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
?
从io.Reader创建io.ReaderAt
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- duanpiyao2734 2016-10-23 18:34关注
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"
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