bytes.Buffer and bufio.Reader are no seekers?

[Ingo Oeser]

Hi there,

is there any reason the in-memory streams bytes.Buffer and bufio.Reader support no Seek()?

Just wondered about it, because it would be very handy for unit tests and benchmarks mocking out file system accesses.

So I did sth. like this to test it:

http://play.golang.org/p/8SjYn-cisj

package main

import (

"bytes"

"fmt"

"io"

"os"

)

type SeekingBuffer struct {

b      []byte

buffer *bytes.Buffer

offset int64

size   int64

}

func NewSeekingBuffer(b []byte) *SeekingBuffer {

if b == nil {

return nil

}

return &SeekingBuffer{

b:      b,

buffer: bytes.NewBuffer(b),

offset: 0,

}

}

func (fb *SeekingBuffer) Read(p []byte) (n int, err error) {

n, err = fb.buffer.Read(p)

fb.offset += int64(n)

return n, err

}

func (fb *SeekingBuffer) Seek(offset int64, whence int) (ret int64, err error) {

var newoffset int64

switch whence {

case 0:

newoffset = offset

case 1:

newoffset = fb.offset + offset

case 2:

newoffset = int64(len(fb.b)) - offset

}

if newoffset == fb.offset {

return newoffset, nil

}

fb.buffer = bytes.NewBuffer(fb.b[newoffset:])

fb.offset = newoffset

return fb.offset, nil

}

func main() {

fb := NewSeekingBuffer([]byte("Hello, playground"))

pos, err := fb.Seek(0, 0)

fmt.Println("pos = ", pos, ", error = ", err)

pos, err = fb.Seek(7, 0)

fmt.Println("pos = ", pos, ", error = ", err)

io.Copy(os.Stdout, fb)

}

[Andrew Gerrand]

On 2 May 2013 10:15, Ingo Oeser <night...@googlemail.com> wrote:

is there any reason the in-memory streams bytes.Buffer and bufio.Reader support no Seek()?

Because they both throw data away after it has been read. They're for potentially boundless streams of data, not in-memory files.

If you want the latter, see bytes.Reader: http://golang.org/pkg/bytes/#Reader

[Ingo Oeser]

Cool thanks! Totally forgot about this bytes.Reader :-)

[Guillaume LE STUM]

I just ran into this as well, when buffering an io.ReadSeeker such as os.File it would be useful to have a bufio.ReadSeeker.

I could work around it by calling Seek on the source os.File and using bufio.Reader.Reset(...) but this is wasteful when seeked data is already buffered