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diff --git a/doc/html/unordered/rationale.html b/doc/html/unordered/rationale.html new file mode 100755 index 0000000000..74c988bc2f --- /dev/null +++ b/doc/html/unordered/rationale.html @@ -0,0 +1,129 @@ +<html> +<head> +<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> +<title>Implementation Rationale</title> +<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> +<meta name="generator" content="DocBook XSL Stylesheets V1.76.1"> +<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> +<link rel="up" href="../unordered.html" title="Chapter 33. Boost.Unordered"> +<link rel="prev" href="compliance.html" title="C++11 Compliance"> +<link rel="next" href="changes.html" title="Change Log"> +</head> +<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> +<table cellpadding="2" width="100%"><tr> +<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> +<td align="center"><a href="../../../index.html">Home</a></td> +<td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> +<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> +<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> +<td align="center"><a href="../../../more/index.htm">More</a></td> +</tr></table> +<hr> +<div class="spirit-nav"> +<a accesskey="p" href="compliance.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../unordered.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="changes.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> +</div> +<div class="section"> +<div class="titlepage"><div><div><h2 class="title" style="clear: both"> +<a name="unordered.rationale"></a><a class="link" href="rationale.html" title="Implementation Rationale">Implementation Rationale</a> +</h2></div></div></div> +<p> + The intent of this library is to implement the unordered containers in the + draft standard, so the interface was fixed. But there are still some implementation + decisions to make. The priorities are conformance to the standard and portability. + </p> +<p> + The <a href="http://en.wikipedia.org/wiki/Hash_table" target="_top">wikipedia article + on hash tables</a> has a good summary of the implementation issues for + hash tables in general. + </p> +<h3> +<a name="unordered.rationale.h0"></a> + <span><a name="unordered.rationale.data_structure"></a></span><a class="link" href="rationale.html#unordered.rationale.data_structure">Data + Structure</a> + </h3> +<p> + By specifying an interface for accessing the buckets of the container the standard + pretty much requires that the hash table uses chained addressing. + </p> +<p> + It would be conceivable to write a hash table that uses another method. For + example, it could use open addressing, and use the lookup chain to act as a + bucket but there are a some serious problems with this: + </p> +<div class="itemizedlist"><ul class="itemizedlist" type="disc"> +<li class="listitem"> + The draft standard requires that pointers to elements aren't invalidated, + so the elements can't be stored in one array, but will need a layer of + indirection instead - losing the efficiency and most of the memory gain, + the main advantages of open addressing. + </li> +<li class="listitem"> + Local iterators would be very inefficient and may not be able to meet the + complexity requirements. + </li> +<li class="listitem"> + There are also the restrictions on when iterators can be invalidated. Since + open addressing degrades badly when there are a high number of collisions + the restrictions could prevent a rehash when it's really needed. The maximum + load factor could be set to a fairly low value to work around this - but + the standard requires that it is initially set to 1.0. + </li> +<li class="listitem"> + And since the standard is written with a eye towards chained addressing, + users will be surprised if the performance doesn't reflect that. + </li> +</ul></div> +<p> + So chained addressing is used. + </p> +<h3> +<a name="unordered.rationale.h1"></a> + <span><a name="unordered.rationale.number_of_buckets"></a></span><a class="link" href="rationale.html#unordered.rationale.number_of_buckets">Number + of Buckets</a> + </h3> +<p> + There are two popular methods for choosing the number of buckets in a hash + table. One is to have a prime number of buckets, another is to use a power + of 2. + </p> +<p> + Using a prime number of buckets, and choosing a bucket by using the modulus + of the hash function's result will usually give a good result. The downside + is that the required modulus operation is fairly expensive. + </p> +<p> + Using a power of 2 allows for much quicker selection of the bucket to use, + but at the expense of loosing the upper bits of the hash value. For some specially + designed hash functions it is possible to do this and still get a good result + but as the containers can take arbitrary hash functions this can't be relied + on. + </p> +<p> + To avoid this a transformation could be applied to the hash function, for an + example see <a href="http://www.concentric.net/~Ttwang/tech/inthash.htm" target="_top">Thomas + Wang's article on integer hash functions</a>. Unfortunately, a transformation + like Wang's requires knowledge of the number of bits in the hash value, so + it isn't portable enough. This leaves more expensive methods, such as Knuth's + Multiplicative Method (mentioned in Wang's article). These don't tend to work + as well as taking the modulus of a prime, and the extra computation required + might negate efficiency advantage of power of 2 hash tables. + </p> +<p> + So, this implementation uses a prime number for the hash table size. + </p> +</div> +<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> +<td align="left"></td> +<td align="right"><div class="copyright-footer">Copyright © 2003, 2004 Jeremy B. Maitin-Shepard<br>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>) + </p> +</div></td> +</tr></table> +<hr> +<div class="spirit-nav"> +<a accesskey="p" href="compliance.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../unordered.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="changes.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> +</div> +</body> +</html> |