从io.Reader创建io.ReaderAt

Question

Is there an implementation of io.ReaderAt that can be created from an implementation of io.Reader without first being read into a []byte or string?

Answer 1

Yes, this is possible. As mentioned in my comment above, the implementation is limited in that you cannot seek backward nor can you re-read a section that has already been read.

Here is a example implementation:

type unbufferedReaderAt struct {
    R io.Reader
    N int64
}

func NewUnbufferedReaderAt(r io.Reader) io.ReaderAt {
    return &unbufferedReaderAt{R: r}
}

func (u *unbufferedReaderAt) ReadAt(p []byte, off int64) (n int, err error) {
    if off < u.N {
        return 0, errors.New("invalid offset")
    }
    diff := off - u.N
    written, err := io.CopyN(ioutil.Discard, u.R, diff)
    u.N += written
    if err != nil {
        return 0, err
    }

    n, err = u.R.Read(p)
    u.N += int64(n)
    return
}

Example usage:

s := strings.NewReader("hello world")

var b [5]byte
ura := NewUnbufferedReaderAt(s)
if _, err := ura.ReadAt(b[:], 0); err != nil {
    panic(err)
}
fmt.Printf("%s
", b[:]) // prints "hello"

/*
if _, err := ura.ReadAt(b[:], 0); err != nil {
    panic(err) // panics
}
fmt.Printf("%s
", b[:])
*/

if _, err := ura.ReadAt(b[:], 6); err != nil {
    panic(err)
}
fmt.Printf("%s
", b[:]) // prints "world"

Answer 2

Something like the below. Note bytes.Reader implements the ReadAt(...) method/function: https://golang.org/pkg/bytes/#Reader.ReadAt. So the line bytes.NewReader is esssentially what you are looking for.

Getting a bytes.Reader:

var ioReader io.Reader
...
buff := bytes.NewBuffer([]byte{})
size, err := io.Copy(buff, ioReader)
if err != nil {
    return err
}

reader := bytes.NewReader(buff.Bytes())
// Do something with `reader`