// Chapter 30 example: package bank. Every rule about an account is here, and
// other packages cannot reach past these methods to the fields.

package bank

import (
	"errors"
	"fmt"
)

// Account is one bank account. Make one with NewAccount.
type Account struct {
	owner   string
	balance int
}

// NewAccount returns an account with a balance of 0. The owner must not be
// empty.
func NewAccount(owner string) (*Account, error) {
	if owner == "" {
		return nil, errors.New("an account needs an owner")
	}
	return &Account{owner: owner}, nil
}

// checkAmount is a helper for this package only.
func checkAmount(amount int) error {
	if amount <= 0 {
		return fmt.Errorf("the amount %d is not above 0", amount)
	}
	return nil
}

// Deposit adds to the balance. The amount must be above 0.
func (a *Account) Deposit(amount int) error {
	err := checkAmount(amount)
	if err != nil {
		return err
	}
	a.balance = a.balance + amount
	return nil
}

// Withdraw takes from the balance. The amount must be above 0, and no more
// than the balance.
func (a *Account) Withdraw(amount int) error {
	err := checkAmount(amount)
	if err != nil {
		return err
	}
	if amount > a.balance {
		return fmt.Errorf("cannot withdraw %d: the balance is %d", amount, a.balance)
	}
	a.balance = a.balance - amount
	return nil
}

// Owner returns the name on the account.
func (a *Account) Owner() string {
	return a.owner
}

// Balance returns the current balance.
func (a *Account) Balance() int {
	return a.balance
}