Revision control
Copy as Markdown
Other Tools
/* origin: FreeBSD /usr/src/lib/msun/src/e_expf.c */
/*
* Conversion to float by Ian Lance Taylor, Cygnus Support, ian@cygnus.com.
*/
/*
* ====================================================
* 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 super::scalbnf;
const HALF: [f32; 2] = [0.5, -0.5];
const LN2_HI: f32 = 6.9314575195e-01; /* 0x3f317200 */
const LN2_LO: f32 = 1.4286067653e-06; /* 0x35bfbe8e */
const INV_LN2: f32 = 1.4426950216e+00; /* 0x3fb8aa3b */
/*
* Domain [-0.34568, 0.34568], range ~[-4.278e-9, 4.447e-9]:
* |x*(exp(x)+1)/(exp(x)-1) - p(x)| < 2**-27.74
*/
const P1: f32 = 1.6666625440e-1; /* 0xaaaa8f.0p-26 */
const P2: f32 = -2.7667332906e-3; /* -0xb55215.0p-32 */
/// Exponential, base *e* (f32)
///
/// Calculate the exponential of `x`, that is, *e* raised to the power `x`
/// (where *e* is the base of the natural system of logarithms, approximately 2.71828).
#[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
pub fn expf(mut x: f32) -> f32 {
let x1p127 = f32::from_bits(0x7f000000); // 0x1p127f === 2 ^ 127
let x1p_126 = f32::from_bits(0x800000); // 0x1p-126f === 2 ^ -126 /*original 0x1p-149f ??????????? */
let mut hx = x.to_bits();
let sign = (hx >> 31) as i32; /* sign bit of x */
let signb: bool = sign != 0;
hx &= 0x7fffffff; /* high word of |x| */
/* special cases */
if hx >= 0x42aeac50 {
/* if |x| >= -87.33655f or NaN */
if hx > 0x7f800000 {
/* NaN */
return x;
}
if (hx >= 0x42b17218) && (!signb) {
/* x >= 88.722839f */
/* overflow */
x *= x1p127;
return x;
}
if signb {
/* underflow */
force_eval!(-x1p_126 / x);
if hx >= 0x42cff1b5 {
/* x <= -103.972084f */
return 0.;
}
}
}
/* argument reduction */
let k: i32;
let hi: f32;
let lo: f32;
if hx > 0x3eb17218 {
/* if |x| > 0.5 ln2 */
if hx > 0x3f851592 {
/* if |x| > 1.5 ln2 */
k = (INV_LN2 * x + i!(HALF, sign as usize)) as i32;
} else {
k = 1 - sign - sign;
}
let kf = k as f32;
hi = x - kf * LN2_HI; /* k*ln2hi is exact here */
lo = kf * LN2_LO;
x = hi - lo;
} else if hx > 0x39000000 {
/* |x| > 2**-14 */
k = 0;
hi = x;
lo = 0.;
} else {
/* raise inexact */
force_eval!(x1p127 + x);
return 1. + x;
}
/* x is now in primary range */
let xx = x * x;
let c = x - xx * (P1 + xx * P2);
let y = 1. + (x * c / (2. - c) - lo + hi);
if k == 0 {
y
} else {
scalbnf(y, k)
}
}