douzhan5058 2016-10-23 15:51
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从io.Reader创建io.ReaderAt

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?

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  • duanpiyao2734 2016-10-23 18:34
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    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"
    
    本回答被题主选为最佳回答 , 对您是否有帮助呢?
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