go源码阅读笔记（math.1）

阳光穿透心脏的1/2处 2022-08-21 04:45 154阅读 0赞

# 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][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

[http_blog.csdn.net_qq_15437667_article_details_50901543]: http://blog.csdn.net/qq_15437667/article/details/50901543