Golang PutUvarint Uvarint

看bigcache库的时候,注意到存放数据用的是PutUvarint、Uvarint,那这两个方法是做什么的呢？

PutUvarint

源码及注释

// PutUvarint encodes a uint64 into buf and returns the number of bytes written.
// If the buffer is too small, PutUvarint will panic.
// PutVarint 将 int64 编码为 buf 并返回写入的字节数。如果缓冲区太小，PutVarint 会panic。
func PutUvarint(buf []byte, x uint64) int {
	i := 0
	// 0x80 128
	for x >= 0x80 {
		buf[i] = byte(x) | 0x80
		// 右移7位,然后将右移后的值，赋值给x
		x >>= 7
		i++
	}
	buf[i] = byte(x)
	return i + 1
}

代码示例

package main

import (
	"encoding/binary"
	"fmt"
)

func main() {
	buf := make([]byte, binary.MaxVarintLen64)

	for _, x := range []int64{-65, -64, -2, -1, 0, 1, 2, 63, 64} {
		n := binary.PutVarint(buf, x)
		fmt.Printf("%x\n", buf[:n])
	}
}

输出结果

8101
7f
03
01
00
02
04
7e
8001

Program exited.

https://go.dev/play/p/yRUoUooVrHm

Uvarint

源码及注释

// Uvarint decodes a uint64 from buf and returns that value and the
// number of bytes read (> 0). If an error occurred, the value is 0
// and the number of bytes n is <= 0 meaning:
//
//	n == 0: buf too small
//	n  < 0: value larger than 64 bits (overflow)
//	        and -n is the number of bytes read
// Uvarint 从 buf 解码 uint64 并返回该值和读取的字节数（> 0）。如果发生错误，则该值为0，并且字节数n <= 0意味着：
// n == 0：buf太小了
// n <0：大于64位的值（溢出）
//       和-n是读取的字节数
func Uvarint(buf []byte) (uint64, int) {
	var x uint64
	var s uint
	for i, b := range buf {
		// MaxVarintLen64 10
		if i == MaxVarintLen64 {
			// Catch byte reads past MaxVarintLen64.
			// See issue https://golang.org/issues/41185
			return 0, -(i + 1) // overflow
		}
		// 0x80 128
		if b < 0x80 {
			if i == MaxVarintLen64-1 && b > 1 {
				return 0, -(i + 1) // overflow
			}
			return x | uint64(b)<<s, i + 1
		}
		// 0x7f 127
		x |= uint64(b&0x7f) << s
		s += 7
	}
	return 0, 0
}

代码示例

package main

import (
	"encoding/binary"
	"fmt"
)

func main() {
	inputs := [][]byte{
		[]byte{0x01},       //1
		[]byte{0x02},       //2
		[]byte{0x7f},       //127
		[]byte{0x80, 0x01}, //128,1
		[]byte{0xff, 0x01}, //255,1
		[]byte{0x80, 0x02}, //128,2
		[]byte{0x80, 0x03}, //128,3
	}
	for _, b := range inputs {
		x, n := binary.Uvarint(b)
		if n != len(b) {
			fmt.Println("Uvarint did not consume all of in")
		}
		fmt.Println(x)
	}
}

输出结果

1
2
127
128
255
256
384

Program exited.

https://go.dev/play/p/df572cFK6Im

在线16进制转10进制:https://jisuan5.com/hexadecimal-to-decimal/
参考文章:https://cloud.tencent.com/developer/section/1141534