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05/09/2007 at 07:07PM PDT, ID: 22563155
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6.8

Code for a rolling checksum (i.e. rsync)

Asked by d_melnyk in Visual Basic Programming, Algorithms, Math & Science

Tags: checksum, rsync

Hi Folks:
  I am trying to find sample code (C++ is OK , VB might be better) for an example of calculating a rolling checksum crc along the lines of rsync. My understanding is that you can take a weak crc calculation of a block of data and by somehow "dropping" the first byte and "adding" the next byte after the block quickly generate a new "weak" check sum of the next block of data. e.g.:  a) calculate a weak crc on a 512 byte block of data; b) "drop" the first byte of the block and "add" byte 513 (next byte) using AND / XOR whatever? - and obtain the weak CRC for the block running from byte 2 (1 based) to byte 513. What I am trying to locate is i) code for the weak crc check sum and ii) what are the instructions to create the "rolling checksum?

Any assitance would be greatly appreciated... I have found many items regarding "rsync" but the low level math eludes me ... a more specific example would be extremely helpful for my project.

Best regards to all ... thanks in advance for any help / suggestions etc. ... Dave Melnyk
[+][-]05/10/07 07:14 AM, ID: 19065376

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[+][-]05/10/07 07:23 AM, ID: 19065457

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[+][-]05/10/07 08:38 AM, ID: 19066192

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[+][-]05/11/07 02:20 PM, ID: 19075628

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[+][-]05/11/07 03:27 PM, ID: 19076055

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About this solution

Zones: Visual Basic Programming, Algorithms, Math & Science
Tags: checksum, rsync
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Solution Provided By: NovaDenizen
Participating Experts: 2
Solution Grade: B
 
 
[+][-]05/11/07 03:29 PM, ID: 19076070

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[+][-]05/22/07 03:00 PM, ID: 19137896

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