| /* origin: FreeBSD /usr/src/lib/msun/src/s_log1pf.c */ |
| /* |
| * ==================================================== |
| * 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. |
| * ==================================================== |
| */ |
| |
| use core::f32; |
| |
| const LN2_HI: f32 = 6.9313812256e-01; /* 0x3f317180 */ |
| const LN2_LO: f32 = 9.0580006145e-06; /* 0x3717f7d1 */ |
| /* |(log(1+s)-log(1-s))/s - Lg(s)| < 2**-34.24 (~[-4.95e-11, 4.97e-11]). */ |
| const LG1: f32 = 0.66666662693; /* 0xaaaaaa.0p-24 */ |
| const LG2: f32 = 0.40000972152; /* 0xccce13.0p-25 */ |
| const LG3: f32 = 0.28498786688; /* 0x91e9ee.0p-25 */ |
| const LG4: f32 = 0.24279078841; /* 0xf89e26.0p-26 */ |
| |
| #[inline] |
| pub fn log1pf(x: f32) -> f32 { |
| let mut ui: u32 = x.to_bits(); |
| let hfsq: f32; |
| let mut f: f32 = 0.; |
| let mut c: f32 = 0.; |
| let s: f32; |
| let z: f32; |
| let r: f32; |
| let w: f32; |
| let t1: f32; |
| let t2: f32; |
| let dk: f32; |
| let ix: u32; |
| let mut iu: u32; |
| let mut k: i32; |
| |
| ix = ui; |
| k = 1; |
| if ix < 0x3ed413d0 || (ix >> 31) > 0 { |
| /* 1+x < sqrt(2)+ */ |
| if ix >= 0xbf800000 { |
| /* x <= -1.0 */ |
| if x == -1. { |
| return x / 0.0; /* log1p(-1)=+inf */ |
| } |
| return (x - x) / 0.0; /* log1p(x<-1)=NaN */ |
| } |
| if ix << 1 < 0x33800000 << 1 { |
| /* |x| < 2**-24 */ |
| /* underflow if subnormal */ |
| if (ix & 0x7f800000) == 0 { |
| force_eval!(x * x); |
| } |
| return x; |
| } |
| if ix <= 0xbe95f619 { |
| /* sqrt(2)/2- <= 1+x < sqrt(2)+ */ |
| k = 0; |
| c = 0.; |
| f = x; |
| } |
| } else if ix >= 0x7f800000 { |
| return x; |
| } |
| if k > 0 { |
| ui = (1. + x).to_bits(); |
| iu = ui; |
| iu += 0x3f800000 - 0x3f3504f3; |
| k = (iu >> 23) as i32 - 0x7f; |
| /* correction term ~ log(1+x)-log(u), avoid underflow in c/u */ |
| if k < 25 { |
| c = if k >= 2 { |
| 1. - (f32::from_bits(ui) - x) |
| } else { |
| x - (f32::from_bits(ui) - 1.) |
| }; |
| c /= f32::from_bits(ui); |
| } else { |
| c = 0.; |
| } |
| /* reduce u into [sqrt(2)/2, sqrt(2)] */ |
| iu = (iu & 0x007fffff) + 0x3f3504f3; |
| ui = iu; |
| f = f32::from_bits(ui) - 1.; |
| } |
| s = f / (2.0 + f); |
| z = s * s; |
| w = z * z; |
| t1 = w * (LG2 + w * LG4); |
| t2 = z * (LG1 + w * LG3); |
| r = t2 + t1; |
| hfsq = 0.5 * f * f; |
| dk = k as f32; |
| return s * (hfsq + r) + (dk * LN2_LO + c) - hfsq + f + dk * LN2_HI; |
| } |
