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26 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
27 <a name="hash.rationale"></a><a class="link" href="rationale.html" title="Rationale">Rationale</a>
28 </h2></div></div></div>
30 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>.
33 <a name="hash.rationale.h0"></a>
34 <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
35 of the hash function</a>
38 Many hash functions strive to have little correlation between the input and
39 output values. They attempt to uniformally distribute the output values for
40 very similar inputs. This hash function makes no such attempt. In fact, for
41 integers, the result of the hash function is often just the input value. So
42 similar but different input values will often result in similar but different
43 output values. This means that it is not appropriate as a general hash function.
44 For example, a hash table may discard bits from the hash function resulting
45 in likely collisions, or might have poor collision resolution when hash values
46 are clustered together. In such cases this hash function will preform poorly.
49 But the standard has no such requirement for the hash function, it just requires
50 that the hashes of two different values are unlikely to collide. Containers
51 or algorithms designed to work with the standard hash function will have to
52 be implemented to work well when the hash function's output is correlated to
53 its input. Since they are paying that cost a higher quality hash function would
57 For other use cases, if you do need a higher quality hash function, then neither
58 the standard hash function or <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">hash</span></code> are
59 appropriate. There are several options available. One is to use a second hash
60 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
61 Wang's hash function</a>. This this may not work as well as a hash algorithm
62 tailored for the input.
65 For strings there are several fast, high quality hash functions available (for
66 example <a href="http://code.google.com/p/smhasher/" target="_top">MurmurHash3</a>
67 and <a href="http://code.google.com/p/cityhash/" target="_top">Google's CityHash</a>),
68 although they tend to be more machine specific. These may also be appropriate
69 for hashing a binary representation of your data - providing that all equal
70 values have an equal representation, which is not always the case (e.g. for
71 floating point values).
73 <div class="footnotes">
74 <br><hr style="width:100; text-align:left;margin-left: 0">
75 <div id="ftn.hash.rationale.f0" class="footnote"><p><a href="#hash.rationale.f0" class="para"><sup class="para">[2] </sup></a>
76 issue 6.18 of the <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1837.pdf" target="_top">Library
77 Extension Technical Report Issues List</a> (page 63)
81 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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83 <td align="right"><div class="copyright-footer">Copyright © 2005-2008 Daniel
85 Distributed under the Boost Software License, Version 1.0. (See accompanying
86 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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