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1 %$Header$
3 \chapter[\ccchzeroshorttitle{}]{\ccchzerolongtitle{}}
5 \label{ccch0}
7 \beginchapterquote{``\ldots{} Beauty is the first test: there is no permanent
8 place in the world for ugly mathematics.''}
9 {G.H. Hardy \cite{bibref:b:mathematiciansapology:1940},
10 p.85}
12 \section{Introduction}
13 %Section Tag: INT
16 \section{crc32}
18 \begin{tclcommandname}{crc32}%
19 generates the CRC-32 of a file or string. This CRC can be reliably used to obtain
20 digital signatures of files or data.
21 \end{tclcommandname}
23 \begin{tclcommandsynopsis}
24 \tclcommandsynopsisline{crc32}{filename}
25 \tclcommandsynopsisline{crc32}{-string binarystringval}
26 \tclcommandsynopsisline{crc32}{-initialstate}
27 \tclcommandsynopsisline{crc32}{-advancestate state filename}
28 \tclcommandsynopsisline{crc32}{-advancestate -string state binarystringval}
29 \tclcommandsynopsisline{crc32}{-crcfromstate state}
30 \end{tclcommandsynopsis}
32 \begin{tclcommanddescription}
33 The \emph{crc32} command forms the CRC-32 of the binary contents of a file
34 or of the binary contents of a string. The CRC-32 is useful as a digital
35 signature, and can be used with unity probability to determine that two
36 files are different, or with a probability of about $1-2^{-32}$ to determine
37 that two files are almost certainly identical.
39 In the invocations below, the CRC-32 is always returned as a 10-character ASCII string
40 of the form \emph{``0xDDDDDDDD''}, where \emph{``DDDDDDDD''} is the hexadecimal representation
41 of the 32-bit CRC-32, and \emph{``0x''} is a constant 2-character prefix which is included for
42 aesthetics. It is guaranteed that:
44 \begin{itemize}
45 \item The string returned will be exclusive ASCII.
46 \item The string will have a length of exactly 10 characters.
47 \item The first two characters of the string will be \emph{``0x''}.
48 \item Any letters in the hexadecimal representation will be upper-case.
49 \end{itemize}
51 \begin{tclcommandinternaldescription}{\tclcommanddescsynopsisline{crc32}{filename}}
52 Returns the CRC-32 of \emph{filename}, treated as an ordered collection of bytes (i.e.
53 newline characters and file termination characters are not treated---the file is
54 treated as a binary file). \emph{filename} must be specified in the form accepted by
55 the Tcl internals (forward slashes only).
56 \end{tclcommandinternaldescription}
58 \begin{tclcommandinternaldescription}{\tclcommanddescsynopsisline{crc32}{-string binarystringval}}
59 Returns the CRC-32 of \emph{binarystringval}, treated as an ordered collection of bytes (i.e.
60 newline characters and string termination characters are not honored---the string is
61 treated as a binary string).\footnote{For an ASCII string, the \emph{crc32} extension will
62 behave as expected, and will process all characters up to but not including the zero
63 terminator. However, the \emph{crc32} extension will also correctly process non-ASCII strings.}
64 \end{tclcommandinternaldescription}
70 \begin{tclcommandinternaldescription}{\tclcommanddescsynopsisline{crc32}{-initialstate}%
71 \tclcommanddescsynopsisline{crc32}{-advancestate state filename}%
72 \tclcommanddescsynopsisline{crc32}{-advancestate -string state filename}%
73 \tclcommanddescsynopsisline{crc32}{-crcfromstate state}%
74 }
75 The four forms above are designed to allow ``running CRCs'' to be calculated; in which
76 the CRC is calculated piecemeal. These forms allow the caller to retain the internal
77 state vector of the CRC calculation algorithm.
79 The first form, \emph{crc32 -initialstate}, returns an ASCII representation of the
80 correct initial state vector of the CRC-32 state machine. The client is required
81 to obtain this initial state before beginning a piecemeal CRC calculation. Although the
82 returned string is a constant (it will always be the same), representational details
83 may change in future versions of the \emph{crc32} extension, and so a caller should never
84 make assumptions about what this invocation will return, as these assumptions may
85 render a script incompatible with future versions of \emph{crc32}.
87 The second and third forms, \emph{crc32 -advancestate state filename}
88 and \emph{crc32 -advancestate -string state filename}, apply a file or a binary string
89 to \emph{state} to produce a new \emph{state}, which is returned. This new \emph{state}
90 must be retained by the caller and used in subsequent calls.
92 The final form, \emph{crc32 -crcfromstate state}, maps from the state vector to the
93 calculated CRC, and will return a 10-character ASCII string as described above.
94 \end{tclcommandinternaldescription}
96 \end{tclcommanddescription}
99 \begin{tclcommandusagenotes}
100 The ``piecemeal'' forms are as efficient as the file and string forms---there is no difference
101 in the internal algorithms. The primary cost of the piecemeal forms is in importing and
102 exporting the algorithm state vector to/from an ASCII string.
103 Thus, the piecemeal forms become less efficient when
104 small files or strings are processed, as there are more exports and imports
105 of the state vector. When using the piecemeal forms, processing the data in
106 larger chunks will give better performance.
107 \end{tclcommandusagenotes}
109 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
110 \noindent\begin{figure}[!b]
111 \noindent\rule[-0.25in]{\textwidth}{1pt}
112 \begin{tiny}
113 \begin{verbatim}
114 $HeadURL$
115 $Revision$
116 $Date$
117 $Author$
118 \end{verbatim}
119 \end{tiny}
120 \noindent\rule[0.25in]{\textwidth}{1pt}
121 \end{figure}
122 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
123 %
124 %End of file C_CCH0.TEX


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