66 lines
2.0 KiB
Rust
66 lines
2.0 KiB
Rust
/* origin: FreeBSD /usr/src/lib/msun/src/e_logf.c */
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/*
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* Conversion to float by Ian Lance Taylor, Cygnus Support, ian@cygnus.com.
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*/
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/*
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* ====================================================
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* Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
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*
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* Developed at SunPro, a Sun Microsystems, Inc. business.
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* Permission to use, copy, modify, and distribute this
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* software is freely granted, provided that this notice
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* is preserved.
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* ====================================================
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*/
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const LN2_HI: f32 = 6.9313812256e-01; /* 0x3f317180 */
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const LN2_LO: f32 = 9.0580006145e-06; /* 0x3717f7d1 */
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/* |(log(1+s)-log(1-s))/s - Lg(s)| < 2**-34.24 (~[-4.95e-11, 4.97e-11]). */
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const LG1: f32 = 0.66666662693; /* 0xaaaaaa.0p-24*/
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const LG2: f32 = 0.40000972152; /* 0xccce13.0p-25 */
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const LG3: f32 = 0.28498786688; /* 0x91e9ee.0p-25 */
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const LG4: f32 = 0.24279078841; /* 0xf89e26.0p-26 */
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#[inline]
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pub fn logf(mut x: f32) -> f32 {
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let x1p25 = f32::from_bits(0x4c000000); // 0x1p25f === 2 ^ 25
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let mut ix = x.to_bits();
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let mut k = 0i32;
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if (ix < 0x00800000) || ((ix >> 31) != 0) {
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/* x < 2**-126 */
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if ix << 1 == 0 {
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return -1. / (x * x); /* log(+-0)=-inf */
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}
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if (ix >> 31) != 0 {
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return (x - x) / 0.; /* log(-#) = NaN */
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}
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/* subnormal number, scale up x */
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k -= 25;
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x *= x1p25;
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ix = x.to_bits();
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} else if ix >= 0x7f800000 {
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return x;
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} else if ix == 0x3f800000 {
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return 0.;
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}
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/* reduce x into [sqrt(2)/2, sqrt(2)] */
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ix += 0x3f800000 - 0x3f3504f3;
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k += ((ix >> 23) as i32) - 0x7f;
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ix = (ix & 0x007fffff) + 0x3f3504f3;
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x = f32::from_bits(ix);
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let f = x - 1.;
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let s = f / (2. + f);
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let z = s * s;
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let w = z * z;
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let t1 = w * (LG2 + w * LG4);
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let t2 = z * (LG1 + w * LG3);
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let r = t2 + t1;
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let hfsq = 0.5 * f * f;
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let dk = k as f32;
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s * (hfsq + r) + dk * LN2_LO - hfsq + f + dk * LN2_HI
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}
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