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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="boost_multiprecision.tut.hash"></a><a class="link" href="hash.html" title="Hash Function Support">Hash Function Support</a>
  28 </h3></div></div></div>
  29 <p>
  30         All of the types in this library support hashing via boost::hash or std::hash.
  31         That means we can use multiprecision types directly in hashed containers
  32         such as std::unordered_set:
  33       </p>
  34 <pre class="programlisting"><span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span 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">&lt;</span><span class="identifier">uint256_t</span><span class="special">&gt;</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">&lt;</span><span class="identifier">uint256_t</span><span class="special">&gt;</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">&lt;</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">&amp;</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">&lt;</span><span class="keyword">const</span> <span class="keyword">char</span><span class="special">*&gt;(</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">&lt;</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">&lt;</span><span class="identifier">uint256_t</span><span class="special">,</span> <span class="identifier">cityhash</span><span class="special">&gt;</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">&lt;</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>
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  73       and Christopher Kormanyos<p>
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