// interest_rate returns the interest rate for the provided balance.
double interest_rate(double balance) {
    if (balance < 0)
        return 3.213;
    if (balance >= 5000)
        return 2.475;
    if (balance < 1000)
        return 0.5;
    return 1.621;
}

// yearly_interest calculates the yearly interest for the provided balance.
double yearly_interest(double balance) {
    return balance * interest_rate(balance) / 100;
}

// annual_balance_update calculates the annual balance update, taking into
// account the interest rate.
double annual_balance_update(double balance) {
    return balance + yearly_interest(balance);
}

// years_until_desired_balance calculates the minimum number of years required
// to reach the desired balance.
int years_until_desired_balance(double balance, double target_balance) {
    int y = 0;

    for (double u{balance}; u < target_balance; u = annual_balance_update(u)) {
        ++y;
    }

    return y;
}