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An O(ND) Difference Algorithm for C#

Moving to GitHub

The source code and brief documentation is available in GitHub https://github.com/mathertel/Diff

Here you can find an C# implementation of the famous algorithm that finds the best diff of 2 inputs.

This implementation is about comparing text files and the proven, best and most famous algorithm to identify the differences between them. The source code that you can find in the download implements a small class with a simple to use API that just does this job. You should have it in the bag of your algorithms.

The algorithm was first published 35 Years ago under "An O(ND) Difference Algorithm and its Variations" by Eugene Myers Algorithmica Vol. 1 No. 2, 1986, p 251 . You can find a copy if it at http://www.xmailserver.org/diff2.pdf.

In this article you can find a abstract recursive definition of the algorithm using some pseudo-code that needs to be transferred to an existing programming language.

History and Why

There are many C, Java, Lisp implementations public available of this algorithm out there on the internet. Before I wrote the C# version I discovered that almost all these implementations seem to come from the same source (GNU diffutils) that is only available under the (unfree) GNU public license and therefore cannot be reused as a source code for a commercial or re-distributable application without being trapped by the GNU license.

There are very old C implementations that use other (worse) heuristic algorithms. Microsoft also published source code of a diff-tool (windiff) that uses some tree structures. Also, a direct transfer from a C source to C# is not easy because there is a lot of pointer arithmetic in the typical C solutions and I wanted a managed solution. I tried a lot sources but at least found no usable solution written for the .NET platform.

These are the reasons why I implemented the original published algorithm from the scratch and made it available without the GNU license limitations under a BSD style license. The history of this implementation is back to 2002 when I published a Visual Studio add-in that also can compare files, see http://www.codeproject.com/KB/macros/WebReports8.aspx.

I found no more bugs in the last 15 years so I think that the code is stable.

I did not need a high performance diff tool. I will do some performance tweaking when needed, so please let me know. I also dropped some hints in the source code on that topic.

How it works (briefely)

Comparing the characters of 2 huge text files is not easy to implement and tends to be slow. Comparing numbers is much easier so the first step is to compute unique numbers for all textlines. If textlines are identical then identical numbers are computed.

The API

To use this functionality you only have to call one of the DiffText methods. They all get a pair of strings and return an array of items that describe the inserts and deletes between the 2 strings. There are no "changes" reported. Instead you can find a "insert" and "deleted" pair.

DiffText(string TextA, string TextB)

Find the difference in 2 texts, comparing by textlines without any conversion. A array of Items containing the differences is returned.

DiffText(string TextA, string TextB, bool trimSpace, bool ignoreSpace, bool ignoreCase)

Find the difference in 2 texts, comparing by textlines with some optional conversions. A array of Items containing the differences is returned.

Diff(int[] ArrayA, int[] ArrayB)

Find the difference in 2 arrays of integers. A array of Items containing the differences is returned.

A Test program

The TestCases.cs can be started on the command line with dotnet run and executes several test cases.

A very pragmatic unit test collection.

Change Log

This work was first published at http://www.gotdotnet.com/Community/UserSamples/Details.aspx?SampleGuid= 96065252-BE1D-4E2F-B7CB-3695FEB0D2C3.

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