[manpage_begin crc16 n 1.1.2]
[see_also cksum(n)]
[see_also crc32(n)]
[see_also sum(n)]
[keywords checksum]
[keywords cksum]
[keywords crc]
[keywords crc16]
[keywords crc32]
[keywords {cyclic redundancy check}]
[keywords {data integrity}]
[keywords security]
[copyright {2002, Pat Thoyts}]
[moddesc   {Cyclic Redundancy Checks}]
[titledesc {Perform a 16bit Cyclic Redundancy Check}]
[category  {Hashes, checksums, and encryption}]
[require Tcl 8.2]
[require crc16 [opt 1.1.2]]
[description]
[para]

This package provides a Tcl-only implementation of the CRC
algorithms based upon information provided at
http://www.microconsultants.com/tips/crc/crc.txt

There are a number of permutations available for calculating CRC
checksums and this package can handle all of them. Defaults are set up
for the most common cases.

[section COMMANDS]

[list_begin definitions]

[call [cmd ::crc::crc16] [opt "-format [arg format]"] \
  [opt "-seed [arg value]"] [opt "-implementation [arg procname]"] [arg message]]
[call [cmd ::crc::crc16] [opt "-format [arg format]"] \
  [opt "-seed [arg value]"] [opt "-implementation [arg procname]"] "-filename [arg file]"]
[call [cmd ::crc::crc-ccitt] [opt "-format [arg format]"] \
  [opt "-seed [arg value]"] [opt "-implementation [arg procname]"] [arg message]]
[call [cmd ::crc::crc-ccitt] [opt "-format [arg format]"] \
  [opt "-seed [arg value]"] [opt "-implementation [arg procname]"] "-filename [arg file]"]
[call [cmd ::crc::xmodem] [opt "-format [arg format]"] \
  [opt "-seed [arg value]"] [opt "-implementation [arg procname]"] [arg message]]
[call [cmd ::crc::xmodem] [opt "-format [arg format]"] \
  [opt "-seed [arg value]"] [opt "-implementation [arg procname]"] "-filename [arg file]"]

The command takes either string data or a file name and returns a checksum
value calculated using the CRC algorithm. The command used sets up the
CRC polynomial, initial value and bit ordering for the desired
standard checksum calculation. The result is formatted
using the [arg format](n) specifier provided or as an unsigned integer
(%u) by default.

[para]

A number of common polynomials are in use with the CRC algorithm and
the most commonly used of these are included in this package. For
convenience each of these has a command alias in the crc namespace.

[para]

It is possible to implement the CRC-32 checksum using this crc16
package as the implementation is sufficiently generic to extend to 32
bit checksums. As an example this has been done already - however this
is not the fastest method to implement this algorithm in Tcl and a
separate [package crc32] package is available.

[list_end]

[section OPTIONS]

[list_begin definitions]

[def "-filename [arg name]"]

Return a checksum for the file contents instead of for parameter data.

[def "-format [arg string]"]

Return the checksum using an alternative format template.

[def "-seed [arg value]"]

Select an alternative seed value for the CRC calculation. The default
is 0 for the CRC16 calculation and 0xFFFF for the CCITT version.
This can be useful for calculating the CRC for data
structures without first converting the whole structure into a
string. The CRC of the previous member can be used as the seed for
calculating the CRC of the next member. It is also used for
accumulating a checksum from fragments of a large message (or file)

[def "-implementation [arg procname]"]

This hook is provided to allow users to provide their own
implementation (perhaps a C compiled extension). The
procedure specfied is called with two parameters. The first is the
data to be checksummed and the second is the seed value. An
integer is expected as the result.
[para]
The package provides some implementations of standard CRC polynomials
for the XMODEM, CCITT and the usual CRC-16 checksum. For convenience,
additional commands have been provided that make use of these
implementations.

[def "--"]

Terminate option processing.

[list_end]

[section EXAMPLES]

[para]
[example {
% crc::crc16 "Hello, World!"
64077
}]

[para]
[example {
% crc::crc-ccitt "Hello, World!"
26586
}]

[para]
[example {
% crc::crc16 -format 0x%X "Hello, World!"
0xFA4D
}]

[para]
[example {
% crc::crc16 -file crc16.tcl
51675
}]

[section AUTHORS]
Pat Thoyts

[vset CATEGORY crc]
[include ../doctools2base/include/feedback.inc]
[manpage_end]
