将额外的数据写入io.Writer

Question

I am trying to figure out how to change the behaviour of jpeg.Encode API (http://golang.org/pkg/image/jpeg/#Encode) that takes an io.Writer as a parameter and writes the file to it.

I need to insert a set of bytes after the first two bytes written in to the file, and I think it is possible to do without keeping everything in memory. I am new to Golang, I found io.Pipe that connects a reader and a writer, but I do not understand how to use it for what I need.

Could someone help?

EDIT: should I just implement my own io.Writer interface?

Answer 1

If you know upfront the bytes you need to add in the stream (whatever the stream, image or not), building a Writer object for this specific purpose is not really hard.

Here is an example:

package main

import (
    "fmt"
    "image"
    "image/draw"
    "image/jpeg"
    "image/color"
    "os"
    "io"
)

type ByteInserter struct {
    n   int
    b   []byte
    pos int
    w   io.Writer
}

func NewByteInserter(w io.Writer) *ByteInserter {
    return &ByteInserter{w:w}
}

func (bi* ByteInserter) Set( b[]byte, pos int ) {
    bi.b = b
    bi.pos = pos
}

func (bi *ByteInserter) Write( p []byte ) (n int, err error) {
    if bi.n>bi.pos || bi.n+len(p)<=bi.pos {
        n, err = bi.w.Write(p)
        bi.n += n
    } else {
        cut := bi.pos-bi.n
        if cut > 0 {
            n,err = bi.w.Write(p[:cut])
            bi.n += n
            if err != nil {
                return
            }
        }
        _,err = bi.w.Write(bi.b)
        if err != nil {
            return
        }
        n2 := 0
        n2, err = bi.w.Write(p[cut:])
        bi.n += n2
        n += n2
    }
    return
}

func main() {

    // Blue rectangle, stolen from Nigel Tao's post
    // http://blog.golang.org/go-imagedraw-package
    img := image.NewRGBA(image.Rect(0, 0, 640, 480))
    blue := color.RGBA{0, 0, 255, 255}
    draw.Draw(img, img.Bounds(), &image.Uniform{blue}, image.ZP, draw.Src)

    file, err := os.Create("file.jpg")
    if err != nil {
        panic(err)
    }

    bi := NewByteInserter(file)
    bi.Set( []byte("XXX"), 2 )   // Three bytes added at position 2
    jpeg.Encode(bi,img,nil)
    file.Close()

    fmt.Println("Written!")
}

That said, editing a stream to customize an image seems really hackish to me.