// Matches returns whether given content matches c.
func (c Checksum) Matches(content []byte, parts int) error {
	if len(c.Encoded) == 0 {
		return nil
	}
	hasher := c.Type.Hasher()
	_, err := hasher.Write(content)
	if err != nil {
		return err
	}
	sum := hasher.Sum(nil)
	if c.WantParts > 0 && c.WantParts != parts {
		return ChecksumMismatch{
			Want: fmt.Sprintf("%s-%d", c.Encoded, c.WantParts),