exercism/cpp/interest-is-interesting/interest_is_interesting.cpp

// interest_rate returns the interest rate for the provided balance.
double interest_rate(double balance) {

  if (balance < 0) {
    return 3.213;

  } else if (balance >= 0 && balance < 1000) {
    return 0.5;

  } else if (balance >= 1000 && balance < 5000) {
    return 1.621;

  } else // balance >= 5000
    return 2.475;
}

// 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 yearly_interest(balance) + 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 years{0};
  while (balance < target_balance) {
    years++;
    balance = annual_balance_update(balance);
  }

  return years;
}