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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="hash.rationale"></a><a class="link" href="rationale.html" title="Rationale">Rationale</a>
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</h2></div></div></div>
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<p>
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The rationale can be found in the original design <a href="#ftn.hash.rationale.f0" class="footnote" name="hash.rationale.f0"><sup class="footnote">[2]</sup></a>.
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<h4>
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<a name="hash.rationale.h0"></a>
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<span class="phrase"><a name="hash.rationale.quality_of_the_hash_function"></a></span><a class="link" href="rationale.html#hash.rationale.quality_of_the_hash_function">Quality
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of the hash function</a>
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</h4>
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<p>
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Many hash functions strive to have little correlation between the input and
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output values. They attempt to uniformally distribute the output values for
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very similar inputs. This hash function makes no such attempt. In fact, for
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integers, the result of the hash function is often just the input value. So
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similar but different input values will often result in similar but different
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output values. This means that it is not appropriate as a general hash function.
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For example, a hash table may discard bits from the hash function resulting
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in likely collisions, or might have poor collision resolution when hash values
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are clustered together. In such cases this hash function will preform poorly.
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</p>
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<p>
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But the standard has no such requirement for the hash function, it just requires
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that the hashes of two different values are unlikely to collide. Containers
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or algorithms designed to work with the standard hash function will have to
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be implemented to work well when the hash function's output is correlated to
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its input. Since they are paying that cost a higher quality hash function would
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be wasteful.
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</p>
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<p>
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For other use cases, if you do need a higher quality hash function, then neither
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the standard hash function or <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">hash</span></code> are
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appropriate. There are several options available. One is to use a second hash
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on the output of this hash function, such as <a href="http://web.archive.org/web/20121102023700/http://www.concentric.net/~Ttwang/tech/inthash.htm" target="_top">Thomas
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Wang's hash function</a>. This this may not work as well as a hash algorithm
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tailored for the input.
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</p>
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<p>
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For strings there are several fast, high quality hash functions available (for
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example <a href="http://code.google.com/p/smhasher/" target="_top">MurmurHash3</a>
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and <a href="http://code.google.com/p/cityhash/" target="_top">Google's CityHash</a>),
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although they tend to be more machine specific. These may also be appropriate
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for hashing a binary representation of your data - providing that all equal
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values have an equal representation, which is not always the case (e.g. for
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floating point values).
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</p>
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<div class="footnotes">
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<br><hr style="width:100; text-align:left;margin-left: 0">
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<div id="ftn.hash.rationale.f0" class="footnote"><p><a href="#hash.rationale.f0" class="para"><sup class="para">[2] </sup></a>
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issue 6.18 of the <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1837.pdf" target="_top">Library
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Extension Technical Report Issues List</a> (page 63)
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2005-2008 Daniel
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James<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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