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%$Header: /uC Software Multi-Volume Book (A)/Chapter, CCH0, CRC, Checksum, Hash Extensions/c_cch0.tex 3 12/31/00 7:42p Dashley1 $ |
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\chapter[\ccchzeroshorttitle{}]{\ccchzerolongtitle{}} |
\chapter[\ccchzeroshorttitle{}]{\ccchzerolongtitle{}} |
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\label{ccch0} |
\label{ccch0} |
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\beginchapterquote{``\ldots{} Beauty is the first test: there is no permanent |
\beginchapterquote{``\ldots{} Beauty is the first test: there is no permanent |
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place in the world for ugly mathematics.''} |
place in the world for ugly mathematics.''} |
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{G.H. Hardy \cite{bibref:b:mathematiciansapology:1940}, |
{G.H. Hardy \cite{bibref:b:mathematiciansapology:1940}, |
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p.85} |
p.85} |
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\section{Introduction} |
\section{Introduction} |
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%Section Tag: INT |
%Section Tag: INT |
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\section{crc32} |
\section{crc32} |
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\begin{tclcommandname}{crc32}% |
\begin{tclcommandname}{crc32}% |
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generates the CRC-32 of a file or string. This CRC can be reliably used to obtain |
generates the CRC-32 of a file or string. This CRC can be reliably used to obtain |
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digital signatures of files or data. |
digital signatures of files or data. |
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\end{tclcommandname} |
\end{tclcommandname} |
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\begin{tclcommandsynopsis} |
\begin{tclcommandsynopsis} |
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\tclcommandsynopsisline{crc32}{filename} |
\tclcommandsynopsisline{crc32}{filename} |
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\tclcommandsynopsisline{crc32}{-string binarystringval} |
\tclcommandsynopsisline{crc32}{-string binarystringval} |
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\tclcommandsynopsisline{crc32}{-initialstate} |
\tclcommandsynopsisline{crc32}{-initialstate} |
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\tclcommandsynopsisline{crc32}{-advancestate state filename} |
\tclcommandsynopsisline{crc32}{-advancestate state filename} |
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\tclcommandsynopsisline{crc32}{-advancestate -string state binarystringval} |
\tclcommandsynopsisline{crc32}{-advancestate -string state binarystringval} |
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\tclcommandsynopsisline{crc32}{-crcfromstate state} |
\tclcommandsynopsisline{crc32}{-crcfromstate state} |
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\end{tclcommandsynopsis} |
\end{tclcommandsynopsis} |
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\begin{tclcommanddescription} |
\begin{tclcommanddescription} |
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The \emph{crc32} command forms the CRC-32 of the binary contents of a file |
The \emph{crc32} command forms the CRC-32 of the binary contents of a file |
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or of the binary contents of a string. The CRC-32 is useful as a digital |
or of the binary contents of a string. The CRC-32 is useful as a digital |
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signature, and can be used with unity probability to determine that two |
signature, and can be used with unity probability to determine that two |
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files are different, or with a probability of about $1-2^{-32}$ to determine |
files are different, or with a probability of about $1-2^{-32}$ to determine |
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that two files are almost certainly identical. |
that two files are almost certainly identical. |
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In the invocations below, the CRC-32 is always returned as a 10-character ASCII string |
In the invocations below, the CRC-32 is always returned as a 10-character ASCII string |
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of the form \emph{``0xDDDDDDDD''}, where \emph{``DDDDDDDD''} is the hexadecimal representation |
of the form \emph{``0xDDDDDDDD''}, where \emph{``DDDDDDDD''} is the hexadecimal representation |
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of the 32-bit CRC-32, and \emph{``0x''} is a constant 2-character prefix which is included for |
of the 32-bit CRC-32, and \emph{``0x''} is a constant 2-character prefix which is included for |
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aesthetics. It is guaranteed that: |
aesthetics. It is guaranteed that: |
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\begin{itemize} |
\begin{itemize} |
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\item The string returned will be exclusive ASCII. |
\item The string returned will be exclusive ASCII. |
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\item The string will have a length of exactly 10 characters. |
\item The string will have a length of exactly 10 characters. |
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\item The first two characters of the string will be \emph{``0x''}. |
\item The first two characters of the string will be \emph{``0x''}. |
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\item Any letters in the hexadecimal representation will be upper-case. |
\item Any letters in the hexadecimal representation will be upper-case. |
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\end{itemize} |
\end{itemize} |
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\begin{tclcommandinternaldescription}{\tclcommanddescsynopsisline{crc32}{filename}} |
\begin{tclcommandinternaldescription}{\tclcommanddescsynopsisline{crc32}{filename}} |
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Returns the CRC-32 of \emph{filename}, treated as an ordered collection of bytes (i.e. |
