go源码阅读笔记（math.1）

go源码阅读笔记（math.1）

abs.go

func Abs(x float64) float64

package math

// Abs returns the absolute value of x.
//
// Special cases are:
//  Abs(±Inf) = +Inf
//  Abs(NaN) = NaN
func Abs(x float64) float64 {
// TODO: once golang.org/issue/13095 is fixed, change this to:
// return Float64frombits(Float64bits(x) &^ (1 << 63))
// But for now, this generates better code and can also be inlined:
if x < 0 {
return -x
}
if x == 0 {
return 0 // return correctly abs(-0)
}
return x
}

求一个数的绝对值Abs()函数

主要地方在于

if x == 0 {

return 0 // return correctly abs(-0)

}

这里是考虑当x=-0的场景，所以返回0

bits.go

const (
uvnan    = 0x7FF8000000000001
uvinf    = 0x7FF0000000000000
uvneginf = 0xFFF0000000000000
mask     = 0x7FF
shift    = 64 - 11 - 1
bias     = 1023
)

数值表示遵循IEEE 754标准

uvnan,NaN, Not a Number

uvinf,正无穷大

uvneginf,负无穷大

函数func Inf(sign int) float64

// Inf returns positive infinity if sign >= 0, negative infinity if sign < 0.
func Inf(sign int) float64 {
var v uint64
if sign >= 0 {
v = uvinf
} else {
v = uvneginf
}
return Float64frombits(v)
}

如果sign是正数，就返回正无穷大，如果是负数就返回负无穷大

func NaN() float64

// NaN returns an IEEE 754 ``not-a-number'' value.
func NaN() float64 { return Float64frombits(uvnan) }

NaN()返回NaN值

func IsNaN(f float64) (is bool)

// IsNaN reports whether f is an IEEE 754 ``not-a-number'' value.
func IsNaN(f float64) (is bool) {
// IEEE 754 says that only NaNs satisfy f != f.
// To avoid the floating-point hardware, could use:
//  x := Float64bits(f);
//  return uint32(x>>shift)&mask == mask && x != uvinf && x != uvneginf
return f != f
}

该函数判断一个数是否是NaN

如果不想使用浮点硬件，可以按照注释所说的这种方式计算

x := Float64bits(f);
return uint32(x>>shift)&mask == mask && x != uvinf && x != uvneginf

uint32(x>>shift)&mask == mask，x的高12位为0x7FF

x != uvinf，x不是正无穷大

x != uvneginf，x不是负无穷大

func IsInf(f float64, sign int) bool

// IsInf reports whether f is an infinity, according to sign.
// If sign > 0, IsInf reports whether f is positive infinity.
// If sign < 0, IsInf reports whether f is negative infinity.
// If sign == 0, IsInf reports whether f is either infinity.
func IsInf(f float64, sign int) bool {
// Test for infinity by comparing against maximum float.
// To avoid the floating-point hardware, could use:
//  x := Float64bits(f);
//  return sign >= 0 && x == uvinf || sign <= 0 && x == uvneginf;
return sign >= 0 && f > MaxFloat64 || sign <= 0 && f < -MaxFloat64
}

判断一个数是不是无穷大数

同样，如果不想使用浮点硬件，可以按照注释所说的这种方式计算

func normalize(x float64) (y float64, exp int)

// normalize returns a normal number y and exponent exp
// satisfying x == y × 2**exp. It assumes x is finite and non-zero.
func normalize(x float64) (y float64, exp int) {
const SmallestNormal = 2.2250738585072014e-308 // 2**-1022
if Abs(x) < SmallestNormal {
return x * (1 << 52), -52
}
return x, 0
}

这个不懂什么意思

遗留问题，MaxFloat64和-MaxFloat64是多少呢？

参考http://blog.csdn.net/qq_15437667/article/details/50901543

另一个

Float64frombits(v)像是把一个数转化成float64，是如何实现的呢？

一些数学函数……

func Acosh(x float64) float64

返回x的反双曲余弦值

func Asin(x float64) float64

func Acos(x float64) float64

func Asinh(x float64) float64

func Atan(x float64) float64

func Atan2(y, x float64) float64

func Atanh(x float64) float64