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