Returns the CRC-32 of \emph{filename}, treated as an ordered collection of bytes (i.e. |
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newline characters and file termination characters are not treated---the file is |
newline characters and file termination characters are not treated---the file is |
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treated as a binary file). \emph{filename} must be specified in the form accepted by |
treated as a binary file). \emph{filename} must be specified in the form accepted by |
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the Tcl internals (forward slashes only). |
the Tcl internals (forward slashes only). |
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\end{tclcommandinternaldescription} |
\end{tclcommandinternaldescription} |
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\begin{tclcommandinternaldescription}{\tclcommanddescsynopsisline{crc32}{-string binarystringval}} |
\begin{tclcommandinternaldescription}{\tclcommanddescsynopsisline{crc32}{-string binarystringval}} |
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Returns the CRC-32 of \emph{binarystringval}, treated as an ordered collection of bytes (i.e. |
Returns the CRC-32 of \emph{binarystringval}, treated as an ordered collection of bytes (i.e. |
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newline characters and string termination characters are not honored---the string is |
newline characters and string termination characters are not honored---the string is |
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treated as a binary string).\footnote{For an ASCII string, the \emph{crc32} extension will |
treated as a binary string).\footnote{For an ASCII string, the \emph{crc32} extension will |
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behave as expected, and will process all characters up to but not including the zero |
behave as expected, and will process all characters up to but not including the zero |
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terminator. However, the \emph{crc32} extension will also correctly process non-ASCII strings.} |
terminator. However, the \emph{crc32} extension will also correctly process non-ASCII strings.} |
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\end{tclcommandinternaldescription} |
\end{tclcommandinternaldescription} |
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\begin{tclcommandinternaldescription}{\tclcommanddescsynopsisline{crc32}{-initialstate}% |
\begin{tclcommandinternaldescription}{\tclcommanddescsynopsisline{crc32}{-initialstate}% |
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\tclcommanddescsynopsisline{crc32}{-advancestate state filename}% |
\tclcommanddescsynopsisline{crc32}{-advancestate state filename}% |
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\tclcommanddescsynopsisline{crc32}{-advancestate -string state filename}% |
\tclcommanddescsynopsisline{crc32}{-advancestate -string state filename}% |
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\tclcommanddescsynopsisline{crc32}{-crcfromstate state}% |
\tclcommanddescsynopsisline{crc32}{-crcfromstate state}% |
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} |
} |
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The four forms above are designed to allow ``running CRCs'' to be calculated; in which |
The four forms above are designed to allow ``running CRCs'' to be calculated; in which |
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the CRC is calculated piecemeal. These forms allow the caller to retain the internal |
the CRC is calculated piecemeal. These forms allow the caller to retain the internal |
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state vector of the CRC calculation algorithm. |
state vector of the CRC calculation algorithm. |
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The first form, \emph{crc32 -initialstate}, returns an ASCII representation of the |
The first form, \emph{crc32 -initialstate}, returns an ASCII representation of the |
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correct initial state vector of the CRC-32 state machine. The client is required |
correct initial state vector of the CRC-32 state machine. The client is required |
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to obtain this initial state before beginning a piecemeal CRC calculation. Although the |
to obtain this initial state before beginning a piecemeal CRC calculation. Although the |
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returned string is a constant (it will always be the same), representational details |
returned string is a constant (it will always be the same), representational details |
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may change in future versions of the \emph{crc32} extension, and so a caller should never |
may change in future versions of the \emph{crc32} extension, and so a caller should never |
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make assumptions about what this invocation will return, as these assumptions may |
make assumptions about what this invocation will return, as these assumptions may |
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render a script incompatible with future versions of \emph{crc32}. |
render a script incompatible with future versions of \emph{crc32}. |
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The second and third forms, \emph{crc32 -advancestate state filename} |
The second and third forms, \emph{crc32 -advancestate state filename} |
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and \emph{crc32 -advancestate -string state filename}, apply a file or a binary string |
and \emph{crc32 -advancestate -string state filename}, apply a file or a binary string |
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to \emph{state} to produce a new \emph{state}, which is returned. This new \emph{state} |
to \emph{state} to produce a new \emph{state}, which is returned. This new \emph{state} |
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must be retained by the caller and used in subsequent calls. |
must be retained by the caller and used in subsequent calls. |
