package pointers

import (
    "fmt"
    "errors"
)

type Bitcoin int

type Stringer interface {
    String() string
}

func (b Bitcoin) String() string {
    return fmt.Sprintf("%d BTC", b)
}

type Wallet struct {
    // when using lowercase properties names, it will be private
    balance Bitcoin
}

func (w *Wallet) Deposit(amount Bitcoin) {
    w.balance += amount
}

func (w *Wallet) Balance() Bitcoin {
    return w.balance
}

func (w *Wallet) Withdraw(amount Bitcoin) error {
    if amount > w.balance {
        return errors.New("cannot withdraw, insufficient funds")
    }

    w.balance -= amount
    return nil
}