golang中的big.Int

在官方的 math/big 包中，Int 类型定义如下：

// An Int represents a signed multi-precision integer.
// The zero value for an Int represents the value 0.
type Int struct {
    neg bool // sign
    abs nat  // absolute value of the integer
}

生成 Int 类型的方法为 NewInt()，如下：

// NewInt allocates and returns a new Int set to x.
func NewInt(x int64) *Int {
    return new(Int).SetInt64(x)
}

可见，NewInt() 函数只对 int64 有效，其他类型必须先转成 int64 才行。
但是，官方还提供了许多 Set 函数，可以方便的把其他类型的整形存入 Int ，因此，我们可以先 new(int) 然后再调用 Set 函数。

// SetInt64 sets z to x and returns z.
func (z *Int) SetInt64(x int64) *Int {
    neg := false
    if x < 0 {
        neg = true
        x = -x
    }
    z.abs = z.abs.setUint64(uint64(x))
    z.neg = neg
    return z
}
​
// SetUint64 sets z to x and returns z.
func (z *Int) SetUint64(x uint64) *Int {
    z.abs = z.abs.setUint64(x)
    z.neg = false
    return z
}
​
// Set sets z to x and returns z.
func (z *Int) Set(x *Int) *Int {
    if z != x {
        z.abs = z.abs.set(x.abs)
        z.neg = x.neg
    }
    return z
}

用法示例

func main() {
    big1 := new(big.Int).SetUint64(uint64(1000))
    fmt.Println("big1 is: ", big1)
    
    big2 := big1.Uint64()
    fmt.Println("big2 is: ", big2)
}

除了上述的 Set 函数，big 库还提供了一个 SetString() 函数，可以指定进制数，比如二进制、十进制或者十六进制等！

// SetString sets z to the value of s, interpreted in the given base,
// and returns z and a boolean indicating success. The entire string
// (not just a prefix) must be valid for success. If SetString fails,
// the value of z is undefined but the returned value is nil.
//
// The base argument must be 0 or a value between 2 and MaxBase. If the base
// is 0, the string prefix determines the actual conversion base. A prefix of
// ``0x'' or ``0X'' selects base 16; the ``0'' prefix selects base 8, and a
// ``0b'' or ``0B'' prefix selects base 2. Otherwise the selected base is 10.
//
func (z *Int) SetString(s string, base int) (*Int, bool) {
    r := strings.NewReader(s)
    if _, _, err := z.scan(r, base); err != nil {
        return nil, false
    }
    // entire string must have been consumed
    if _, err := r.ReadByte(); err != io.EOF {
        return nil, false
    }
    return z, true // err == io.EOF => scan consumed all of s
}

用法示例

func main() {
    big1 := new(big.Int).SetString("1000", 10)
    fmt.Println("big1 is: ", big1)
    
    big2 := big1.Uint64()
    fmt.Println("big2 is: ", big2)
}

uint64, int64 等普通类型与 big.Int 类型的转换
如上，直接调用 big 库提供的 Int64(), Uint64() 等函数就可以进行转换。
Int 对象上的运算函数

Mul(im, in)
Add(ip, im)
Div(ip, io)

import (
    "fmt"
    "math"
    "math/big"
)

func main() {
    // Here are some calculations with bigInts:
    im := big.NewInt(math.MaxInt64)
    in := im
    io := big.NewInt(1956)
    ip := big.NewInt(1)
    ip.Mul(im, in).Add(ip, im).Div(ip, io)
    fmt.Printf("Big Int: %v\n", ip)
    // Here are some calculations with bigInts:
    rm := big.NewRat(math.MaxInt64, 1956)
    rn := big.NewRat(-1956, math.MaxInt64)
    ro := big.NewRat(19, 56)
    rp := big.NewRat(1111, 2222)
    rq := big.NewRat(1, 1)
    rq.Mul(rm, rn).Add(rq, ro).Mul(rq, rp)
    fmt.Printf("Big Rat: %v\n", rq)
}

/* Output:
Big Int: 43492122561469640008497075573153004
Big Rat: -37/112
*/

复制

输出结果：

Big Int: 43492122561469640008497075573153004
Big Rat: -37/112