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| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="sort.sort_hpp.float_sort"></a><a class="link" href="float_sort.html" title="Float Sort">Float Sort</a> |
| </h3></div></div></div> |
| <p> |
| <code class="literal"><code class="computeroutput"><a class="link" href="../../boost/sort/spreadsort/float_sort_idp47034528.html" title="Function template float_sort">float_sort</a></code></code> |
| is a fast templated in-place hybrid radix/comparison algorithm much like |
| <code class="literal"><code class="computeroutput"><a class="link" href="../../boost/sort/spreadsort/integer_sort_idp41299456.html" title="Function template integer_sort">integer_sort</a></code></code>, |
| but sorts IEEE floating-point numbers (positive, zero, NaN, and negative) |
| into ascending order by casting them to integers. This works because positive |
| IEEE floating-point numbers sort like integers with the same bits, and negative |
| IEEE floating-point numbers sort in the reverse order of integers with the |
| same bits. float_sort is roughly 2X as fast as std::sort. |
| </p> |
| <p> |
| -0.0 vs. 0.0 and NaN are given definitive ordered positions by the radix-based |
| portion of this algorithm, where comparison-based sorting does not guarantee |
| their relative position. The included tests avoid creating NaN and -0.0 so |
| that results match std::sort, which is not consistent in how it handles these |
| numbers, as they compare as equal to numbers with different values. |
| </p> |
| <p> |
| float_sort checks the size of the data type and whether it is castable, picking |
| an integer type to cast to, if a casting functor isn't provided by the user. |
| </p> |
| <p> |
| float_mem_cast casts IEEE floating-point numbers (positive, zero, NaN, and |
| negative) into integers. This is an essential utility for creating a custom |
| rightshift functor for float_sort, when one is needed. Only IEEE floating-point |
| numbers of the same size as the integer type being cast to should be used |
| in this specialized method call. Worst-case performance is <span class="emphasis"><em>𝑶(N * |
| (log2(range)/s + s))</em></span>, so <code class="literal"><code class="computeroutput"><a class="link" href="../../boost/sort/spreadsort/float_sort_idp47034528.html" title="Function template float_sort">float_sort</a></code></code> |
| is asymptotically faster than pure comparison-based algorithms. <span class="emphasis"><em>s</em></span> |
| is <span class="emphasis"><em>max_splits</em></span>, which defaults to 11, so its worst-case |
| with default settings for 32-bit integers is <span class="emphasis"><em>𝑶(N * ((32/11)</em></span> |
| slow radix-based iterations + 11 fast comparison-based iterations). |
| </p> |
| <p> |
| Some performance plots of runtime vs. n and log2(range) are provided below: |
| </p> |
| <p> |
| <a href="../../../../doc/graph/windows_float_sort.htm" target="_top">Windows Float Sort</a> |
| </p> |
| <p> |
| <a href="../../../../doc/graph/osx_float_sort.htm" target="_top">OSX Float Sort</a> |
| </p> |
| <div class="section"> |
| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="sort.sort_hpp.float_sort.floatsort_examples"></a><a class="link" href="float_sort.html#sort.sort_hpp.float_sort.floatsort_examples" title="Float Sort Examples">Float |
| Sort Examples</a> |
| </h4></div></div></div> |
| <p> |
| See <a href="../../../../example/floatfunctorsample.cpp" target="_top">floatfunctorsample.cpp</a> |
| for a working example of how to sort structs with a float key: |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">CAST_TYPE</span> <span class="keyword">int</span> |
| <span class="preprocessor">#define</span> <span class="identifier">KEY_TYPE</span> <span class="keyword">float</span> |
| </pre> |
| <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">DATA_TYPE</span> <span class="special">{</span> |
| <span class="identifier">KEY_TYPE</span> <span class="identifier">key</span><span class="special">;</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">data</span><span class="special">;</span> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| Right-shift functor: |
| </p> |
| <pre class="programlisting"><span class="comment">// Casting to an integer before bitshifting</span> |
| <span class="keyword">struct</span> <span class="identifier">rightshift</span> <span class="special">{</span> |
| <span class="keyword">int</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">const</span> <span class="identifier">DATA_TYPE</span> <span class="special">&</span><span class="identifier">x</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">unsigned</span> <span class="identifier">offset</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">{</span> |
| <span class="keyword">return</span> <span class="identifier">float_mem_cast</span><span class="special"><</span><span class="identifier">KEY_TYPE</span><span class="special">,</span> <span class="identifier">CAST_TYPE</span><span class="special">>(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">key</span><span class="special">)</span> <span class="special">>></span> <span class="identifier">offset</span><span class="special">;</span> |
| <span class="special">}</span> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| Comparison lessthan <code class="computeroutput"><span class="keyword">operator</span><span class="special"><</span></code> functor: |
| </p> |
| <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">lessthan</span> <span class="special">{</span> |
| <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">const</span> <span class="identifier">DATA_TYPE</span> <span class="special">&</span><span class="identifier">x</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">DATA_TYPE</span> <span class="special">&</span><span class="identifier">y</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">{</span> |
| <span class="keyword">return</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">key</span> <span class="special"><</span> <span class="identifier">y</span><span class="special">.</span><span class="identifier">key</span><span class="special">;</span> |
| <span class="special">}</span> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| Other examples: |
| </p> |
| <p> |
| <a href="../../../../example/double.cpp" target="_top">Sort doubles.</a> |
| </p> |
| <p> |
| <a href="../../../../example/shiftfloatsample.cpp" target="_top">Sort floats using a rightshift |
| functor.</a> |
| </p> |
| </div> |
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| <td align="right"><div class="copyright-footer">Copyright © 2014 Steven Ross<p> |
| Distributed under the <a href="http://boost.org/LICENSE_1_0.txt" target="_top">Boost |
| Software License, Version 1.0</a>. |
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