196 lines
6.7 KiB
HTML
196 lines
6.7 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html><head>
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<meta http-equiv="content-type" content="text/html; charset=windows-1252">
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<title>Natural Order String Comparison</title>
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</head>
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<body>
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<h1>Natural Order String Comparison</h1>
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<p>by <a href="http://sourcefrog.net/">Martin Pool</a>
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</p><p>Computer string sorting algorithms generally don't order strings
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containing numbers in the same way that a human would do. Consider:
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</p><blockquote><pre>rfc1.txt
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rfc2086.txt
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rfc822.txt
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</pre></blockquote>
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<p>It would be more friendly if the program listed the files as
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</p><blockquote><pre>rfc1.txt
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rfc822.txt
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rfc2086.txt
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</pre></blockquote>
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<p>Filenames sort properly if people insert leading zeros, but they
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don't always do that.
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</p><p>I've written a subroutine that compares strings according to this
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natural ordering. You can use this routine in your own software, or
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download a patch to add it to your favourite Unix program.
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</p><h2>Sorting</h2>
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<p>Strings are sorted as usual, except that decimal integer substrings
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are compared on their numeric value. For example,
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</p><blockquote>
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a < a0 < a1 < a1a < a1b < a2 < a10 < a20
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</blockquote>
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<p>Strings can contain several number parts:
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</p><blockquote>
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x2-g8 < x2-y7 < x2-y08 < x8-y8
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</blockquote>
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in which case numeric fields are separated by nonnumeric characters.
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Leading spaces are ignored. This works very well for IP addresses
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from log files, for example.
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<p>
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Leading zeros are <u>not</u> ignored, which tends to give more
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reasonable results on decimal fractions.
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</p>
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<blockquote>
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1.001 < 1.002 < 1.010 < 1.02 < 1.1 < 1.3
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</blockquote>
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<p>Some applications may wish to change this by modifying the test
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that calls <code>isspace</code>.
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</p><p>
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Performance is linear: each character of the string is scanned
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at most once, and only as many characters as necessary to decide
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are considered.
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</p>
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<p><a href="http://sourcefrog.net/projects/natsort/example-out.txt">Longer example of the results</a>
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</p><h2>Licensing</h2>
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<p>This software is copyright by Martin Pool, and made available under
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the same licence as zlib:
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</p><blockquote>
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<p> This software is provided 'as-is', without any express or implied
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warranty. In no event will the authors be held liable for any damages
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arising from the use of this software.
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</p><p> Permission is granted to anyone to use this software for any purpose,
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including commercial applications, and to alter it and redistribute it
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freely, subject to the following restrictions:
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</p><p> 1. The origin of this software must not be misrepresented; you must not
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claim that you wrote the original software. If you use this software
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in a product, an acknowledgment in the product documentation would be
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appreciated but is not required.
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</p><p> 2. Altered source versions must be plainly marked as such, and must not be
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misrepresented as being the original software.
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</p><p> 3. This notice may not be removed or altered from any source distribution.
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</p></blockquote>
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<p>This licence applies only to the C implementation. You are free to
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reimplement the idea fom scratch in any language.
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</p><h2>Related Work</h2>
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<p>
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POSIX sort(1) has the -n option to sort numbers, but this doesn't
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work if there is a non-numeric prefix.
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</p>
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<p>
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GNU ls(1) has the <tt>--sort=version</tt> option, which works
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the same way.
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</p>
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<p>
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The PHP scripting language now has a
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<a href="http://us3.php.net/manual/en/function.strnatcmp.php">strnatcmp</a>
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function based on this code.
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The PHP wrapper was done by Andrei Zimievsky.
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</p>
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<p>
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<a href="http://www.naturalordersort.org/">Stuart
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Cheshire</a> has a Macintosh <q>system extension</q> to do natural ordering.
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I indepdendently reinvented the algorithm, but Stuart had it
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first. I borrowed the term <q>natural sort</q> from him.
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</p>
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<p>
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<a href="http://search.cpan.org/src/EDAVIS/Sort-Versions-1.4/README"><tt>Sort::Versions</tt></a>
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in Perl. "The code has some special magic to deal with common
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conventions in program version numbers, like the difference between
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'decimal' versions (eg perl 5.005) and the Unix kind (eg perl 5.6.1)."
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</p><p><a href="http://www.cpan.org/modules/by-module/Sort/Sort-Naturally-1.01.readme"><tt>Sort::Naturally</tt></a>
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is also in Perl, by Sean M. Burke. It uses locale-sensitive character classes to sort words and numeric substrings
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in a way similar to natsort.
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</p><p>
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Ed Avis wrote <a href="http://membled.com/work/apps/todo/numsort">something similar in Haskell</a>.
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</p><p>
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Pierre-Luc Paour wrote a <a href="http://pierre-luc.paour.9online.fr/NaturalOrderComparator.java"><tt>NaturalOrderComparator</tt>
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in Java</a>
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</p><p>Kristof Coomans wrote a <a href="http://sourcefrog.net/projects/natsort/natcompare.js">natural sort comparison in Javascript</a></p>
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<p>Alan Davies wrote
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<a href="http://sourcefrog.net/projects/natsort/natcmp.rb"><tt>natcmp.rb</tt></a>,
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an implementation in <a href="http://www.ruby-lang.org/">Ruby</a>.
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</p><p><a href="http://sourceforge.net/projects/numacomp">Numacomp</a>
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- similar thing in Python.
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</p><p><a href="http://code.google.com/p/as3natcompare/">as3natcompare</a>
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implementation in Flash ActionScript 3.
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</p><h2>Get It!</h2>
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<ul>
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<li><a href="http://sourcefrog.net/projects/natsort/strnatcmp.c">strnatcmp.c</a>,
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<a href="http://sourcefrog.net/projects/natsort/strnatcmp.h">strnatcmp.h</a> - the algorithm itself
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</li><li><a href="http://sourcefrog.net/projects/natsort/natsort.c">natsort.c</a> - example driver program.
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(Try <tt>ls -F /proc | natsort</tt>)
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</li><li><a href="http://sourcefrog.net/projects/natsort/textutils.diff">textutils.diff</a> - patch to add
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natural sort to sort(1) from GNU textutils-2.0; use the new
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<tt>-N</tt> option.</li>
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<li>Natural ordering is now in PHP4rc2, through the <a href="http://php.net/manual/html/function.strnatcasecmp.html">strnatcasecmp</a>
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and <a href="http://php.net/manual/html/function.strnatcmp.html">strnatcmp</a>
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functions.</li>
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</ul>
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<h2>To Do</h2>
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<p>
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Comparison of characters is purely numeric, without taking
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character set or locale into account. So it is only correct for
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ASCII. This should probably be a separate function because doing
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the comparisons will probably introduce a dependency on the OS
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mechanism for finding the locale and comparing characters.
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</p><p>
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It might be good to support multibyte character sets too.
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</p><p>
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If you fix either of these, please mail me. They should not be
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very hard.
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</p></body></html>
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