去二进制编码变体与固定切片长度

Question

I need to encode integer keys as byte slices for a KV database. I want to make the encoding smaller and cut the zero padding. I thought the variant encoding from the binary package would be the way to go.

But in both cases, variant and fixed, the byte slice length is the same. Just different bits arrangement since first bit is used as a flag. I assumed the variant encoding would cut the "extra fat". No.

package main

import (
    "encoding/binary"
    "fmt"
)

func main() {

    x := 16
    y := 106547

    fmt.Println(x)
    fmt.Println(y)

    // Variant
    bvx := make([]byte, 8)
    bvy := make([]byte, 8)

    xbts := binary.PutUvarint(bvx, uint64(x))
    ybts := binary.PutUvarint(bvy, uint64(y))

    fmt.Println("Variant bytes written x: ", xbts)
    fmt.Println("Variant bytes written y: ", ybts)

    fmt.Println(bvx)
    fmt.Println(bvy)

    fmt.Println("bvx length: ", len(bvx))
    fmt.Println("bvy length: ", len(bvy))

    // Fixed
    bfx := make([]byte, 8)
    bfy := make([]byte, 8)

    binary.LittleEndian.PutUint64(bfx, uint64(x))
    binary.LittleEndian.PutUint64(bfy, uint64(y))

    fmt.Println(bfx)
    fmt.Println(bfy)

    fmt.Println("bfx length: ", len(bfx))
    fmt.Println("bfy length: ", len(bfy))

}

My question is. Do I have to splice the byte slice manually with variant encoding to get rid of the extra bytes? Since put PutUvariant returns the number of bytes written, I can just splice the byte slice.

Is this the right way to do it? If not, what is the correct way to make the slices smaller?

Thanks

Answer 1

Package binary

import "encoding/binary"

func PutUvarint

func PutUvarint(buf []byte, x uint64) int

PutUvarint encodes a uint64 into buf and returns the number of bytes written. If the buffer is too small, PutUvarint will panic.

---

Fix your code:

bvx := make([]byte, binary.MaxVarintLen64)
bvy := make([]byte, binary.MaxVarintLen64)
bvx = bvx[:binary.PutUvarint(bvx[:cap(bvx)], uint64(x))]
bvy = bvy[:binary.PutUvarint(bvy[:cap(bvy)], uint64(y))]

---

package main

import (
    "encoding/binary"
    "fmt"
)

func main() {

    x := 16
    y := 106547

    fmt.Println(x)
    fmt.Println(y)

    // Variant
    bvx := make([]byte, binary.MaxVarintLen64)
    bvy := make([]byte, binary.MaxVarintLen64)

    bvx = bvx[:binary.PutUvarint(bvx[:cap(bvx)], uint64(x))]
    bvy = bvy[:binary.PutUvarint(bvy[:cap(bvy)], uint64(y))]

    fmt.Println("Variant bytes written x: ", len(bvx))
    fmt.Println("Variant bytes written y: ", len(bvy))

    fmt.Println(bvx)
    fmt.Println(bvy)

    fmt.Println("bvx length: ", len(bvx))
    fmt.Println("bvy length: ", len(bvy))

    // Fixed
    bfx := make([]byte, 8)
    bfy := make([]byte, 8)

    binary.LittleEndian.PutUint64(bfx, uint64(x))
    binary.LittleEndian.PutUint64(bfy, uint64(y))

    fmt.Println(bfx)
    fmt.Println(bfy)

    fmt.Println("bfx length: ", len(bfx))
    fmt.Println("bfy length: ", len(bfy))
}

Playground: https://play.golang.org/p/XN46KafMY23

Output:

16
106547
Variant bytes written x:  1
Variant bytes written y:  3
[16]
[179 192 6]
bvx length:  1
bvy length:  3
[16 0 0 0 0 0 0 0]
[51 160 1 0 0 0 0 0]
bfx length:  8
bfy length:  8