1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Hash Function Support</title> 5<link rel="stylesheet" href="../../multiprecision.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../index.html" title="Chapter 1. Boost.Multiprecision"> 8<link rel="up" href="../tut.html" title="Tutorial"> 9<link rel="prev" href="input_output.html" title="Input Output"> 10<link rel="next" href="eigen.html" title="Eigen Interoperability"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="input_output.html"><img 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class="special">::</span><span class="identifier">multiprecision</span><span class="special">;</span> 35<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">random</span><span class="special">;</span> 36 37<span class="identifier">mt19937</span> <span class="identifier">mt</span><span class="special">;</span> 38<span class="identifier">uniform_int_distribution</span><span class="special"><</span><span class="identifier">uint256_t</span><span class="special">></span> <span class="identifier">ui</span><span class="special">;</span> 39 40<span class="identifier">std</span><span class="special">::</span><span class="identifier">unordered_set</span><span class="special"><</span><span class="identifier">uint256_t</span><span class="special">></span> <span class="identifier">set</span><span class="special">;</span> 41<span class="comment">// Put 1000 random values into the container:</span> 42<span class="keyword">for</span><span class="special">(</span><span class="keyword">unsigned</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="number">1000</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> 43 <span class="identifier">set</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">ui</span><span class="special">(</span><span class="identifier">mt</span><span class="special">));</span> 44</pre> 45<p> 46 Or we can define our own hash function, for example in this case based on 47 Google's CityHash: 48 </p> 49<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">cityhash</span> 50<span class="special">{</span> 51 <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">multiprecision</span><span class="special">::</span><span class="identifier">uint256_t</span><span class="special">&</span> <span class="identifier">val</span><span class="special">)</span><span class="keyword">const</span> 52 <span class="special">{</span> 53 <span class="comment">// create a hash from all the limbs of the argument, this function is probably x64 specific,</span> 54 <span class="comment">// and requires that we access the internals of the data type:</span> 55 <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">CityHash64</span><span class="special">(</span><span class="keyword">reinterpret_cast</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">char</span><span class="special">*>(</span><span class="identifier">val</span><span class="special">.</span><span class="identifier">backend</span><span class="special">().</span><span class="identifier">limbs</span><span class="special">()),</span> <span class="identifier">val</span><span class="special">.</span><span class="identifier">backend</span><span class="special">().</span><span class="identifier">size</span><span class="special">()</span> <span class="special">*</span> <span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">val</span><span class="special">.</span><span class="identifier">backend</span><span class="special">().</span><span class="identifier">limbs</span><span class="special">()[</span><span class="number">0</span><span class="special">]));</span> 56 <span class="comment">// modify the returned hash based on sign:</span> 57 <span class="keyword">return</span> <span class="identifier">val</span> <span class="special"><</span> <span class="number">0</span> <span class="special">?</span> <span class="special">~</span><span class="identifier">result</span> <span class="special">:</span> <span class="identifier">result</span><span class="special">;</span> 58 <span class="special">}</span> 59<span class="special">};</span> 60</pre> 61<p> 62 As before insert some values into a container, this time using our custom 63 hasher: 64 </p> 65<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">unordered_set</span><span class="special"><</span><span class="identifier">uint256_t</span><span class="special">,</span> <span class="identifier">cityhash</span><span class="special">></span> <span class="identifier">set2</span><span class="special">;</span> 66<span class="keyword">for</span><span class="special">(</span><span class="keyword">unsigned</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="number">1000</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> 67 <span class="identifier">set2</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">ui</span><span class="special">(</span><span class="identifier">mt</span><span class="special">));</span> 68</pre> 69</div> 70<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 71<td align="left"></td> 72<td align="right"><div class="copyright-footer">Copyright © 2002-2020 John 73 Maddock and Christopher Kormanyos<p> 74 Distributed under the Boost Software License, Version 1.0. (See accompanying 75 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>) 76 </p> 77</div></td> 78</tr></table> 79<hr> 80<div class="spirit-nav"> 81<a accesskey="p" href="input_output.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../tut.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="eigen.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 82</div> 83</body> 84</html> 85
