A Tcl implementation of the unix sum(1) command is now part of [Tcllib] and should appear in the 1.2 release. Both the BSD and SysV algorithms are implemented although more consistent results across many platforms may be obtained using the [cksum] command.

The [sum] algorithms return a 16 bit checksum of the input data calculated by summing the values of each byte of data. The BSD algorithm includes a right bit-rotation which ensures that the ''order'' of the bytes is also significant.

The SysV algorithm is expressed as:
  s = sum of bytes
  r = s % 2^16 + (s % 2^32) / 2^16
  cksum = (r % 2^16) + r / 2^16
or more usefully on a 32 bit architecture as:
  r = (s & 0xffff) + (s >> 8)
  cksum = (r & 0xffff) + (r >> 8)

For example using the BSD algorithm:

  % package require sum
  1.0
  % crc::sum "Hello, World!"
  37287
  % crc::sum -file sum.tcl
  16358
  % crc::sum -format 0x%x -file sum.tcl
  0x3fe6

The SysV algorithm is less robust - meaning that different data may give the same checksum. [cksum] is more robust still and [md5sum] is yet more so. 

  % crc::sum -sysv "Hello, World!"
  1129
  % crc::sum -sysv "World, Hello!"
  1129

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See the code at http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/tcllib/tcllib/modules/crc/sum.tcl?rev=HEAD&content-type=text/vnd.viewcvs-markup

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[Category Package] subset [Tcllib] See also [crc] and [cksum]