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26 <div class=
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27 <a name=
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"Hash Function Support">Hash Function Support
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28 </h3></div></div></div>
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:
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>
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>
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>
46 Or we can define our own hash function, for example in this case based on
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>
62 As before insert some values into a container, this time using our custom
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>
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© 2002-
2013 John Maddock and Christopher Kormanyos
<p>
73 Distributed under the Boost Software License, Version
1.0. (See accompanying
74 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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