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| </pre><pre class="rust"><code><span class="comment">/* origin: FreeBSD /usr/src/lib/msun/src/s_cbrtf.c */</span> |
| <span class="comment">/* |
| * Conversion to float by Ian Lance Taylor, Cygnus Support, ian@cygnus.com. |
| * Debugged and optimized by Bruce D. Evans. |
| */</span> |
| <span class="comment">/* |
| * ==================================================== |
| * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved. |
| * |
| * Developed at SunPro, a Sun Microsystems, Inc. business. |
| * Permission to use, copy, modify, and distribute this |
| * software is freely granted, provided that this notice |
| * is preserved. |
| * ==================================================== |
| */</span> |
| <span class="comment">/* cbrtf(x) |
| * Return cube root of x |
| */</span> |
| |
| <span class="kw">use</span> <span class="ident">core::f32</span>; |
| |
| <span class="kw">const</span> <span class="ident">B1</span>: <span class="ident">u32</span> <span class="op">=</span> <span class="number">709958130</span>; <span class="comment">/* B1 = (127-127.0/3-0.03306235651)*2**23 */</span> |
| <span class="kw">const</span> <span class="ident">B2</span>: <span class="ident">u32</span> <span class="op">=</span> <span class="number">642849266</span>; <span class="comment">/* B2 = (127-127.0/3-24/3-0.03306235651)*2**23 */</span> |
| |
| <span class="doccomment">/// Cube root (f32)</span> |
| <span class="doccomment">///</span> |
| <span class="doccomment">/// Computes the cube root of the argument.</span> |
| <span class="attribute">#[<span class="ident">cfg_attr</span>(<span class="ident">all</span>(<span class="ident">test</span>, <span class="ident">assert_no_panic</span>), <span class="ident">no_panic::no_panic</span>)]</span> |
| <span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">cbrtf</span>(<span class="ident">x</span>: <span class="ident">f32</span>) -> <span class="ident">f32</span> { |
| <span class="kw">let</span> <span class="ident">x1p24</span> <span class="op">=</span> <span class="ident">f32::from_bits</span>(<span class="number">0x4b800000</span>); <span class="comment">// 0x1p24f === 2 ^ 24</span> |
| |
| <span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">r</span>: <span class="ident">f64</span>; |
| <span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">t</span>: <span class="ident">f64</span>; |
| <span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">ui</span>: <span class="ident">u32</span> <span class="op">=</span> <span class="ident">x</span>.<span class="ident">to_bits</span>(); |
| <span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">hx</span>: <span class="ident">u32</span> <span class="op">=</span> <span class="ident">ui</span> <span class="op">&</span> <span class="number">0x7fffffff</span>; |
| |
| <span class="kw">if</span> <span class="ident">hx</span> <span class="op">></span><span class="op">=</span> <span class="number">0x7f800000</span> { |
| <span class="comment">/* cbrt(NaN,INF) is itself */</span> |
| <span class="kw">return</span> <span class="ident">x</span> <span class="op">+</span> <span class="ident">x</span>; |
| } |
| |
| <span class="comment">/* rough cbrt to 5 bits */</span> |
| <span class="kw">if</span> <span class="ident">hx</span> <span class="op"><</span> <span class="number">0x00800000</span> { |
| <span class="comment">/* zero or subnormal? */</span> |
| <span class="kw">if</span> <span class="ident">hx</span> <span class="op">==</span> <span class="number">0</span> { |
| <span class="kw">return</span> <span class="ident">x</span>; <span class="comment">/* cbrt(+-0) is itself */</span> |
| } |
| <span class="ident">ui</span> <span class="op">=</span> (<span class="ident">x</span> <span class="op">*</span> <span class="ident">x1p24</span>).<span class="ident">to_bits</span>(); |
| <span class="ident">hx</span> <span class="op">=</span> <span class="ident">ui</span> <span class="op">&</span> <span class="number">0x7fffffff</span>; |
| <span class="ident">hx</span> <span class="op">=</span> <span class="ident">hx</span> <span class="op">/</span> <span class="number">3</span> <span class="op">+</span> <span class="ident">B2</span>; |
| } <span class="kw">else</span> { |
| <span class="ident">hx</span> <span class="op">=</span> <span class="ident">hx</span> <span class="op">/</span> <span class="number">3</span> <span class="op">+</span> <span class="ident">B1</span>; |
| } |
| <span class="ident">ui</span> <span class="op">&=</span> <span class="number">0x80000000</span>; |
| <span class="ident">ui</span> <span class="op">|</span><span class="op">=</span> <span class="ident">hx</span>; |
| |
| <span class="comment">/* |
| * First step Newton iteration (solving t*t-x/t == 0) to 16 bits. In |
| * double precision so that its terms can be arranged for efficiency |
| * without causing overflow or underflow. |
| */</span> |
| <span class="ident">t</span> <span class="op">=</span> <span class="ident">f32::from_bits</span>(<span class="ident">ui</span>) <span class="kw">as</span> <span class="ident">f64</span>; |
| <span class="ident">r</span> <span class="op">=</span> <span class="ident">t</span> <span class="op">*</span> <span class="ident">t</span> <span class="op">*</span> <span class="ident">t</span>; |
| <span class="ident">t</span> <span class="op">=</span> <span class="ident">t</span> <span class="op">*</span> (<span class="ident">x</span> <span class="kw">as</span> <span class="ident">f64</span> <span class="op">+</span> <span class="ident">x</span> <span class="kw">as</span> <span class="ident">f64</span> <span class="op">+</span> <span class="ident">r</span>) <span class="op">/</span> (<span class="ident">x</span> <span class="kw">as</span> <span class="ident">f64</span> <span class="op">+</span> <span class="ident">r</span> <span class="op">+</span> <span class="ident">r</span>); |
| |
| <span class="comment">/* |
| * Second step Newton iteration to 47 bits. In double precision for |
| * efficiency and accuracy. |
| */</span> |
| <span class="ident">r</span> <span class="op">=</span> <span class="ident">t</span> <span class="op">*</span> <span class="ident">t</span> <span class="op">*</span> <span class="ident">t</span>; |
| <span class="ident">t</span> <span class="op">=</span> <span class="ident">t</span> <span class="op">*</span> (<span class="ident">x</span> <span class="kw">as</span> <span class="ident">f64</span> <span class="op">+</span> <span class="ident">x</span> <span class="kw">as</span> <span class="ident">f64</span> <span class="op">+</span> <span class="ident">r</span>) <span class="op">/</span> (<span class="ident">x</span> <span class="kw">as</span> <span class="ident">f64</span> <span class="op">+</span> <span class="ident">r</span> <span class="op">+</span> <span class="ident">r</span>); |
| |
| <span class="comment">/* rounding to 24 bits is perfect in round-to-nearest mode */</span> |
| <span class="ident">t</span> <span class="kw">as</span> <span class="ident">f32</span> |
| } |
| </code></pre></div> |
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