Here is an example of usage:

func Example() {
	pass := "my_pass"
	hash := "my_pass"

	pwd := passwd.New(
		&unix.MD5{}, // first is preferred type.
		&plainPasswd{},
	)

	_, err := pwd.Passwd(pass, hash)
	if err != nil {
		fmt.Println("fail: ", err)
	}

	// Check if we want to update.
	if !pwd.IsPreferred(hash) {
		newHash, err := pwd.Passwd(pass, "")
		if err != nil {
			fmt.Println("fail: ", err)
		}

		fmt.Println("new hash:", newHash)
	}

	// Output:
	//  new hash: $1$81ed91e1131a3a5a50d8a68e8ef85fa0
}

This shows how one would set a preferred hashing type and if the current version of ones password is not the preferred type updates it to enhance the security of the hashed password when someone logs in.

https://github.com/sour-is/go-passwd/blob/main/passwd_test.go#L33-L59