1 // Copyright 2009 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
8 Floating-point logarithm.
11 // The original C code, the long comment, and the constants
12 // below are from FreeBSD's /usr/src/lib/msun/src/e_log.c
13 // and came with this notice. The go code is a simpler
14 // version of the original C.
16 // ====================================================
17 // Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
19 // Developed at SunPro, a Sun Microsystems, Inc. business.
20 // Permission to use, copy, modify, and distribute this
21 // software is freely granted, provided that this notice
23 // ====================================================
26 // Return the logrithm of x
29 // 1. Argument Reduction: find k and f such that
31 // where sqrt(2)/2 < 1+f < sqrt(2) .
33 // 2. Approximation of log(1+f).
34 // Let s = f/(2+f) ; based on log(1+f) = log(1+s) - log(1-s)
35 // = 2s + 2/3 s**3 + 2/5 s**5 + .....,
37 // We use a special Reme algorithm on [0,0.1716] to generate
38 // a polynomial of degree 14 to approximate R. The maximum error
39 // of this polynomial approximation is bounded by 2**-58.45. In
42 // R(z) ~ L1*s +L2*s +L3*s +L4*s +L5*s +L6*s +L7*s
43 // (the values of L1 to L7 are listed in the program) and
45 // | L1*s +...+L7*s - R(z) | <= 2
47 // Note that 2s = f - s*f = f - hfsq + s*hfsq, where hfsq = f*f/2.
48 // In order to guarantee error in log below 1ulp, we compute log by
49 // log(1+f) = f - s*(f - R) (if f is not too large)
50 // log(1+f) = f - (hfsq - s*(hfsq+R)). (better accuracy)
52 // 3. Finally, log(x) = k*Ln2 + log(1+f).
53 // = k*Ln2_hi+(f-(hfsq-(s*(hfsq+R)+k*Ln2_lo)))
54 // Here Ln2 is split into two floating point number:
56 // where n*Ln2_hi is always exact for |n| < 2000.
59 // log(x) is NaN with signal if x < 0 (including -INF) ;
60 // log(+INF) is +INF; log(0) is -INF with signal;
61 // log(NaN) is that NaN with no signal.
64 // according to an error analysis, the error is always less than
65 // 1 ulp (unit in the last place).
68 // The hexadecimal values are the intended ones for the following
69 // constants. The decimal values may be used, provided that the
70 // compiler will convert from decimal to binary accurately enough
71 // to produce the hexadecimal values shown.
73 // Log returns the natural logarithm of x.
80 func Log(x float64) float64 {
82 Ln2Hi = 6.93147180369123816490e-01 /* 3fe62e42 fee00000 */
83 Ln2Lo = 1.90821492927058770002e-10 /* 3dea39ef 35793c76 */
84 L1 = 6.666666666666735130e-01 /* 3FE55555 55555593 */
85 L2 = 3.999999999940941908e-01 /* 3FD99999 9997FA04 */
86 L3 = 2.857142874366239149e-01 /* 3FD24924 94229359 */
87 L4 = 2.222219843214978396e-01 /* 3FCC71C5 1D8E78AF */
88 L5 = 1.818357216161805012e-01 /* 3FC74664 96CB03DE */
89 L6 = 1.531383769920937332e-01 /* 3FC39A09 D078C69F */
90 L7 = 1.479819860511658591e-01 /* 3FC2F112 DF3E5244 */
93 // TODO(rsc): Remove manual inlining of IsNaN, IsInf
94 // when compiler does it for us
97 case x != x || x > MaxFloat64: // IsNaN(x) || IsInf(x, 1):
118 t1 := s2 * (L1 + s4*(L3+s4*(L5+s4*L7)))
119 t2 := s4 * (L2 + s4*(L4+s4*L6))
122 return k*Ln2Hi - ((hfsq - (s*(hfsq+R) + k*Ln2Lo)) - f)