package generic

import "io"

// io.CopyBuffer has extra tests for interface like io.ReaderFrom and io.WriterTo
// which is not efficient in memory management from tests
func CopyBuffer(dst io.Writer, src io.Reader, buf []byte) (written int64, err error) {
	if buf != nil && len(buf) == 0 {
		panic("empty buffer in copyBuffer")
	}

	for {
		nr, er := src.Read(buf)
		if nr > 0 {
			nw, ew := dst.Write(buf[0:nr])
			if nw > 0 {
				written += int64(nw)
			}
			if ew != nil {
				err = ew
				break
			}
			if nr != nw {
				err = io.ErrShortWrite
				break
			}
		}
		if er != nil {
			if er != io.EOF {
				err = er
			}
			break
		}
	}
	return written, err
}