ceph/src/boost/libs/multiprecision/doc/html/boost_multiprecision/tut/ints/egs/bitops.html

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  27 <a name="boost_multiprecision.tut.ints.egs.bitops"></a><a class="link" href="bitops.html" title="Bit Operations">Bit Operations</a>
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  29 <p>
  30             In this example we'll show how individual bits within an integer may
  31             be manipulated, we'll start with an often needed calculation of <span class="emphasis"><em>2<sup>n</sup> -
  32             1</em></span>, which we could obviously implement like this:
  33           </p>
  34 <pre class="programlisting"><span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">multiprecision</span><span class="special">::</span><span class="identifier">cpp_int</span><span class="special">;</span>
  35
  36 <span class="identifier">cpp_int</span> <span class="identifier">b1</span><span class="special">(</span><span class="keyword">unsigned</span> <span class="identifier">n</span><span class="special">)</span>
  37 <span class="special">{</span>
  38    <span class="identifier">cpp_int</span> <span class="identifier">r</span><span class="special">(</span><span class="number">1</span><span class="special">);</span>
  39    <span class="keyword">return</span> <span class="special">(</span><span class="identifier">r</span> <span class="special">&lt;&lt;</span> <span class="identifier">n</span><span class="special">)</span> <span class="special">-</span> <span class="number">1</span><span class="special">;</span>
  40 <span class="special">}</span>
  41 </pre>
  42 <p>
  43             Calling:
  44           </p>
  45 <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">hex</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">showbase</span> <span class="special">&lt;&lt;</span> <span class="identifier">b1</span><span class="special">(</span><span class="number">200</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
  46 </pre>
  47 <p>
  48             Yields as expected:
  49           </p>
  50 <pre class="programlisting">0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF</pre>
  51 <p>
  52             However, we could equally just set the n'th bit in the result, like this:
  53           </p>
  54 <pre class="programlisting"><span class="identifier">cpp_int</span> <span class="identifier">b2</span><span class="special">(</span><span class="keyword">unsigned</span> <span class="identifier">n</span><span class="special">)</span>
  55 <span class="special">{</span>
  56    <span class="identifier">cpp_int</span> <span class="identifier">r</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
  57    <span class="keyword">return</span> <span class="special">--</span><span class="identifier">bit_set</span><span class="special">(</span><span class="identifier">r</span><span class="special">,</span> <span class="identifier">n</span><span class="special">);</span>
  58 <span class="special">}</span>
  59 </pre>
  60 <p>
  61             Note how the <code class="computeroutput"><span class="identifier">bit_set</span></code>
  62             function sets the specified bit in its argument and then returns a reference
  63             to the result - which we can then simply decrement. The result from a
  64             call to <code class="computeroutput"><span class="identifier">b2</span></code> is the same
  65             as that to <code class="computeroutput"><span class="identifier">b1</span></code>.
  66           </p>
  67 <p>
  68             We can equally test bits, so for example the n'th bit of the result returned
  69             from <code class="computeroutput"><span class="identifier">b2</span></code> shouldn't be
  70             set unless we increment it first:
  71           </p>
  72 <pre class="programlisting"><span class="identifier">assert</span><span class="special">(!</span><span class="identifier">bit_test</span><span class="special">(</span><span class="identifier">b1</span><span class="special">(</span><span class="number">200</span><span class="special">),</span> <span class="number">200</span><span class="special">));</span>     <span class="comment">// OK</span>
  73 <span class="identifier">assert</span><span class="special">(</span><span class="identifier">bit_test</span><span class="special">(++</span><span class="identifier">b1</span><span class="special">(</span><span class="number">200</span><span class="special">),</span> <span class="number">200</span><span class="special">));</span>    <span class="comment">// OK</span>
  74 </pre>
  75 <p>
  76             And of course if we flip the n'th bit after increment, then we should
  77             get back to zero:
  78           </p>
  79 <pre class="programlisting"><span class="identifier">assert</span><span class="special">(!</span><span class="identifier">bit_flip</span><span class="special">(++</span><span class="identifier">b1</span><span class="special">(</span><span class="number">200</span><span class="special">),</span> <span class="number">200</span><span class="special">));</span>   <span class="comment">// OK</span>
  80 </pre>
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  84 <td align="right"><div class="copyright-footer">Copyright &#169; 2002-2013 John Maddock and Christopher Kormanyos<p>
  85         Distributed under the Boost Software License, Version 1.0. (See accompanying
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