Go Varint返回预期值的一半

Question

Why is the output of this code:

package main

import (
    "fmt"
    "encoding/binary"
)

func main() {
    var myByte byte = 18
    array := []byte{myByte}
    val, n := binary.Varint(array)
    fmt.Printf("value: %d, num bytes: %d
", val, n)
}

value: 9, num bytes: 1 instead of value: 18, num bytes: 1

It probably has something to do with two's complement, but I'm not seeing how.

Answer 1

Yes. Check this modified version: http://play.golang.org/p/AyP2a4gue8

package main

import (
    "encoding/binary"
    "fmt"
)

func main() {
    for i := 0; i < 100; i++ {
        var myByte byte = byte(i)
        array := []byte{myByte}
        val, n := binary.Varint(array)
        fmt.Printf("int %d value: %d, num bytes: %d
", i, val, n)

    }    
}

which produces the following output:

int 0 value: 0, num bytes: 1
int 1 value: -1, num bytes: 1
int 2 value: 1, num bytes: 1
int 3 value: -2, num bytes: 1
int 4 value: 2, num bytes: 1
int 5 value: -3, num bytes: 1
int 6 value: 3, num bytes: 1
int 7 value: -4, num bytes: 1
int 8 value: 4, num bytes: 1
int 9 value: -5, num bytes: 1
int 10 value: 5, num bytes: 1
int 11 value: -6, num bytes: 1
int 12 value: 6, num bytes: 1
int 13 value: -7, num bytes: 1
int 14 value: 7, num bytes: 1
int 15 value: -8, num bytes: 1
int 16 value: 8, num bytes: 1
int 17 value: -9, num bytes: 1
int 18 value: 9, num bytes: 1

You can see "zig-zagging" between the negatives and positives. This is because, according to the documented binary format, varints use "zig-zag" encoding so that values of small absolute value are encoded with small values.