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DIFF: Your IRdcLibrary API for Remote Differential Compression

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14 Feb 2019CPOL3 min read 32.5K   8   13
A quick library for differential memory/file processing


It seems to me that you've always wanted to create some backup/restore feature with incremental processing of each file. Here is a small library that demonstrates the power of IRdcLibrary COM API, ready to be used.


COM, Win32 and C++, as usual. I don't tolerate more beginners than me here :P.

Using the Code

The DIFF library can read/write from files and memory. For reading data, you use Microsoft's IRdcFileReader interface. For your convenience, I provide 2 helper classes in the code that can provide the IRdcFileReader interface:

MemoryRdcFileReader, which can read from memory. The memory must persist while this interface exists (i.e., the interface does not copy the memory).


MemoryRdcFileReader(const char* dd,unsigned long long size);

FileRdcFileReader, which can read from an existing HANDLE. The handle must have been opened with GENERIC_READ and must be seekable.


FileRdcFileReader(HANDLE hF);

For writing data, my functions require an instance of a DIFFWRITER class. I also provide a FileDiffWriter (which can provide a DIFFWRITER from a HANDLE which must have been opened with GENERIC_WRITE) and a MemoryDiffWriter which writes to a vector<char>. These helper classes provide the member function GetReader(IRdcFileReader**) that returns a MemoryRdcFileReader or a FileRdcFileReader.

Incremental Backup

Let's say you have a file named 1.txt in c:\test, and you have backed it in c:\test2. Now let's say that you have modified 1.txt in the original c:\test directory, so c:\test\1.txt is newer. Your incremental backup steps now are (error checking is removed for simplicity):

// Prereqs: file readers for both files. 
// Say that hX1 and hX2 are handles to these two files
CComPtr<IRdcFileReader> fil1;
CComPtr<IRdcFileReader> fil2;
fil1.Attach(new DIFF::FileRdcFileReader(hX1));
fil2.Attach(new DIFF::FileRdcFileReader(hX2));

// Instantiate the library

// Create a signature for both files
// Say that hS1 and hS2 are handles to new files that the signature is to be stored
DIFF::FileDiffWriter sig1(hS1);
DIFF::FileDiffWriter sig2(hS2);

// Close handles

This will save in the files the RDC signature for both old 1.txt and new 1.txt.

Your next step is now to generate a diff between 2 files. My library provides you with 2 options: Either you create a self-contained diff, or a standalone-diff.

A self-contained diff allows you to reconstruct the updated file from the source file offline.
A standalone diff is a lot smaller, but it needs parts of the remote file. So if you are to use standalone diffs, you have to implement an IRdcFileReader yourself that downloads the requested portions of the updated file from a server. Since this needs a server, we will focus on self-contained diffs here.

Now, you want to create the diff between the old and the new file.

CComPtr<IRdcFileReader> xsig1;
CComPtr<IRdcFileReader> xsig2;
DIFF::MemoryDiffWriter diff1_2_sc;

This generates a self-contained diff (if you pass NULL instead of r2 in the 3rd parameter, it generates a standalone diff). Instead of uploading now your new file c:\test\1.txt, you can upload this diff file which is smaller. How smaller it is depends on how much the files differ.

Diff Restore

Now say that you have the original (master) backup of 1.txt and you want to restore the newer 1.txt. You need the original 1.txt and a self-contained diff file:

// r1 is the reader for the original file.
// xdiff2 is reader for the diff.
// Say hX3 is a handle to the file we want to reconstruct
DIFF::FileDiffWriter reco1(hX3);

And that's all. Your newer 1.txt file has been reconstructed by the original file and the diff. If the diff is a standalone diff, you would have to pass a reader to DIFF::Reconstruct's 3rd parameter which would be asked to provide portions of the file.

Points of Interest

  • The API requires Vista or newer.
  • The library runs ONLY in multithreaded COM environment (CoInitializeEx(0, COINIT_MULTITHREADED);)
  • The constructor throws an exception if the COM object cannot be created.
  • There's a similarity API I must investigate more.
  • The library uses smart pointers, both COM ATL and C++.

Have fun!


  • 21st March, 2016: Implemented as .h file, with some bugfixes
  • 21st October, 2015: A few bugfixes and some experimental multiple-sig stuff
  • 6th January, 2015: First release (and Happy NY:))


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Written By
Software Developer
Greece Greece
I'm working in C++, PHP , Java, Windows, iOS, Android and Web (HTML/Javascript/CSS).

I 've a PhD in Digital Signal Processing and Artificial Intelligence and I specialize in Pro Audio and AI applications.

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Comments and Discussions

QuestionOther source of information Pin
hi reddit9-May-16 0:09
professionalhi reddit9-May-16 0:09 
I'm following the project since its creation, and alas there's no big source of information on the matter of RDC.
I've found something that might be useful tho: A similar (but abandoned) project by
GitHub - gonmf/rdc: .NET implementation of RSYNC remote differential compression algorithm[^]
There is some useful resource there, I hope you can make use of it
I'd really like to see a working implementation of an RDC sync (as an utility, I'm not really a programmer)
Hope to help, thanks!
AnswerRe: Other source of information Pin
Michael Chourdakis9-May-16 3:07
mvaMichael Chourdakis9-May-16 3:07 
GeneralRe: Other source of information Pin
hi reddit11-May-16 11:43
professionalhi reddit11-May-16 11:43 
QuestionAny update? Pin
WHAK.com7-Mar-15 0:06
professionalWHAK.com7-Mar-15 0:06 
QuestionUpdated Microsoft Docs on RDC Pin
WHAK.com12-Jan-15 12:41
professionalWHAK.com12-Jan-15 12:41 
AnswerRe: Updated Microsoft Docs on RDC Pin
Michael Chourdakis12-Jan-15 13:04
mvaMichael Chourdakis12-Jan-15 13:04 
QuestionInteresting material Pin
WHAK.com8-Jan-15 8:31
professionalWHAK.com8-Jan-15 8:31 
AnswerRe: Interesting material Pin
Michael Chourdakis8-Jan-15 8:35
mvaMichael Chourdakis8-Jan-15 8:35 
QuestionRDC enabled copy tool Pin
WHAK.com7-Jan-15 9:08
professionalWHAK.com7-Jan-15 9:08 
AnswerRe: RDC enabled copy tool Pin
Michael Chourdakis7-Jan-15 10:14
mvaMichael Chourdakis7-Jan-15 10:14 
AnswerRe: RDC enabled copy tool Pin
Michael Chourdakis21-Mar-16 10:43
mvaMichael Chourdakis21-Mar-16 10:43 source code Pin
Lucian Todor6-Jan-15 21:51
Lucian Todor6-Jan-15 21:51 
GeneralRe: source code Pin
Michael Chourdakis6-Jan-15 22:25
mvaMichael Chourdakis6-Jan-15 22:25 

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