You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: hdescobarh.rs
    path of file: ./repos/retos-programacion-2023/Retos/Reto #44 - ADIVINANZAS MATEMÁTICAS [Media]/rust
    
    the code of the file until where you have to start completion: // author: Hans D. Escobar H. (hdescobarh)

/*
Solución con el mínimo número de bibliotecas externas.

rand es necesario porque en la
    
    fn operand_digits_works() {
        let mut game = Game::default();
        assert_eq!((1, 1), game.generate_operands_digits());
        game.difficulty_level = 2;
        assert_eq!((2, 1), game.generate_operands_digits());
        game.difficulty_level = 3;
        assert_eq!((2, 2), game.generate_operands_digits());
        game.difficulty_level = 4;
        assert_eq!((3, 2), game.generate_operands_digits());
        game.difficulty_level = 12;
        assert_eq!((7, 6), game.generate_operands_digits());
        game.difficulty_level = 13;
        assert_eq!((7, 7), game.generate_operands_digits());
        game.difficulty_level = 81;
        assert_eq!((41, 41), game.generate_operands_digits());
    }