package mpb

import (
	"errors"
	"io"
	"time"
)

// errInvalidWrite means that a write returned an impossible count.
var errInvalidWrite = errors.New("invalid write result")

// copyBuffer is a shameless copy paste of the same function from std lib's io.go with necessary
// changes to track ewma counters.
func copyBuffer(bar *Bar, dst io.Writer, src io.Reader, buf []byte) (written int64, err error) {
	if buf == nil {
		size := 32 * 1024
		if l, ok := src.(*io.LimitedReader); ok && int64(size) > l.N {
			if l.N < 1 {
				size = 1
			} else {
				size = int(l.N)
			}
		}
		buf = make([]byte, size)
	}
	for {
		start := time.Now()
		nr, er := src.Read(buf)
		if nr > 0 {
			nw, ew := dst.Write(buf[0:nr])
			if nw < 0 || nr < nw {
				nw = 0
				if ew == nil {
					ew = errInvalidWrite
				}
			}
			written += int64(nw)
			bar.EwmaIncrBy(nw, time.Since(start))
			if ew != nil {
				err = ew
				break
			}
			if nr != nw {
				err = io.ErrShortWrite
				break
			}
		} else {
			bar.EwmaIncrBy(0, time.Since(start))
		}
		if er != nil {
			if er != io.EOF {
				err = er
			}
			break
		}
	}
	return written, err
}