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    <p>You've omitted critical information about the number of strings you intend to index.</p> <p>But given that you say you expect the <em>minimum</em> length of an indexed string to be 256, storing the indices as 64% incurs <em>at most</em> 3% overhead. If the total length of the string file is less than 4GB, you could use 32-bit indices and incur 1.5% overhead. These numbers suggest to me that if compression matters, <strong>you're better off compressing the strings, not the indices</strong>. For that problem a variation on LZ77 seems in order.</p> <p>If you want to try a wild idea, put each string in a separate file, pull them all into a zip file, and see how you can do with <a href="http://zziplib.sourceforge.net/" rel="nofollow noreferrer"><code>zziplib</code></a>. This probably won't be great, but it's nearly zero work on your part.</p> <p>More data on the problem would be welcome:</p> <ul> <li>Number of strings</li> <li>Average length of a string</li> <li>Maximum length of a string</li> <li>Median length of strings</li> <li>Degree to which the strings file compresses with <code>gzip</code></li> <li>Whether you are allowed to change the order of strings to improve compression</li> </ul> <h2>EDIT</h2> <p>The comment and revised question makes the problem much clearer. I like your idea of grouping, and I would try a simple delta encoding, group the deltas, and use a variable-length code within each group. I wouldn't wire in 64 as the group size&ndash;I think you will probably want to determine that empirically. </p> <p>You asked for existing libraries. For the grouping and delta encoding I doubt you will find much. For variable-length integer codes, I'm not seeing much in the way of C libraries, but you can find variable-length codings in <a href="http://www.dlugosz.com/ZIP2/VLI.html" rel="nofollow noreferrer">Perl</a> and <a href="http://www.codecodex.com/wiki/index.php?title=Variable-Length_Integers" rel="nofollow noreferrer">Python</a>. There are a ton of papers and some patents on this topic, and I suspect you're going to wind up having to roll your own. But there are some simple codes out there, and you could give UTF-8 a try&mdash;it can code unsigned integers up to 32 bits, and you can grab C code from <a href="http://plan9.bell-labs.com/sources/plan9/sys/src/" rel="nofollow noreferrer">Plan&nbsp;9</a> and I'm sure many other sources.</p>
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    1. POC Library for compressing sequential positive integers
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    1. USNorman Ramsey
    1. USNorman Ramsey
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    1. COHi Norman, The number of strings is in the order of hundreds of millions and average length would be 10k. Maximum length is 2^31. The full set of strings (the values) do not fit in memory and they cannot be re-ordered. More on the practical side, I am using this to build a library, so I don't really have control over the input. These numbers represent use cases I have seen in the past (web pages).
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    2. COYow! OK, your edit makes the problem much clearer. If you find a solution off the shelf I'll be very impressed. I've added a couple of links to my answer.
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