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.
7 // The go code is a modified version of the original C code from
8 // http://www.netlib.org/fdlibm/s_cbrt.c and came with this notice.
10 // ====================================================
11 // Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
13 // Developed at SunSoft, a Sun Microsystems, Inc. business.
14 // Permission to use, copy, modify, and distribute this
15 // software is freely granted, provided that this notice
17 // ====================================================
19 // Cbrt returns the cube root of x.
26 func Cbrt(x float64) float64 {
33 func cbrt(x float64) float64 {
35 B1 = 715094163 // (682-0.03306235651)*2**20
36 B2 = 696219795 // (664-0.03306235651)*2**20
37 C = 5.42857142857142815906e-01 // 19/35 = 0x3FE15F15F15F15F1
38 D = -7.05306122448979611050e-01 // -864/1225 = 0xBFE691DE2532C834
39 E = 1.41428571428571436819e+00 // 99/70 = 0x3FF6A0EA0EA0EA0F
40 F = 1.60714285714285720630e+00 // 45/28 = 0x3FF9B6DB6DB6DB6E
41 G = 3.57142857142857150787e-01 // 5/14 = 0x3FD6DB6DB6DB6DB7
42 SmallestNormal = 2.22507385850720138309e-308 // 2**-1022 = 0x0010000000000000
46 case x == 0 || IsNaN(x) || IsInf(x, 0):
56 // rough cbrt to 5 bits
57 t := Float64frombits(Float64bits(x)/3 + B1<<32)
58 if x < SmallestNormal {
60 t = float64(1 << 54) // set t= 2**54
62 t = Float64frombits(Float64bits(t)/3 + B2<<32)
65 // new cbrt to 23 bits
70 // chop to 22 bits, make larger than cbrt(x)
71 t = Float64frombits(Float64bits(t)&(0xFFFFFFFFC<<28) + 1<<30)
73 // one step newton iteration to 53 bits with error less than 0.667ulps
74 s = t * t // t*t is exact
77 r = (r - t) / (w + r) // r-s is exact
80 // restore the sign bit