package cowutils

import (
	"errors"
	"io"
)

// sectionWriter is a helper used when we need to fallback into copying data manually
type sectionWriter struct {
	w    io.WriterAt // target file
	base int64       // base position in file
	off  int64       // current relative offset
}

// Write writes & updates offset
func (s *sectionWriter) Write(p []byte) (int, error) {
	n, err := s.w.WriteAt(p, s.base+s.off)
	s.off += int64(n)
	return n, err
}

func (s *sectionWriter) Seek(offset int64, whence int) (int64, error) {
	switch whence {
	case io.SeekStart:
		// nothing needed
	case io.SeekCurrent:
		offset += s.off
	case io.SeekEnd:
		// we don't support io.SeekEnd
		fallthrough
	default:
		return s.off, errors.New("Seek: invalid whence")
	}
	if offset < 0 {
		return s.off, errors.New("Seek: invalid offset")
	}
	s.off = offset
	return offset, nil
}