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Diffstat (limited to 'vendor/libm/src/math/log.rs')
| -rw-r--r-- | vendor/libm/src/math/log.rs | 118 |
1 files changed, 118 insertions, 0 deletions
diff --git a/vendor/libm/src/math/log.rs b/vendor/libm/src/math/log.rs new file mode 100644 index 000000000..4126e413b --- /dev/null +++ b/vendor/libm/src/math/log.rs @@ -0,0 +1,118 @@ +/* origin: FreeBSD /usr/src/lib/msun/src/e_log.c */ +/* + * ==================================================== + * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved. + * + * Developed at SunSoft, a Sun Microsystems, Inc. business. + * Permission to use, copy, modify, and distribute this + * software is freely granted, provided that this notice + * is preserved. + * ==================================================== + */ +/* log(x) + * Return the logarithm of x + * + * Method : + * 1. Argument Reduction: find k and f such that + * x = 2^k * (1+f), + * where sqrt(2)/2 < 1+f < sqrt(2) . + * + * 2. Approximation of log(1+f). + * Let s = f/(2+f) ; based on log(1+f) = log(1+s) - log(1-s) + * = 2s + 2/3 s**3 + 2/5 s**5 + ....., + * = 2s + s*R + * We use a special Remez algorithm on [0,0.1716] to generate + * a polynomial of degree 14 to approximate R The maximum error + * of this polynomial approximation is bounded by 2**-58.45. In + * other words, + * 2 4 6 8 10 12 14 + * R(z) ~ Lg1*s +Lg2*s +Lg3*s +Lg4*s +Lg5*s +Lg6*s +Lg7*s + * (the values of Lg1 to Lg7 are listed in the program) + * and + * | 2 14 | -58.45 + * | Lg1*s +...+Lg7*s - R(z) | <= 2 + * | | + * Note that 2s = f - s*f = f - hfsq + s*hfsq, where hfsq = f*f/2. + * In order to guarantee error in log below 1ulp, we compute log + * by + * log(1+f) = f - s*(f - R) (if f is not too large) + * log(1+f) = f - (hfsq - s*(hfsq+R)). (better accuracy) + * + * 3. Finally, log(x) = k*ln2 + log(1+f). + * = k*ln2_hi+(f-(hfsq-(s*(hfsq+R)+k*ln2_lo))) + * Here ln2 is split into two floating point number: + * ln2_hi + ln2_lo, + * where n*ln2_hi is always exact for |n| < 2000. + * + * Special cases: + * log(x) is NaN with signal if x < 0 (including -INF) ; + * log(+INF) is +INF; log(0) is -INF with signal; + * log(NaN) is that NaN with no signal. + * + * Accuracy: + * according to an error analysis, the error is always less than + * 1 ulp (unit in the last place). + * + * Constants: + * The hexadecimal values are the intended ones for the following + * constants. The decimal values may be used, provided that the + * compiler will convert from decimal to binary accurately enough + * to produce the hexadecimal values shown. + */ + +const LN2_HI: f64 = 6.93147180369123816490e-01; /* 3fe62e42 fee00000 */ +const LN2_LO: f64 = 1.90821492927058770002e-10; /* 3dea39ef 35793c76 */ +const LG1: f64 = 6.666666666666735130e-01; /* 3FE55555 55555593 */ +const LG2: f64 = 3.999999999940941908e-01; /* 3FD99999 9997FA04 */ +const LG3: f64 = 2.857142874366239149e-01; /* 3FD24924 94229359 */ +const LG4: f64 = 2.222219843214978396e-01; /* 3FCC71C5 1D8E78AF */ +const LG5: f64 = 1.818357216161805012e-01; /* 3FC74664 96CB03DE */ +const LG6: f64 = 1.531383769920937332e-01; /* 3FC39A09 D078C69F */ +const LG7: f64 = 1.479819860511658591e-01; /* 3FC2F112 DF3E5244 */ + +#[inline] +#[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)] +pub fn log(mut x: f64) -> f64 { + let x1p54 = f64::from_bits(0x4350000000000000); // 0x1p54 === 2 ^ 54 + + let mut ui = x.to_bits(); + let mut hx: u32 = (ui >> 32) as u32; + let mut k: i32 = 0; + + if (hx < 0x00100000) || ((hx >> 31) != 0) { + /* x < 2**-126 */ + if ui << 1 == 0 { + return -1. / (x * x); /* log(+-0)=-inf */ + } + if hx >> 31 != 0 { + return (x - x) / 0.0; /* log(-#) = NaN */ + } + /* subnormal number, scale x up */ + k -= 54; + x *= x1p54; + ui = x.to_bits(); + hx = (ui >> 32) as u32; + } else if hx >= 0x7ff00000 { + return x; + } else if hx == 0x3ff00000 && ui << 32 == 0 { + return 0.; + } + + /* reduce x into [sqrt(2)/2, sqrt(2)] */ + hx += 0x3ff00000 - 0x3fe6a09e; + k += ((hx >> 20) as i32) - 0x3ff; + hx = (hx & 0x000fffff) + 0x3fe6a09e; + ui = ((hx as u64) << 32) | (ui & 0xffffffff); + x = f64::from_bits(ui); + + let f: f64 = x - 1.0; + let hfsq: f64 = 0.5 * f * f; + let s: f64 = f / (2.0 + f); + let z: f64 = s * s; + let w: f64 = z * z; + let t1: f64 = w * (LG2 + w * (LG4 + w * LG6)); + let t2: f64 = z * (LG1 + w * (LG3 + w * (LG5 + w * LG7))); + let r: f64 = t2 + t1; + let dk: f64 = k as f64; + s * (hfsq + r) + dk * LN2_LO - hfsq + f + dk * LN2_HI +} |