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The final form, \emph{crc32 -crcfromstate state}, maps from the state vector to the |
The final form, \emph{crc32 -crcfromstate state}, maps from the state vector to the |
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calculated CRC, and will return a 10-character ASCII string as described above. |
calculated CRC, and will return a 10-character ASCII string as described above. |
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\end{tclcommandinternaldescription} |
\end{tclcommandinternaldescription} |
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\end{tclcommanddescription} |
\end{tclcommanddescription} |
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\begin{tclcommandusagenotes} |
\begin{tclcommandusagenotes} |
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The ``piecemeal'' forms are as efficient as the file and string forms---there is no difference |
The ``piecemeal'' forms are as efficient as the file and string forms---there is no difference |
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in the internal algorithms. The primary cost of the piecemeal forms is in importing and |
in the internal algorithms. The primary cost of the piecemeal forms is in importing and |
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exporting the algorithm state vector to/from an ASCII string. |
exporting the algorithm state vector to/from an ASCII string. |
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Thus, the piecemeal forms become less efficient when |
Thus, the piecemeal forms become less efficient when |
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small files or strings are processed, as there are more exports and imports |
small files or strings are processed, as there are more exports and imports |
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of the state vector. When using the piecemeal forms, processing the data in |
of the state vector. When using the piecemeal forms, processing the data in |
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larger chunks will give better performance. |
larger chunks will give better performance. |
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\end{tclcommandusagenotes} |
\end{tclcommandusagenotes} |
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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\noindent\begin{figure}[!b] |
\noindent\begin{figure}[!b] |
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\noindent\rule[-0.25in]{\textwidth}{1pt} |
\noindent\rule[-0.25in]{\textwidth}{1pt} |
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\begin{tiny} |
\begin{tiny} |
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\begin{verbatim} |
\begin{verbatim} |
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$Workfile: c_cch0.tex $ |
$Workfile: c_cch0.tex $ |
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$Archive: /uC Software Multi-Volume Book (A)/Chapter, CCH0, CRC, Checksum, Hash Extensions/c_cch0.tex $ |
$Archive: /uC Software Multi-Volume Book (A)/Chapter, CCH0, CRC, Checksum, Hash Extensions/c_cch0.tex $ |
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$Revision: 3 $ |
$Revision: 3 $ |
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$Author: Dashley1 $ |
$Author: Dashley1 $ |
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$Date: 12/31/00 7:42p $ |
$Date: 12/31/00 7:42p $ |
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$Modtime: 12/24/00 4:05a $ |
$Modtime: 12/24/00 4:05a $ |
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\end{verbatim} |
\end{verbatim} |
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\end{tiny} |
\end{tiny} |
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\noindent\rule[0.25in]{\textwidth}{1pt} |
\noindent\rule[0.25in]{\textwidth}{1pt} |
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\end{figure} |
\end{figure} |
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%$History: c_cch0.tex $ |
%$History: c_cch0.tex $ |
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% |
% |
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% ***************** Version 3 ***************** |
% ***************** Version 3 ***************** |
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% User: Dashley1 Date: 12/31/00 Time: 7:42p |
% User: Dashley1 Date: 12/31/00 Time: 7:42p |
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% Updated in $/uC Software Multi-Volume Book (A)/Chapter, CCH0, CRC, Checksum, Hash Extensions |
% Updated in $/uC Software Multi-Volume Book (A)/Chapter, CCH0, CRC, Checksum, Hash Extensions |
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% |
% |
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% ***************** Version 2 ***************** |
% ***************** Version 2 ***************** |
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% User: Dashley1 Date: 12/22/00 Time: 12:54a |
% User: Dashley1 Date: 12/22/00 Time: 12:54a |
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% Updated in $/uC Software Multi-Volume Book (A)/Chapter, CCH0, CRC, Checksum, Hash Extensions |
% Updated in $/uC Software Multi-Volume Book (A)/Chapter, CCH0, CRC, Checksum, Hash Extensions |
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% Tcl automated method of build refined. |
% Tcl automated method of build refined. |
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% |
% |
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% ***************** Version 1 ***************** |
% ***************** Version 1 ***************** |
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% User: David Ashley Date: 11/29/00 Time: 7:17p |
% User: David Ashley Date: 11/29/00 Time: 7:17p |
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% Created in $/uC Software Multi-Volume Book (A)/Chapter, CCH0, CRC, Checksum, Hash Extensions |
% Created in $/uC Software Multi-Volume Book (A)/Chapter, CCH0, CRC, Checksum, Hash Extensions |
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% Initial check-in. |
% Initial check-in. |
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% |
% |
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%End of file C_CCH0.TEX |
%End of file C_CCH0.TEX |
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