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Task 24: elink template removal following a TFD

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{{About|data object, text, and file comparisons in computing||Comparison (disambiguation){{!}}Comparison}}
[[File:Kompare.png|thumb|The [[KDE]] [[diff]] tool ''[[Kompare]]'']]

Editing documents, program code, or any data always risks introducing errors. Displaying the differences between two or more sets of data, '''file comparison''' tools can make computing simpler, and more efficient by focusing on new data and ignoring what did not change. Generically known as a '''diff'''<ref>[http://catb.org/jargon/html/D/diff.html "diff", The Jargon File]</ref> after the [[Unix]] [[diff utility|<code>diff</code> utility]], there are a range of ways to compare data sources and display the results.

Some widely used file comparison programs are [[diff]], [[cmp (Unix)|cmp]], [[Apple Developer Tools#FileMerge|FileMerge]], [[WinMerge]], [[Beyond Compare]], and [[File Compare]].

Because understanding changes is important to writers of code or documents, many [[text editor]]s and [[word processor]]s include the functionality necessary to see the changes between different versions of a file or document.

== Method types ==

The most efficient method of finding differences depends on the source data, and the nature of the changes. One approach is to find the [[longest common subsequence problem|longest common subsequence]] between two files, then regard the non-common data as an insertion, or a deletion.

In 1978, Paul Heckel published an algorithm that identifies most moved blocks of text.<ref>{{Citation|first1=Paul|last1=Heckel|year=1978|title=A Technique for Isolating Differences Between Files|journal=Communications of the ACM|volume=21|issue=4|pages=264–268|url=http://documents.scribd.com/docs/10ro9oowpo1h81pgh1as.pdf|access-date=2011-12-04|doi=10.1145/359460.359467|s2cid=207683976}}</ref> This is used in the [[IBM History Flow tool]].<ref>{{Citation|last1=Viégas|first1=Fernanda B.|last2=Wattenberg|first2=Martin|last3=Kushal|first3=Kushal Dave|year=2004|title=Studying Cooperation and Conflict between Authors with history flow Visualizations|publisher=CHI|volume=6|pages=575–582|publication-place=Vienna|url=http://domino.watson.ibm.com/cambridge/research.nsf/58bac2a2a6b05a1285256b30005b3953/53240210b04ea0eb85256f7300567f7e/$FILE/TR2004-19.pdf|access-date=2011-12-01}}</ref> Other file comparison programs find block moves.{{Clarify|date=January 2012}}

Some specialized file comparison tools find the [[longest increasing subsequence]] between two files.<ref name="PatentUS7031972B2">{{cite web |author1=Liwei Ren |author2=Jinsheng Gu |author3=Luosheng Peng |title=Algorithms for block-level code alignment of software binary files |url=https://patents.google.com/patent/US7031972 |website=Google Patents |publisher=USPTO |access-date=10 May 2019 |date=18 April 2006}}</ref> The [[rsync]] protocol uses a [[rolling hash]] function to compare two files on two distant computers with low communication overhead.

File comparison in word processors is typically at the word level, while comparison in most programming tools is at the line level. Byte or character-level comparison is useful in some specialized applications.

== Display ==

The optimal way to display the results of a file comparison depends on many factors, including the type of source data. The fixed lines of programming code provide a clear unit of comparison. This does not work with documents, where adding a single word may cause the following lines to wrap differently, but still not change the content.

The most popular ways to display changes are either side-by-side, or a consolidating view that highlights data inserts, and deletes. In either side-by-side viewing, [[code folding]] or [[folding editor|text folding]], for the sake of efficiency, the interface may hide portions of the file that did not change and show only the changes.{{Clarify|date=January 2012}}

== Reasoning ==

There are various reasons to use comparison tools, and tools themselves use different approaches. To compare binary files, a tool may use byte-level comparison. Comparing [[text file]]s or [[computer program]]s, many tools use a side-by-side visual comparison.<ref>{{Cite book |last1=MacKenzie |first1=David |url=https://books.google.com/books?id=oIINAAAACAAJ |title=Comparing and Merging Files with Gnu Diff and Patch |last2=Eggert |first2=Paul |last3=Stallman |first3=Richard |date=2003 |publisher=Network Theory |isbn=978-0-9541617-5-0 |language=en}}</ref> This gives the user the chance to choose which changes to keep or reject before merging the files into a new version.<ref>{{Cite web |title=File comparison software: vc-dwim and vc-chlog |url=http://www.gnu.org/software/vc-dwim/vc-dwim.html |access-date=2023-04-16 |website=www.gnu.org}}</ref> Or perhaps to keep them both as-is for later reference, through some form of "versioning" control.

File comparison is an important, and integral process of [[file synchronization]] and [[backup]]. In backup methodologies, the issue of [[data corruption]] is important. Rarely is there a warning before corruption occurs, this can make recovery difficult or impossible. Often, the problem is only apparent the next time someone tries to open a file. In this circumstance, a comparison tool can help to isolate the introduction of the problem.<ref>{{Cite web |title=SystemRescue - System Rescue Homepage |url=https://www.system-rescue.org/ |access-date=2023-04-16 |website=www.system-rescue.org}}</ref>

== Historical uses ==

Prior to file comparison, machines existed to compare magnetic tapes or punch cards. The [[IBM 519|IBM 519 Card Reproducer]] could determine whether a deck of [[punched card]]s were equivalent. In 1957, [[John Van Gardner]] developed a system to compare the [[check sum]]s of loaded sections of [[Fortran]] programs to [[debugging|debug]] compilation problems on the [[IBM 704]].<ref>{{cite web |url=http://www.softwarepreservation.org/projects/FORTRAN/paper/John%20Van%20Gardner%20-%20Fortran%20And%20The%20Genesis%20Of%20Project%20Intercept.pdf |title=Fortran And The Genesis Of Project Intercept |author=John Van Gardner |access-date=2011-12-06}}</ref>

== See also ==

* {{Annotated link |Comparison of file comparison tools}}
* {{Annotated link |Computer-assisted reviewing}}
* {{Annotated link |Data differencing}}
* {{Annotated link |Delta encoding}}
* {{Annotated link |Document comparison}}
* {{Annotated link |Edit distance}}

== References ==
{{Reflist}}

== External links ==
{{commons category|File comparison}}

{{Computer files}}
{{Version control software}}

[[Category:File comparison tools| ]]
[[Category:Data differencing]]
[[Category:Utility software types]]

File comparison - Revision history

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