> over the augmented +message, <i>i.e.</i> the message bytes and the appended CRC bytes. Then +assign the result to the CRC. To later check a received message, either +use <code>augmented_crc</code> (with the same parameters as +transmission, of course) on the received <em>unaugmented</em> message +and check if the result equals the CRC, or use +<code>augmented_crc</code> on the received <em>augmented</em> message +and check if the result equals zero. Note that the CRC has to be stored +with the more-significant bytes first (big-endian).</p> + +<p>Interruptions in the CRC data can be handled by feeding the result of +<code>augmented_crc</code> of the previous data block as the +<var>initial_remainder</var> when calling <code>augmented_crc</code> on +the next data block. Remember that the actual CRC can only be +determined after feeding the augmented bytes. Since this method uses +modulo-2 polynominal division at its most raw, neither final XOR values +nor reflection can be used.</p> + +<p>Note that for the same CRC system, the initial remainder for +augmented message method will be different than for the unaugmented +message method. The main exception is zero; if the augmented-CRC +algorithm uses a zero initial remainder, the equivalent unaugmented-CRC +algorithm will also use a zero initial remainder. Given an initial +remainder for a augmented-CRC algorithm, the result from processing just +zero-valued CRC bytes without any message bytes is the equivalent inital +remainder for the unaugmented-CRC algorithm. An example follows:</p> + +<blockquote><pre>#include <boost/crc.hpp> <i>// for boost::crc_basic, boost::augmented_crc</i> +#include <boost/cstdint.hpp> <i>// for boost::uint16_t</i> + +#include <cassert> <i>// for assert</i> +#include <iostream> <i>// for std::cout</i> +#include <ostream> <i>// for std::endl</i> + + +// Main function +int +main () +{ + using boost::uint16_t; + using boost::augmented_crc; + + uint16_t data[6] = { 2, 4, 31, 67, 98, 0 }; + uint16_t const init_rem = 0x123; + + uint16_t crc1 = augmented_crc<16, 0x8005>( data, sizeof(data), init_rem ); + + uint16_t const zero = 0; + uint16_t const new_init_rem = augmented_crc<16, 0x8005>( &zero, sizeof(zero) ); + + boost::crc_basic<16> crc2( 0x8005, new_init_rem ); + crc2.process_block( data, &data[5] ); // don't include CRC + assert( crc2.checksum() == crc1 ); + + std::cout << "All tests passed." << std::endl; + return 0; +} +</pre></blockquote> + +<h2><a name="crc_ex">Pre-Defined CRC Samples</a></h2> + +<p>Four sample CRC types are given, representing several common CRC +algorithms. For example, computations from <code>boost::crc_32_type</code> +can be used for implementing the PKZip standard. Note that, in general, this +library is concerned with CRC implementation, and not with determining +"good" sets of CRC parameters.</p> + +<table cellpadding="5" border="1"> + <caption>Common CRCs</caption> + <tr> + <th>Algorithm</th> + <th>Example Protocols</th> + </tr> + <tr> + <td><code>crc_16_type</code></td> + <td>BISYNCH, ARC</td> + </tr> + <tr> + <td><code>crc_ccitt_type</code></td> + <td>designated by CCITT (Comité Consultatif International + Télégraphique et Téléphonique)</td> + </tr> + <tr> + <td><code>crc_xmodem_type</code></td> + <td>XMODEM</td> + </tr> + <tr> + <td><code>crc_32_type</code></td> + <td>PKZip, AUTODIN II, Ethernet, FDDI</td> + </tr> +</table> + +<hr> + +<h2><a name="references">References</a></h2> + +<ul> + <li>The CRC header itself: <cite><a href="../../boost/crc.hpp">crc.hpp</a></cite> + <li>Some test code: <cite><a href="test/crc_test.cpp">crc_test.cpp</a></cite> + <li>Some example code: <cite><a href="crc_example.cpp">crc_example.cpp</a></cite> +</ul> + +<h2><a name="credits">Credits</a></h2> + +<h3><a name="contributors">Contributors</a></h3> + +<dl> + <dt>Michael Barr (<a + href="mailto:mbarr@netrino.com">mbarr@netrino.com</a>) + <dd>Wrote <a + href="http://www.netrino.com/Embedded-Systems/How-To/CRC-Calculation-C-Code">CRC + Implementation Code in C</a>, a less-confusing guide to implementing CRC + algorithms. (Originally published as "Slow and Steady + Never Lost the Race" in the January 2000 issue of <cite><a + href="http://www.embedded.com/">Embedded Systems + Programming</a></cite>, pages 37–46. The web version used to be + known as <cite><a href="http://www.netrino.com/Connecting/2000-01/">Easier + Said Than Done</a></cite>.) + + <dt>Daryle Walker + <dd>Started the library and contributed the theoretical and optimal + CRC computation class templates and the CRC computing function + template. Contributed <cite><a + href="test/crc_test.cpp">crc_test.cpp</a></cite> and <cite><a + href="crc_example.cpp">crc_example.cpp</a></cite>. + + <dt>Ross N. Williams + <dd>Wrote <cite><a href="http://www.ross.net/crc/crcpaper.html">A + Painless Guide to CRC Error Detection Algorithms</a></cite>, a + definitive source of CRC information. +</dl> + +<h3><a name="acknowledgements">Acknowledgements</a></h3> + +<p>For giving advice on compiler/C++ compliance, implementation, +interface, algorithms, and bug reports:</p> + +<ul> + <li>Darin Adler</li> + <li>Beman Dawes</li> + <li>Doug Gregor</li> + <li>John Maddock</li> + <li>Joe Mariadassou</li> + <li>Jens Maurer</li> + <li>Vladimir Prus</li> + <li>Joel Young</li> +</ul> + +<h3><a name="history">History</a></h3> + +<dl> + <dt>18 Dec 2011, Daryle Walker + <dd>Folded the two versions of <code>boost::augmented_crc</code> together. + + <dt>15 Jun 2003, Daryle Walker + <dd>Added example program. + + <dt>14 May 2001, Daryle Walker + <dd>Initial version. +</dl> + +<hr> + +<p>Revised: 18 December 2011</p> + +<p>Copyright 2001, 2003, 2011 Daryle Walker. Use, modification, and distribution +are subject to the Boost Software License, Version 1.0. (See accompanying +file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or a copy at +<<a href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>>.)</p> + +</body> +</html> |